[PATCH v2 0/3] pidfs: implement file handle support

[PATCH v2 0/3] pidfs: implement file handle support

[Erin Shepherd]

Since the introduction of pidfs, we have had 64-bit process identifiers
that will not be reused for the entire uptime of the system. This greatly
facilitates process tracking in userspace.

There are two limitations at present:

 * These identifiers are currently only exposed to processes on 64-bit
   systems. On 32-bit systems, inode space is also limited to 32 bits and
   therefore is subject to the same reuse issues.
 * There is no way to go from one of these unique identifiers to a pid or
   pidfd.

This patch implements fh_export and fh_to_dentry which enables userspace to
convert PIDs to and from PID file handles. A process can convert a pidfd into
a file handle using name_to_handle_at, store it (in memory, on disk, or 
elsewhere) and then convert it back into a pidfd suing open_by_handle_at.

To support us going from a file handle to a pidfd, we have to store a pid
inside the file handle. To ensure file handles are invariant and can move
between pid namespaces, we stash a pid from the initial namespace inside
the file handle.
	
  (There has been some discussion as to whether or not it is OK to include
  the PID in the initial pid namespace, but so far there hasn't been any
  conclusive reason given as to why this would be a bad idea)	

Signed-off-by: Erin Shepherd <erin.shepherd@e43.eu>
---
Changes in v2:
- Permit filesystems to opt out of CAP_DAC_READ_SEARCH
- Inline find_pid_ns/get_pid logic; remove unnecessary put_pid
- Squash fh_export & fh_to_dentry into one commit
- Link to v1: https://lore.kernel.org/r/2aa94713-c12a-4344-a45c-a01f26e16a0d@e43.eu

Remaining: To address the PIDFD_THREAD question
 - Do we want to stash it in file handles (IMO no but there may be merits to
   doing so)
 - If not do we want PIDFD_THREAD/O_EXCL to open_by_handle_at to work or do we
   do something else?

   (Perhaps we could just add an ioctl which lets an app change the PIDFD_THREAD flag?)

If PIDFD_SELF lands <https://lore.kernel.org/r/cover.1727644404.git.lorenzo.stoakes@oracle.com>:
 - Support for PIDFD_SELF(_THREAD) as reference fd in open_by_handle_at?
   (We can even detect that special case early there and bypass most of the file handle logic)

---
Erin Shepherd (3):
      pseudofs: add support for export_ops
      exportfs: allow fs to disable CAP_DAC_READ_SEARCH check
      pidfs: implement file handle support

 fs/fhandle.c              | 36 +++++++++++++++------------
 fs/libfs.c                |  1 +
 fs/pidfs.c                | 62 ++++++++++++++++++++++++++++++++++++++++++++++-
 include/linux/exportfs.h  |  3 +++
 include/linux/pseudo_fs.h |  1 +
 5 files changed, 87 insertions(+), 16 deletions(-)
---
base-commit: 14b6320953a3f856a3f93bf9a0e423395baa593d
change-id: 20241112-pidfs_fh-fbb04b108a2b

Best regards,
-- 
Erin Shepherd <erin.shepherd@e43.eu>

[PATCH v2 1/3] pseudofs: add support for export_ops

[Erin Shepherd]

Pseudo-filesystems might reasonably wish to implement the export ops
(particularly for name_to_handle_at/open_by_handle_at); plumb this
through pseudo_fs_context

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Erin Shepherd <erin.shepherd@e43.eu>
---
 fs/libfs.c                | 1 +
 include/linux/pseudo_fs.h | 1 +
 2 files changed, 2 insertions(+)

diff --git a/fs/libfs.c b/fs/libfs.c
index 46966fd8bcf9f042e85d0b66134e59fbef83abfd..698a2ddfd0cb94a8927d1d8a3bb3b3226d6d5476 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -669,6 +669,7 @@ static int pseudo_fs_fill_super(struct super_block *s, struct fs_context *fc)
 	s->s_blocksize_bits = PAGE_SHIFT;
 	s->s_magic = ctx->magic;
 	s->s_op = ctx->ops ?: &simple_super_operations;
+	s->s_export_op = ctx->eops;
 	s->s_xattr = ctx->xattr;
 	s->s_time_gran = 1;
 	root = new_inode(s);
diff --git a/include/linux/pseudo_fs.h b/include/linux/pseudo_fs.h
index 730f77381d55f1816ef14adf7dd2cf1d62bb912c..2503f7625d65e7b1fbe9e64d5abf06cd8f017b5f 100644
--- a/include/linux/pseudo_fs.h
+++ b/include/linux/pseudo_fs.h
@@ -5,6 +5,7 @@
 
 struct pseudo_fs_context {
 	const struct super_operations *ops;
+	const struct export_operations *eops;
 	const struct xattr_handler * const *xattr;
 	const struct dentry_operations *dops;
 	unsigned long magic;

-- 
2.46.1

[PATCH v2 2/3] exportfs: allow fs to disable CAP_DAC_READ_SEARCH check

[Erin Shepherd]

For pidfs, there is no reason to restrict file handle decoding by
CAP_DAC_READ_SEARCH. Introduce an export_ops flag that can indicate
this

Signed-off-by: Erin Shepherd <erin.shepherd@e43.eu>
---
 fs/fhandle.c             | 36 +++++++++++++++++++++---------------
 include/linux/exportfs.h |  3 +++
 2 files changed, 24 insertions(+), 15 deletions(-)

diff --git a/fs/fhandle.c b/fs/fhandle.c
index 82df28d45cd70a7df525f50bbb398d646110cd99..056116e58f43983bc7bb86da170fb554c7a2fac7 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -235,26 +235,32 @@ static int do_handle_to_path(struct file_handle *handle, struct path *path,
 	return 0;
 }
 
-/*
- * Allow relaxed permissions of file handles if the caller has the
- * ability to mount the filesystem or create a bind-mount of the
- * provided @mountdirfd.
- *
- * In both cases the caller may be able to get an unobstructed way to
- * the encoded file handle. If the caller is only able to create a
- * bind-mount we need to verify that there are no locked mounts on top
- * of it that could prevent us from getting to the encoded file.
- *
- * In principle, locked mounts can prevent the caller from mounting the
- * filesystem but that only applies to procfs and sysfs neither of which
- * support decoding file handles.
- */
 static inline bool may_decode_fh(struct handle_to_path_ctx *ctx,
 				 unsigned int o_flags)
 {
 	struct path *root = &ctx->root;
+	struct export_operations *nop = root->mnt->mnt_sb->s_export_op;
+
+	if (nop && nop->flags & EXPORT_OP_UNRESTRICTED_OPEN)
+		return true;
+
+	if (capable(CAP_DAC_READ_SEARCH))
+		return true;
 
 	/*
+	 * Allow relaxed permissions of file handles if the caller has the
+	 * ability to mount the filesystem or create a bind-mount of the
+	 * provided @mountdirfd.
+	 *
+	 * In both cases the caller may be able to get an unobstructed way to
+	 * the encoded file handle. If the caller is only able to create a
+	 * bind-mount we need to verify that there are no locked mounts on top
+	 * of it that could prevent us from getting to the encoded file.
+	 *
+	 * In principle, locked mounts can prevent the caller from mounting the
+	 * filesystem but that only applies to procfs and sysfs neither of which
+	 * support decoding file handles.
+	 *
 	 * Restrict to O_DIRECTORY to provide a deterministic API that avoids a
 	 * confusing api in the face of disconnected non-dir dentries.
 	 *
@@ -293,7 +299,7 @@ static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
 	if (retval)
 		goto out_err;
 
-	if (!capable(CAP_DAC_READ_SEARCH) && !may_decode_fh(&ctx, o_flags)) {
+	if (!may_decode_fh(&ctx, o_flags)) {
 		retval = -EPERM;
 		goto out_path;
 	}
diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
index 893a1d21dc1c4abc7e52325d7a4cf0adb407f039..459508b53e77ed0597cee217ffe3d82cc7cc11a4 100644
--- a/include/linux/exportfs.h
+++ b/include/linux/exportfs.h
@@ -247,6 +247,9 @@ struct export_operations {
 						*/
 #define EXPORT_OP_FLUSH_ON_CLOSE	(0x20) /* fs flushes file data on close */
 #define EXPORT_OP_ASYNC_LOCK		(0x40) /* fs can do async lock request */
+#define EXPORT_OP_UNRESTRICTED_OPEN	(0x80) /* FS allows open_by_handle_at
+						  without CAP_DAC_READ_SEARCH
+						*/
 	unsigned long	flags;
 };
 

-- 
2.46.1

[PATCH v2 3/3] pidfs: implement file handle support

[Erin Shepherd]

On 64-bit platforms, userspace can read the pidfd's inode in order to
get a never-repeated PID identifier. On 32-bit platforms this identifier
is not exposed, as inodes are limited to 32 bits. Instead expose the
identifier via export_fh, which makes it available to userspace via
name_to_handle_at

In addition we implement fh_to_dentry, which allows userspace to
recover a pidfd from a PID file handle.

We stash the process' PID in the root pid namespace inside the handle,
and use that to recover the pid (validating that pid->ino matches the
value in the handle, i.e. that the pid has not been reused).

We use the root namespace in order to ensure that file handles can be
moved across namespaces; however, we validate that the PID exists in
the current namespace before returning the inode.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Erin Shepherd <erin.shepherd@e43.eu>
---
 fs/pidfs.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 61 insertions(+), 1 deletion(-)

diff --git a/fs/pidfs.c b/fs/pidfs.c
index 80675b6bf88459c22787edaa68db360bdc0d0782..0684a9b8fe71c5205fb153b2714bc9c672045fd5 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/anon_inodes.h>
+#include <linux/exportfs.h>
 #include <linux/file.h>
 #include <linux/fs.h>
 #include <linux/magic.h>
@@ -347,11 +348,69 @@ static const struct dentry_operations pidfs_dentry_operations = {
 	.d_prune	= stashed_dentry_prune,
 };
 
+#define PIDFD_FID_LEN 3
+
+struct pidfd_fid {
+	u64 ino;
+	s32 pid;
+} __packed;
+
+static int pidfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
+			   struct inode *parent)
+{
+	struct pid *pid = inode->i_private;
+	struct pidfd_fid *fid = (struct pidfd_fid *)fh;
+
+	if (*max_len < PIDFD_FID_LEN) {
+		*max_len = PIDFD_FID_LEN;
+		return FILEID_INVALID;
+	}
+
+	fid->ino = pid->ino;
+	fid->pid = pid_nr(pid);
+	*max_len = PIDFD_FID_LEN;
+	return FILEID_INO64_GEN;
+}
+
+static struct dentry *pidfs_fh_to_dentry(struct super_block *sb,
+					 struct fid *gen_fid,
+					 int fh_len, int fh_type)
+{
+	int ret;
+	struct path path;
+	struct pidfd_fid *fid = (struct pidfd_fid *)gen_fid;
+	struct pid *pid;
+
+	if (fh_type != FILEID_INO64_GEN || fh_len < PIDFD_FID_LEN)
+		return NULL;
+
+	scoped_guard(rcu) {
+		pid = find_pid_ns(fid->pid, &init_pid_ns);
+		if (!pid || pid->ino != fid->ino || pid_vnr(pid) == 0)
+			return NULL;
+
+		pid = get_pid(pid);
+	}
+
+	ret = path_from_stashed(&pid->stashed, pidfs_mnt, pid, &path);
+	if (ret < 0)
+		return ERR_PTR(ret);
+
+	mntput(path.mnt);
+	return path.dentry;
+}
+
+static const struct export_operations pidfs_export_operations = {
+	.encode_fh = pidfs_encode_fh,
+	.fh_to_dentry = pidfs_fh_to_dentry,
+	.flags = EXPORT_OP_UNRESTRICTED_OPEN,
+};
+
 static int pidfs_init_inode(struct inode *inode, void *data)
 {
 	inode->i_private = data;
 	inode->i_flags |= S_PRIVATE;
-	inode->i_mode |= S_IRWXU;
+	inode->i_mode |= S_IRWXU | S_IRWXG | S_IRWXO;
 	inode->i_op = &pidfs_inode_operations;
 	inode->i_fop = &pidfs_file_operations;
 	/*
@@ -382,6 +441,7 @@ static int pidfs_init_fs_context(struct fs_context *fc)
 		return -ENOMEM;
 
 	ctx->ops = &pidfs_sops;
+	ctx->eops = &pidfs_export_operations;
 	ctx->dops = &pidfs_dentry_operations;
 	fc->s_fs_info = (void *)&pidfs_stashed_ops;
 	return 0;

-- 
2.46.1

Re: [PATCH v2 2/3] exportfs: allow fs to disable CAP_DAC_READ_SEARCH check

[kernel test robot]

Hi Erin,

kernel test robot noticed the following build errors:

[auto build test ERROR on 14b6320953a3f856a3f93bf9a0e423395baa593d]

url:    https://github.com/intel-lab-lkp/linux/commits/Erin-Shepherd/pseudofs-add-support-for-export_ops/20241114-020539
base:   14b6320953a3f856a3f93bf9a0e423395baa593d
patch link:    https://lore.kernel.org/r/20241113-pidfs_fh-v2-2-9a4d28155a37%40e43.eu
patch subject: [PATCH v2 2/3] exportfs: allow fs to disable CAP_DAC_READ_SEARCH check
config: x86_64-allnoconfig (https://download.01.org/0day-ci/archive/20241114/202411140603.E03vXsdz-lkp@intel.com/config)
compiler: clang version 19.1.3 (https://github.com/llvm/llvm-project ab51eccf88f5321e7c60591c5546b254b6afab99)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20241114/202411140603.E03vXsdz-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202411140603.E03vXsdz-lkp@intel.com/

All errors (new ones prefixed by >>):

   In file included from fs/fhandle.c:2:
   In file included from include/linux/syscalls.h:93:
   In file included from include/trace/syscall.h:7:
   In file included from include/linux/trace_events.h:6:
   In file included from include/linux/ring_buffer.h:5:
   In file included from include/linux/mm.h:2213:
   include/linux/vmstat.h:518:36: warning: arithmetic between different enumeration types ('enum node_stat_item' and 'enum lru_list') [-Wenum-enum-conversion]
     518 |         return node_stat_name(NR_LRU_BASE + lru) + 3; // skip "nr_"
         |                               ~~~~~~~~~~~ ^ ~~~
>> fs/fhandle.c:242:28: error: initializing 'struct export_operations *' with an expression of type 'const struct export_operations *' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
     242 |         struct export_operations *nop = root->mnt->mnt_sb->s_export_op;
         |                                   ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   1 warning and 1 error generated.


vim +242 fs/fhandle.c

   237	
   238	static inline bool may_decode_fh(struct handle_to_path_ctx *ctx,
   239					 unsigned int o_flags)
   240	{
   241		struct path *root = &ctx->root;
 > 242		struct export_operations *nop = root->mnt->mnt_sb->s_export_op;
   243	
   244		if (nop && nop->flags & EXPORT_OP_UNRESTRICTED_OPEN)
   245			return true;
   246	
   247		if (capable(CAP_DAC_READ_SEARCH))
   248			return true;
   249	
   250		/*
   251		 * Allow relaxed permissions of file handles if the caller has the
   252		 * ability to mount the filesystem or create a bind-mount of the
   253		 * provided @mountdirfd.
   254		 *
   255		 * In both cases the caller may be able to get an unobstructed way to
   256		 * the encoded file handle. If the caller is only able to create a
   257		 * bind-mount we need to verify that there are no locked mounts on top
   258		 * of it that could prevent us from getting to the encoded file.
   259		 *
   260		 * In principle, locked mounts can prevent the caller from mounting the
   261		 * filesystem but that only applies to procfs and sysfs neither of which
   262		 * support decoding file handles.
   263		 *
   264		 * Restrict to O_DIRECTORY to provide a deterministic API that avoids a
   265		 * confusing api in the face of disconnected non-dir dentries.
   266		 *
   267		 * There's only one dentry for each directory inode (VFS rule)...
   268		 */
   269		if (!(o_flags & O_DIRECTORY))
   270			return false;
   271	
   272		if (ns_capable(root->mnt->mnt_sb->s_user_ns, CAP_SYS_ADMIN))
   273			ctx->flags = HANDLE_CHECK_PERMS;
   274		else if (is_mounted(root->mnt) &&
   275			 ns_capable(real_mount(root->mnt)->mnt_ns->user_ns,
   276				    CAP_SYS_ADMIN) &&
   277			 !has_locked_children(real_mount(root->mnt), root->dentry))
   278			ctx->flags = HANDLE_CHECK_PERMS | HANDLE_CHECK_SUBTREE;
   279		else
   280			return false;
   281	
   282		/* Are we able to override DAC permissions? */
   283		if (!ns_capable(current_user_ns(), CAP_DAC_READ_SEARCH))
   284			return false;
   285	
   286		ctx->fh_flags = EXPORT_FH_DIR_ONLY;
   287		return true;
   288	}
   289	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH v2 2/3] exportfs: allow fs to disable CAP_DAC_READ_SEARCH check

[kernel test robot]

Hi Erin,

kernel test robot noticed the following build warnings:

[auto build test WARNING on 14b6320953a3f856a3f93bf9a0e423395baa593d]

url:    https://github.com/intel-lab-lkp/linux/commits/Erin-Shepherd/pseudofs-add-support-for-export_ops/20241114-020539
base:   14b6320953a3f856a3f93bf9a0e423395baa593d
patch link:    https://lore.kernel.org/r/20241113-pidfs_fh-v2-2-9a4d28155a37%40e43.eu
patch subject: [PATCH v2 2/3] exportfs: allow fs to disable CAP_DAC_READ_SEARCH check
config: openrisc-defconfig (https://download.01.org/0day-ci/archive/20241114/202411140905.a0ntnQQG-lkp@intel.com/config)
compiler: or1k-linux-gcc (GCC) 14.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20241114/202411140905.a0ntnQQG-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202411140905.a0ntnQQG-lkp@intel.com/

All warnings (new ones prefixed by >>):

   fs/fhandle.c: In function 'may_decode_fh':
>> fs/fhandle.c:242:41: warning: initialization discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
     242 |         struct export_operations *nop = root->mnt->mnt_sb->s_export_op;
         |                                         ^~~~


vim +/const +242 fs/fhandle.c

   237	
   238	static inline bool may_decode_fh(struct handle_to_path_ctx *ctx,
   239					 unsigned int o_flags)
   240	{
   241		struct path *root = &ctx->root;
 > 242		struct export_operations *nop = root->mnt->mnt_sb->s_export_op;
   243	
   244		if (nop && nop->flags & EXPORT_OP_UNRESTRICTED_OPEN)
   245			return true;
   246	
   247		if (capable(CAP_DAC_READ_SEARCH))
   248			return true;
   249	
   250		/*
   251		 * Allow relaxed permissions of file handles if the caller has the
   252		 * ability to mount the filesystem or create a bind-mount of the
   253		 * provided @mountdirfd.
   254		 *
   255		 * In both cases the caller may be able to get an unobstructed way to
   256		 * the encoded file handle. If the caller is only able to create a
   257		 * bind-mount we need to verify that there are no locked mounts on top
   258		 * of it that could prevent us from getting to the encoded file.
   259		 *
   260		 * In principle, locked mounts can prevent the caller from mounting the
   261		 * filesystem but that only applies to procfs and sysfs neither of which
   262		 * support decoding file handles.
   263		 *
   264		 * Restrict to O_DIRECTORY to provide a deterministic API that avoids a
   265		 * confusing api in the face of disconnected non-dir dentries.
   266		 *
   267		 * There's only one dentry for each directory inode (VFS rule)...
   268		 */
   269		if (!(o_flags & O_DIRECTORY))
   270			return false;
   271	
   272		if (ns_capable(root->mnt->mnt_sb->s_user_ns, CAP_SYS_ADMIN))
   273			ctx->flags = HANDLE_CHECK_PERMS;
   274		else if (is_mounted(root->mnt) &&
   275			 ns_capable(real_mount(root->mnt)->mnt_ns->user_ns,
   276				    CAP_SYS_ADMIN) &&
   277			 !has_locked_children(real_mount(root->mnt), root->dentry))
   278			ctx->flags = HANDLE_CHECK_PERMS | HANDLE_CHECK_SUBTREE;
   279		else
   280			return false;
   281	
   282		/* Are we able to override DAC permissions? */
   283		if (!ns_capable(current_user_ns(), CAP_DAC_READ_SEARCH))
   284			return false;
   285	
   286		ctx->fh_flags = EXPORT_FH_DIR_ONLY;
   287		return true;
   288	}
   289	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH v2 2/3] exportfs: allow fs to disable CAP_DAC_READ_SEARCH check

[Christoph Hellwig]

On Wed, Nov 13, 2024 at 05:55:24PM +0000, Erin Shepherd wrote:
> For pidfs, there is no reason to restrict file handle decoding by
> CAP_DAC_READ_SEARCH.

Why is there no reason, i.e. why do you think it is safe.

>Introduce an export_ops flag that can indicate
> this

Also why is is desirable?

To be this looks more than sketchy with the actual exporting hat on,
but I guess that's now how the cool kids use open by handle these days.

Re: [PATCH v2 2/3] exportfs: allow fs to disable CAP_DAC_READ_SEARCH check

[Amir Goldstein]

On Wed, Nov 13, 2024 at 8:11 PM Erin Shepherd <erin.shepherd@e43.eu> wrote:
>
> For pidfs, there is no reason to restrict file handle decoding by
> CAP_DAC_READ_SEARCH. Introduce an export_ops flag that can indicate
> this
>
> Signed-off-by: Erin Shepherd <erin.shepherd@e43.eu>
> ---
>  fs/fhandle.c             | 36 +++++++++++++++++++++---------------
>  include/linux/exportfs.h |  3 +++
>  2 files changed, 24 insertions(+), 15 deletions(-)
>
> diff --git a/fs/fhandle.c b/fs/fhandle.c
> index 82df28d45cd70a7df525f50bbb398d646110cd99..056116e58f43983bc7bb86da170fb554c7a2fac7 100644
> --- a/fs/fhandle.c
> +++ b/fs/fhandle.c
> @@ -235,26 +235,32 @@ static int do_handle_to_path(struct file_handle *handle, struct path *path,
>         return 0;
>  }
>
> -/*
> - * Allow relaxed permissions of file handles if the caller has the
> - * ability to mount the filesystem or create a bind-mount of the
> - * provided @mountdirfd.
> - *
> - * In both cases the caller may be able to get an unobstructed way to
> - * the encoded file handle. If the caller is only able to create a
> - * bind-mount we need to verify that there are no locked mounts on top
> - * of it that could prevent us from getting to the encoded file.
> - *
> - * In principle, locked mounts can prevent the caller from mounting the
> - * filesystem but that only applies to procfs and sysfs neither of which
> - * support decoding file handles.
> - */
>  static inline bool may_decode_fh(struct handle_to_path_ctx *ctx,
>                                  unsigned int o_flags)
>  {
>         struct path *root = &ctx->root;
> +       struct export_operations *nop = root->mnt->mnt_sb->s_export_op;
> +
> +       if (nop && nop->flags & EXPORT_OP_UNRESTRICTED_OPEN)
> +               return true;
> +
> +       if (capable(CAP_DAC_READ_SEARCH))
> +               return true;
>
>         /*
> +        * Allow relaxed permissions of file handles if the caller has the
> +        * ability to mount the filesystem or create a bind-mount of the
> +        * provided @mountdirfd.
> +        *
> +        * In both cases the caller may be able to get an unobstructed way to
> +        * the encoded file handle. If the caller is only able to create a
> +        * bind-mount we need to verify that there are no locked mounts on top
> +        * of it that could prevent us from getting to the encoded file.
> +        *
> +        * In principle, locked mounts can prevent the caller from mounting the
> +        * filesystem but that only applies to procfs and sysfs neither of which
> +        * support decoding file handles.
> +        *
>          * Restrict to O_DIRECTORY to provide a deterministic API that avoids a
>          * confusing api in the face of disconnected non-dir dentries.
>          *
> @@ -293,7 +299,7 @@ static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
>         if (retval)
>                 goto out_err;
>
> -       if (!capable(CAP_DAC_READ_SEARCH) && !may_decode_fh(&ctx, o_flags)) {
> +       if (!may_decode_fh(&ctx, o_flags)) {
>                 retval = -EPERM;
>                 goto out_path;
>         }
> diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
> index 893a1d21dc1c4abc7e52325d7a4cf0adb407f039..459508b53e77ed0597cee217ffe3d82cc7cc11a4 100644
> --- a/include/linux/exportfs.h
> +++ b/include/linux/exportfs.h
> @@ -247,6 +247,9 @@ struct export_operations {
>                                                 */
>  #define EXPORT_OP_FLUSH_ON_CLOSE       (0x20) /* fs flushes file data on close */
>  #define EXPORT_OP_ASYNC_LOCK           (0x40) /* fs can do async lock request */
> +#define EXPORT_OP_UNRESTRICTED_OPEN    (0x80) /* FS allows open_by_handle_at
> +                                                 without CAP_DAC_READ_SEARCH
> +                                               */

Don't love the name, but I wonder, isn't SB_NOUSER already a good
enough indication that CAP_DAC_READ_SEARCH is irrelevant?

Essentially, mnt_fd is the user's proof that they can access the mount
and CAP_DAC_READ_SEARCH is the legacy "proof" that the user can
reach from mount the inode by path lookup.

Which reminds me, what is the mnt_fd expected for opening a pidfd
file by handle?

Thanks,
Amir.

Re: [PATCH v2 0/3] pidfs: implement file handle support

[Amir Goldstein]

On Wed, Nov 13, 2024 at 7:01 PM Erin Shepherd <erin.shepherd@e43.eu> wrote:
>
> Since the introduction of pidfs, we have had 64-bit process identifiers
> that will not be reused for the entire uptime of the system. This greatly
> facilitates process tracking in userspace.
>
> There are two limitations at present:
>
>  * These identifiers are currently only exposed to processes on 64-bit
>    systems. On 32-bit systems, inode space is also limited to 32 bits and
>    therefore is subject to the same reuse issues.
>  * There is no way to go from one of these unique identifiers to a pid or
>    pidfd.
>
> This patch implements fh_export and fh_to_dentry which enables userspace to
> convert PIDs to and from PID file handles. A process can convert a pidfd into
> a file handle using name_to_handle_at, store it (in memory, on disk, or
> elsewhere) and then convert it back into a pidfd suing open_by_handle_at.
>
> To support us going from a file handle to a pidfd, we have to store a pid
> inside the file handle. To ensure file handles are invariant and can move
> between pid namespaces, we stash a pid from the initial namespace inside
> the file handle.
>
>   (There has been some discussion as to whether or not it is OK to include
>   the PID in the initial pid namespace, but so far there hasn't been any
>   conclusive reason given as to why this would be a bad idea)

IIUC, this is already exposed as st_ino on a 64bit arch?
If that is the case, then there is certainly no new info leak in this patch.

>
> Signed-off-by: Erin Shepherd <erin.shepherd@e43.eu>
> ---
> Changes in v2:
> - Permit filesystems to opt out of CAP_DAC_READ_SEARCH
> - Inline find_pid_ns/get_pid logic; remove unnecessary put_pid
> - Squash fh_export & fh_to_dentry into one commit

Not sure why you did that.
It was pretty nice as separate commits if you ask me. Whatever.

Thanks,
Amir.

Re: [PATCH v2 3/3] pidfs: implement file handle support

[Amir Goldstein]

On Wed, Nov 13, 2024 at 7:11 PM Erin Shepherd <erin.shepherd@e43.eu> wrote:
>
> On 64-bit platforms, userspace can read the pidfd's inode in order to
> get a never-repeated PID identifier. On 32-bit platforms this identifier
> is not exposed, as inodes are limited to 32 bits. Instead expose the
> identifier via export_fh, which makes it available to userspace via
> name_to_handle_at
>
> In addition we implement fh_to_dentry, which allows userspace to
> recover a pidfd from a PID file handle.

"In addition" is a good indication that a separate patch was a good idea..

>
> We stash the process' PID in the root pid namespace inside the handle,
> and use that to recover the pid (validating that pid->ino matches the
> value in the handle, i.e. that the pid has not been reused).
>
> We use the root namespace in order to ensure that file handles can be
> moved across namespaces; however, we validate that the PID exists in
> the current namespace before returning the inode.
>
> Reviewed-by: Amir Goldstein <amir73il@gmail.com>

This patch has changed enough that you should not have kept my RVB.

BTW, please refrain from using the same subject for the cover letter and
a single patch. They are not the same thing, so if they get the same
name, one of them has an inaccurate description.

> Signed-off-by: Erin Shepherd <erin.shepherd@e43.eu>
> ---
>  fs/pidfs.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 61 insertions(+), 1 deletion(-)
>
> diff --git a/fs/pidfs.c b/fs/pidfs.c
> index 80675b6bf88459c22787edaa68db360bdc0d0782..0684a9b8fe71c5205fb153b2714bc9c672045fd5 100644
> --- a/fs/pidfs.c
> +++ b/fs/pidfs.c
> @@ -1,5 +1,6 @@
>  // SPDX-License-Identifier: GPL-2.0
>  #include <linux/anon_inodes.h>
> +#include <linux/exportfs.h>
>  #include <linux/file.h>
>  #include <linux/fs.h>
>  #include <linux/magic.h>
> @@ -347,11 +348,69 @@ static const struct dentry_operations pidfs_dentry_operations = {
>         .d_prune        = stashed_dentry_prune,
>  };
>
> +#define PIDFD_FID_LEN 3
> +
> +struct pidfd_fid {
> +       u64 ino;
> +       s32 pid;
> +} __packed;
> +
> +static int pidfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
> +                          struct inode *parent)
> +{
> +       struct pid *pid = inode->i_private;
> +       struct pidfd_fid *fid = (struct pidfd_fid *)fh;
> +
> +       if (*max_len < PIDFD_FID_LEN) {
> +               *max_len = PIDFD_FID_LEN;
> +               return FILEID_INVALID;
> +       }
> +
> +       fid->ino = pid->ino;
> +       fid->pid = pid_nr(pid);
> +       *max_len = PIDFD_FID_LEN;
> +       return FILEID_INO64_GEN;
> +}
> +
> +static struct dentry *pidfs_fh_to_dentry(struct super_block *sb,
> +                                        struct fid *gen_fid,
> +                                        int fh_len, int fh_type)
> +{
> +       int ret;
> +       struct path path;
> +       struct pidfd_fid *fid = (struct pidfd_fid *)gen_fid;
> +       struct pid *pid;
> +
> +       if (fh_type != FILEID_INO64_GEN || fh_len < PIDFD_FID_LEN)
> +               return NULL;
> +
> +       scoped_guard(rcu) {
> +               pid = find_pid_ns(fid->pid, &init_pid_ns);
> +               if (!pid || pid->ino != fid->ino || pid_vnr(pid) == 0)
> +                       return NULL;
> +
> +               pid = get_pid(pid);
> +       }
> +
> +       ret = path_from_stashed(&pid->stashed, pidfs_mnt, pid, &path);
> +       if (ret < 0)
> +               return ERR_PTR(ret);

How come no need to put_pid() in case of error?

> +
> +       mntput(path.mnt);
> +       return path.dentry;
> +}
> +
> +static const struct export_operations pidfs_export_operations = {
> +       .encode_fh = pidfs_encode_fh,
> +       .fh_to_dentry = pidfs_fh_to_dentry,
> +       .flags = EXPORT_OP_UNRESTRICTED_OPEN,
> +};
> +
>  static int pidfs_init_inode(struct inode *inode, void *data)
>  {
>         inode->i_private = data;
>         inode->i_flags |= S_PRIVATE;
> -       inode->i_mode |= S_IRWXU;
> +       inode->i_mode |= S_IRWXU | S_IRWXG | S_IRWXO;

This change is not explained.
Why is it here?

Thanks,
Amir.

Re: [PATCH v2 3/3] pidfs: implement file handle support

[Erin Shepherd]

On 14/11/2024 08:07, Amir Goldstein wrote:
> On Wed, Nov 13, 2024 at 7:11 PM Erin Shepherd <erin.shepherd@e43.eu> wrote:
>> On 64-bit platforms, userspace can read the pidfd's inode in order to
>> get a never-repeated PID identifier. On 32-bit platforms this identifier
>> is not exposed, as inodes are limited to 32 bits. Instead expose the
>> identifier via export_fh, which makes it available to userspace via
>> name_to_handle_at
>>
>> In addition we implement fh_to_dentry, which allows userspace to
>> recover a pidfd from a PID file handle.
> "In addition" is a good indication that a separate patch was a good idea..
>
>> We stash the process' PID in the root pid namespace inside the handle,
>> and use that to recover the pid (validating that pid->ino matches the
>> value in the handle, i.e. that the pid has not been reused).
>>
>> We use the root namespace in order to ensure that file handles can be
>> moved across namespaces; however, we validate that the PID exists in
>> the current namespace before returning the inode.
>>
>> Reviewed-by: Amir Goldstein <amir73il@gmail.com>
> This patch has changed enough that you should not have kept my RVB.
>
> BTW, please refrain from using the same subject for the cover letter and
> a single patch. They are not the same thing, so if they get the same
> name, one of them has an inaccurate description.
>
ACK to all three.


>> Signed-off-by: Erin Shepherd <erin.shepherd@e43.eu>
>> ---
>>  fs/pidfs.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>>  1 file changed, 61 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/pidfs.c b/fs/pidfs.c
>> index 80675b6bf88459c22787edaa68db360bdc0d0782..0684a9b8fe71c5205fb153b2714bc9c672045fd5 100644
>> --- a/fs/pidfs.c
>> +++ b/fs/pidfs.c
>> @@ -1,5 +1,6 @@
>>  // SPDX-License-Identifier: GPL-2.0
>>  #include <linux/anon_inodes.h>
>> +#include <linux/exportfs.h>
>>  #include <linux/file.h>
>>  #include <linux/fs.h>
>>  #include <linux/magic.h>
>> @@ -347,11 +348,69 @@ static const struct dentry_operations pidfs_dentry_operations = {
>>         .d_prune        = stashed_dentry_prune,
>>  };
>>
>> +#define PIDFD_FID_LEN 3
>> +
>> +struct pidfd_fid {
>> +       u64 ino;
>> +       s32 pid;
>> +} __packed;
>> +
>> +static int pidfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
>> +                          struct inode *parent)
>> +{
>> +       struct pid *pid = inode->i_private;
>> +       struct pidfd_fid *fid = (struct pidfd_fid *)fh;
>> +
>> +       if (*max_len < PIDFD_FID_LEN) {
>> +               *max_len = PIDFD_FID_LEN;
>> +               return FILEID_INVALID;
>> +       }
>> +
>> +       fid->ino = pid->ino;
>> +       fid->pid = pid_nr(pid);
>> +       *max_len = PIDFD_FID_LEN;
>> +       return FILEID_INO64_GEN;
>> +}
>> +
>> +static struct dentry *pidfs_fh_to_dentry(struct super_block *sb,
>> +                                        struct fid *gen_fid,
>> +                                        int fh_len, int fh_type)
>> +{
>> +       int ret;
>> +       struct path path;
>> +       struct pidfd_fid *fid = (struct pidfd_fid *)gen_fid;
>> +       struct pid *pid;
>> +
>> +       if (fh_type != FILEID_INO64_GEN || fh_len < PIDFD_FID_LEN)
>> +               return NULL;
>> +
>> +       scoped_guard(rcu) {
>> +               pid = find_pid_ns(fid->pid, &init_pid_ns);
>> +               if (!pid || pid->ino != fid->ino || pid_vnr(pid) == 0)
>> +                       return NULL;
>> +
>> +               pid = get_pid(pid);
>> +       }
>> +
>> +       ret = path_from_stashed(&pid->stashed, pidfs_mnt, pid, &path);
>> +       if (ret < 0)
>> +               return ERR_PTR(ret);
> How come no need to put_pid() in case of error?

This one confused me at first too, but path_from_stashed frees it (via
stashed_ops.put_data) on error. You can see the same pattern in
pidfs_alloc_file.

(It already needs to know how to free it for the case where a stashed
dentry already exists)

>> +
>> +       mntput(path.mnt);
>> +       return path.dentry;
>> +}
>> +
>> +static const struct export_operations pidfs_export_operations = {
>> +       .encode_fh = pidfs_encode_fh,
>> +       .fh_to_dentry = pidfs_fh_to_dentry,
>> +       .flags = EXPORT_OP_UNRESTRICTED_OPEN,
>> +};
>> +
>>  static int pidfs_init_inode(struct inode *inode, void *data)
>>  {
>>         inode->i_private = data;
>>         inode->i_flags |= S_PRIVATE;
>> -       inode->i_mode |= S_IRWXU;
>> +       inode->i_mode |= S_IRWXU | S_IRWXG | S_IRWXO;
> This change is not explained.
> Why is it here?

open_by_handle_at eventually passes through the may_open permission check.
The existing permissions only permits root to open them (since the owning
uid & gid is 0). So, it was necessary to widen them to align with how
pidfd_open works.

If I stick with this approach (see [1]) I'll ensure to document this change
better in the commit message.

[1] https://lore.kernel.org/all/6a3ed633-311d-47ff-8a7e-5121d6186139@e43.eu/

Re: [PATCH v2 0/3] pidfs: implement file handle support

[Erin Shepherd]

   

On 14/11/2024 08:02, Amir Goldstein wrote:
> On Wed, Nov 13, 2024 at 7:01 PM Erin Shepherd <erin.shepherd@e43.eu> wrote:
>> Since the introduction of pidfs, we have had 64-bit process identifiers
>> that will not be reused for the entire uptime of the system. This greatly
>> facilitates process tracking in userspace.
>>
>> There are two limitations at present:
>>
>>  * These identifiers are currently only exposed to processes on 64-bit
>>    systems. On 32-bit systems, inode space is also limited to 32 bits and
>>    therefore is subject to the same reuse issues.
>>  * There is no way to go from one of these unique identifiers to a pid or
>>    pidfd.
>>
>> This patch implements fh_export and fh_to_dentry which enables userspace to
>> convert PIDs to and from PID file handles. A process can convert a pidfd into
>> a file handle using name_to_handle_at, store it (in memory, on disk, or
>> elsewhere) and then convert it back into a pidfd suing open_by_handle_at.
>>
>> To support us going from a file handle to a pidfd, we have to store a pid
>> inside the file handle. To ensure file handles are invariant and can move
>> between pid namespaces, we stash a pid from the initial namespace inside
>> the file handle.
>>
>>   (There has been some discussion as to whether or not it is OK to include
>>   the PID in the initial pid namespace, but so far there hasn't been any
>>   conclusive reason given as to why this would be a bad idea)
> IIUC, this is already exposed as st_ino on a 64bit arch?
> If that is the case, then there is certainly no new info leak in this patch.

pid.ino is exposed, but the init-ns pid isn't exposed anywhere to my knowledge.

>> Signed-off-by: Erin Shepherd <erin.shepherd@e43.eu>
>> ---
>> Changes in v2:
>> - Permit filesystems to opt out of CAP_DAC_READ_SEARCH
>> - Inline find_pid_ns/get_pid logic; remove unnecessary put_pid
>> - Squash fh_export & fh_to_dentry into one commit
> Not sure why you did that.
> It was pretty nice as separate commits if you ask me. Whatever.

I can revert that if you prefer. I squashed them because there was some churn
when adding the init-ns-pid necessary to restore them, but I am happy to do
things in two steps.

Do you prefer having the final handle format in the first step, or letting it
evolve into final form over the series?

Re: [PATCH v2 3/3] pidfs: implement file handle support

[Christian Brauner]

On Wed, Nov 13, 2024 at 05:55:25PM +0000, Erin Shepherd wrote:
> On 64-bit platforms, userspace can read the pidfd's inode in order to
> get a never-repeated PID identifier. On 32-bit platforms this identifier
> is not exposed, as inodes are limited to 32 bits. Instead expose the
> identifier via export_fh, which makes it available to userspace via
> name_to_handle_at
> 
> In addition we implement fh_to_dentry, which allows userspace to
> recover a pidfd from a PID file handle.
> 
> We stash the process' PID in the root pid namespace inside the handle,
> and use that to recover the pid (validating that pid->ino matches the
> value in the handle, i.e. that the pid has not been reused).
> 
> We use the root namespace in order to ensure that file handles can be
> moved across namespaces; however, we validate that the PID exists in
> the current namespace before returning the inode.
> 
> Reviewed-by: Amir Goldstein <amir73il@gmail.com>
> Signed-off-by: Erin Shepherd <erin.shepherd@e43.eu>
> ---

I think you need at least something like the following completely
untested draft on top:

- the pidfs_finish_open_by_handle_at() is somewhat of a clutch to handle
  thread vs thread-group pidfds but it works.

- In contrast to pidfd_open() that uses dentry_open() to create a pidfd
  open_by_handle_at() uses file_open_root(). That's overall fine but
  makes pidfds subject to security hooks which they aren't via
  pidfd_open(). It also necessitats pidfs_finish_open_by_handle_at().
  There's probably other solutions I'm not currently seeing.

- The exportfs_decode_fh_raw() call that's used to decode the pidfd is
  passed vfs_dentry_acceptable() as acceptability callback. For pidfds
  we don't need any of that functionality and we don't need any of the
  disconnected dentry handling logic. So the easiest way to fix that is
  to rely on EXPORT_OP_UNRESTRICTED_OPEN to skip everything. That in
  turns means the only acceptability we have is the nop->fh_to_dentry()
  callback for pidfs.

- This all really needs rigorous selftests before we can even think of
  merging any of this.

Anway, here's the _completely untested, quickly drafted_ diff on top:

diff --git a/fs/exportfs/expfs.c b/fs/exportfs/expfs.c
index 4f2dd4ab4486..65c93f7132d4 100644
--- a/fs/exportfs/expfs.c
+++ b/fs/exportfs/expfs.c
@@ -450,6 +450,13 @@ exportfs_decode_fh_raw(struct vfsmount *mnt, struct fid *fid, int fh_len,
 		goto err_result;
 	}
 
+	/*
+	 * The filesystem has no acceptance criteria other than those in
+	 * nop->fh_to_dentry().
+	 */
+	if (nop->flags & EXPORT_OP_UNRESTRICTED_OPEN)
+		return result;
+
 	/*
 	 * If no acceptance criteria was specified by caller, a disconnected
 	 * dentry is also accepatable. Callers may use this mode to query if
diff --git a/fs/fhandle.c b/fs/fhandle.c
index 056116e58f43..89c2efacc0c3 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -11,6 +11,7 @@
 #include <linux/personality.h>
 #include <linux/uaccess.h>
 #include <linux/compat.h>
+#include <linux/pidfs.h>
 #include "internal.h"
 #include "mount.h"
 
@@ -218,20 +219,21 @@ static int do_handle_to_path(struct file_handle *handle, struct path *path,
 {
 	int handle_dwords;
 	struct vfsmount *mnt = ctx->root.mnt;
+	struct dentry *dentry;
 
 	/* change the handle size to multiple of sizeof(u32) */
 	handle_dwords = handle->handle_bytes >> 2;
-	path->dentry = exportfs_decode_fh_raw(mnt,
-					  (struct fid *)handle->f_handle,
-					  handle_dwords, handle->handle_type,
-					  ctx->fh_flags,
-					  vfs_dentry_acceptable, ctx);
-	if (IS_ERR_OR_NULL(path->dentry)) {
-		if (path->dentry == ERR_PTR(-ENOMEM))
+	dentry = exportfs_decode_fh_raw(mnt, (struct fid *)handle->f_handle,
+					handle_dwords, handle->handle_type,
+					ctx->fh_flags, vfs_dentry_acceptable,
+					ctx);
+	if (IS_ERR_OR_NULL(dentry)) {
+		if (dentry == ERR_PTR(-ENOMEM))
 			return -ENOMEM;
 		return -ESTALE;
 	}
 	path->mnt = mntget(mnt);
+	path->dentry = dentry;
 	return 0;
 }
 
@@ -239,7 +241,7 @@ static inline bool may_decode_fh(struct handle_to_path_ctx *ctx,
 				 unsigned int o_flags)
 {
 	struct path *root = &ctx->root;
-	struct export_operations *nop = root->mnt->mnt_sb->s_export_op;
+	const struct export_operations *nop = root->mnt->mnt_sb->s_export_op;
 
 	if (nop && nop->flags & EXPORT_OP_UNRESTRICTED_OPEN)
 		return true;
@@ -342,7 +344,7 @@ static long do_handle_open(int mountdirfd, struct file_handle __user *ufh,
 			   int open_flag)
 {
 	long retval = 0;
-	struct path path;
+	struct path path __free(path_put) = {};
 	struct file *file;
 	int fd;
 
@@ -351,19 +353,24 @@ static long do_handle_open(int mountdirfd, struct file_handle __user *ufh,
 		return retval;
 
 	fd = get_unused_fd_flags(open_flag);
-	if (fd < 0) {
-		path_put(&path);
+	if (fd < 0)
 		return fd;
-	}
+
 	file = file_open_root(&path, "", open_flag, 0);
 	if (IS_ERR(file)) {
 		put_unused_fd(fd);
-		retval =  PTR_ERR(file);
-	} else {
-		retval = fd;
-		fd_install(fd, file);
+		return PTR_ERR(file);
 	}
-	path_put(&path);
+
+	retval = pidfs_finish_open_by_handle_at(file, open_flag);
+	if (retval) {
+		put_unused_fd(fd);
+		fput(file);
+		return retval;
+	}
+
+	retval = fd;
+	fd_install(fd, file);
 	return retval;
 }
 
diff --git a/fs/pidfs.c b/fs/pidfs.c
index 0684a9b8fe71..19948002f395 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -237,6 +237,24 @@ struct pid *pidfd_pid(const struct file *file)
 	return file_inode(file)->i_private;
 }
 
+int pidfs_finish_open_by_handle_at(struct file *file, unsigned int oflags)
+{
+	struct pid *pid;
+	bool thread = oflags & PIDFD_THREAD;
+
+	pid = pidfd_pid(file);
+	if (IS_ERR(pid))
+		return 0;
+
+	if (!pid_has_task(pid, thread ? PIDTYPE_PID : PIDTYPE_TGID))
+		return -EINVAL;
+
+	if (thread)
+		file->f_flags |= PIDFD_THREAD;
+
+	return 0;
+}
+
 static struct vfsmount *pidfs_mnt __ro_after_init;
 
 #if BITS_PER_LONG == 32
@@ -377,7 +395,7 @@ static struct dentry *pidfs_fh_to_dentry(struct super_block *sb,
 					 int fh_len, int fh_type)
 {
 	int ret;
-	struct path path;
+	struct path path __free(path_put) = {};
 	struct pidfd_fid *fid = (struct pidfd_fid *)gen_fid;
 	struct pid *pid;
 
@@ -393,11 +411,12 @@ static struct dentry *pidfs_fh_to_dentry(struct super_block *sb,
 	}
 
 	ret = path_from_stashed(&pid->stashed, pidfs_mnt, pid, &path);
-	if (ret < 0)
+	if (ret < 0) {
+		put_pid(pid);
 		return ERR_PTR(ret);
+	}
 
-	mntput(path.mnt);
-	return path.dentry;
+	return dget(path.dentry);
 }
 
 static const struct export_operations pidfs_export_operations = {
diff --git a/include/linux/pidfs.h b/include/linux/pidfs.h
index 75bdf9807802..9a4130056e7d 100644
--- a/include/linux/pidfs.h
+++ b/include/linux/pidfs.h
@@ -3,6 +3,7 @@
 #define _LINUX_PID_FS_H
 
 struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags);
+int pidfs_finish_open_by_handle_at(struct file *file, unsigned int oflags);
 void __init pidfs_init(void);
 
 #endif /* _LINUX_PID_FS_H */


>  fs/pidfs.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 61 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/pidfs.c b/fs/pidfs.c
> index 80675b6bf88459c22787edaa68db360bdc0d0782..0684a9b8fe71c5205fb153b2714bc9c672045fd5 100644
> --- a/fs/pidfs.c
> +++ b/fs/pidfs.c
> @@ -1,5 +1,6 @@
>  // SPDX-License-Identifier: GPL-2.0
>  #include <linux/anon_inodes.h>
> +#include <linux/exportfs.h>
>  #include <linux/file.h>
>  #include <linux/fs.h>
>  #include <linux/magic.h>
> @@ -347,11 +348,69 @@ static const struct dentry_operations pidfs_dentry_operations = {
>  	.d_prune	= stashed_dentry_prune,
>  };
>  
> +#define PIDFD_FID_LEN 3
> +
> +struct pidfd_fid {
> +	u64 ino;
> +	s32 pid;
> +} __packed;
> +
> +static int pidfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
> +			   struct inode *parent)
> +{
> +	struct pid *pid = inode->i_private;
> +	struct pidfd_fid *fid = (struct pidfd_fid *)fh;
> +
> +	if (*max_len < PIDFD_FID_LEN) {
> +		*max_len = PIDFD_FID_LEN;
> +		return FILEID_INVALID;
> +	}
> +
> +	fid->ino = pid->ino;
> +	fid->pid = pid_nr(pid);
> +	*max_len = PIDFD_FID_LEN;
> +	return FILEID_INO64_GEN;
> +}
> +
> +static struct dentry *pidfs_fh_to_dentry(struct super_block *sb,
> +					 struct fid *gen_fid,
> +					 int fh_len, int fh_type)
> +{
> +	int ret;
> +	struct path path;
> +	struct pidfd_fid *fid = (struct pidfd_fid *)gen_fid;
> +	struct pid *pid;
> +
> +	if (fh_type != FILEID_INO64_GEN || fh_len < PIDFD_FID_LEN)
> +		return NULL;
> +
> +	scoped_guard(rcu) {
> +		pid = find_pid_ns(fid->pid, &init_pid_ns);
> +		if (!pid || pid->ino != fid->ino || pid_vnr(pid) == 0)
> +			return NULL;
> +
> +		pid = get_pid(pid);
> +	}
> +
> +	ret = path_from_stashed(&pid->stashed, pidfs_mnt, pid, &path);
> +	if (ret < 0)
> +		return ERR_PTR(ret);
> +
> +	mntput(path.mnt);
> +	return path.dentry;
> +}
> +
> +static const struct export_operations pidfs_export_operations = {
> +	.encode_fh = pidfs_encode_fh,
> +	.fh_to_dentry = pidfs_fh_to_dentry,
> +	.flags = EXPORT_OP_UNRESTRICTED_OPEN,
> +};
> +
>  static int pidfs_init_inode(struct inode *inode, void *data)
>  {
>  	inode->i_private = data;
>  	inode->i_flags |= S_PRIVATE;
> -	inode->i_mode |= S_IRWXU;
> +	inode->i_mode |= S_IRWXU | S_IRWXG | S_IRWXO;
>  	inode->i_op = &pidfs_inode_operations;
>  	inode->i_fop = &pidfs_file_operations;
>  	/*
> @@ -382,6 +441,7 @@ static int pidfs_init_fs_context(struct fs_context *fc)
>  		return -ENOMEM;
>  
>  	ctx->ops = &pidfs_sops;
> +	ctx->eops = &pidfs_export_operations;
>  	ctx->dops = &pidfs_dentry_operations;
>  	fc->s_fs_info = (void *)&pidfs_stashed_ops;
>  	return 0;
> 
> -- 
> 2.46.1
> 

Re: [PATCH v2 2/3] exportfs: allow fs to disable CAP_DAC_READ_SEARCH check

[Erin Shepherd]

On 14/11/2024 05:37, Christoph Hellwig wrote:
> On Wed, Nov 13, 2024 at 05:55:24PM +0000, Erin Shepherd wrote:
>> For pidfs, there is no reason to restrict file handle decoding by
>> CAP_DAC_READ_SEARCH.
> Why is there no reason, i.e. why do you think it is safe.

A process can use both open_by_handle_at to open the exact same set of
pidfds as they can by pidfd_open. i.e. there is no reason to additionally
restrict access to the former API.

>> Introduce an export_ops flag that can indicate
>> this
> Also why is is desirable?
>
> To be this looks more than sketchy with the actual exporting hat on,
> but I guess that's now how the cool kids use open by handle these days.
Right - we have a bunch of API file systems where userspace wants stable
non-reused file references for the same reasons network filesystems do.
The first example of this was cgroupfs, but the same rationale exists for
pidfs and process tracking.

Re: [PATCH v2 3/3] pidfs: implement file handle support

[Erin Shepherd]

On 14/11/2024 13:52, Christian Brauner wrote:

> I think you need at least something like the following completely
> untested draft on top:
>
> - the pidfs_finish_open_by_handle_at() is somewhat of a clutch to handle
>   thread vs thread-group pidfds but it works.
>
> - In contrast to pidfd_open() that uses dentry_open() to create a pidfd
>   open_by_handle_at() uses file_open_root(). That's overall fine but
>   makes pidfds subject to security hooks which they aren't via
>   pidfd_open(). It also necessitats pidfs_finish_open_by_handle_at().
>   There's probably other solutions I'm not currently seeing.

These two concerns combined with the special flag make me wonder if pidfs
is so much of a special snowflake we should just special case it up front
and skip all of the shared handle decode logic?

> - The exportfs_decode_fh_raw() call that's used to decode the pidfd is
>   passed vfs_dentry_acceptable() as acceptability callback. For pidfds
>   we don't need any of that functionality and we don't need any of the
>   disconnected dentry handling logic. So the easiest way to fix that is
>   to rely on EXPORT_OP_UNRESTRICTED_OPEN to skip everything. That in
>   turns means the only acceptability we have is the nop->fh_to_dentry()
>   callback for pidfs.

With the current logic we go exportfs_decode_fh_raw(...) ->
find_acceptable_alias(result) -> vfs_dentry_acceptable(context, result).


vfs_dentry_acceptable immediately returns 1 if ctx->flags is 0, which will
always be the case if EXPORT_OP_UNRESTRICTED_OPEN was set, so we immediately
fall back out of the call tree and return result.

So I'm not 100% sure we actually need this special case but I'm not opposed.

> - This all really needs rigorous selftests before we can even think of
>   merging any of this.

Re: [PATCH v2 3/3] pidfs: implement file handle support

[Christian Brauner]

On Thu, Nov 14, 2024 at 02:13:06PM +0100, Erin Shepherd wrote:
> On 14/11/2024 13:52, Christian Brauner wrote:
> 
> > I think you need at least something like the following completely
> > untested draft on top:
> >
> > - the pidfs_finish_open_by_handle_at() is somewhat of a clutch to handle
> >   thread vs thread-group pidfds but it works.
> >
> > - In contrast to pidfd_open() that uses dentry_open() to create a pidfd
> >   open_by_handle_at() uses file_open_root(). That's overall fine but
> >   makes pidfds subject to security hooks which they aren't via
> >   pidfd_open(). It also necessitats pidfs_finish_open_by_handle_at().
> >   There's probably other solutions I'm not currently seeing.
> 
> These two concerns combined with the special flag make me wonder if pidfs
> is so much of a special snowflake we should just special case it up front
> and skip all of the shared handle decode logic?

Care to try a patch and see what it looks like?

> 
> > - The exportfs_decode_fh_raw() call that's used to decode the pidfd is
> >   passed vfs_dentry_acceptable() as acceptability callback. For pidfds
> >   we don't need any of that functionality and we don't need any of the
> >   disconnected dentry handling logic. So the easiest way to fix that is
> >   to rely on EXPORT_OP_UNRESTRICTED_OPEN to skip everything. That in
> >   turns means the only acceptability we have is the nop->fh_to_dentry()
> >   callback for pidfs.
> 
> With the current logic we go exportfs_decode_fh_raw(...) ->
> find_acceptable_alias(result) -> vfs_dentry_acceptable(context, result).
> vfs_dentry_acceptable immediately returns 1 if ctx->flags is 0, which will
> always be the case if EXPORT_OP_UNRESTRICTED_OPEN was set, so we immediately
> fall back out of the call tree and return result.
> 
> So I'm not 100% sure we actually need this special case but I'm not opposed.

Oh right, I completely forgot that find_acceptable_alias() is a no-op
for pidfs.

Re: [PATCH v2 2/3] exportfs: allow fs to disable CAP_DAC_READ_SEARCH check

[Christian Brauner]

On Thu, Nov 14, 2024 at 07:37:19AM +0100, Amir Goldstein wrote:
> On Wed, Nov 13, 2024 at 8:11 PM Erin Shepherd <erin.shepherd@e43.eu> wrote:
> >
> > For pidfs, there is no reason to restrict file handle decoding by
> > CAP_DAC_READ_SEARCH. Introduce an export_ops flag that can indicate
> > this
> >
> > Signed-off-by: Erin Shepherd <erin.shepherd@e43.eu>
> > ---
> >  fs/fhandle.c             | 36 +++++++++++++++++++++---------------
> >  include/linux/exportfs.h |  3 +++
> >  2 files changed, 24 insertions(+), 15 deletions(-)
> >
> > diff --git a/fs/fhandle.c b/fs/fhandle.c
> > index 82df28d45cd70a7df525f50bbb398d646110cd99..056116e58f43983bc7bb86da170fb554c7a2fac7 100644
> > --- a/fs/fhandle.c
> > +++ b/fs/fhandle.c
> > @@ -235,26 +235,32 @@ static int do_handle_to_path(struct file_handle *handle, struct path *path,
> >         return 0;
> >  }
> >
> > -/*
> > - * Allow relaxed permissions of file handles if the caller has the
> > - * ability to mount the filesystem or create a bind-mount of the
> > - * provided @mountdirfd.
> > - *
> > - * In both cases the caller may be able to get an unobstructed way to
> > - * the encoded file handle. If the caller is only able to create a
> > - * bind-mount we need to verify that there are no locked mounts on top
> > - * of it that could prevent us from getting to the encoded file.
> > - *
> > - * In principle, locked mounts can prevent the caller from mounting the
> > - * filesystem but that only applies to procfs and sysfs neither of which
> > - * support decoding file handles.
> > - */
> >  static inline bool may_decode_fh(struct handle_to_path_ctx *ctx,
> >                                  unsigned int o_flags)
> >  {
> >         struct path *root = &ctx->root;
> > +       struct export_operations *nop = root->mnt->mnt_sb->s_export_op;
> > +
> > +       if (nop && nop->flags & EXPORT_OP_UNRESTRICTED_OPEN)
> > +               return true;
> > +
> > +       if (capable(CAP_DAC_READ_SEARCH))
> > +               return true;
> >
> >         /*
> > +        * Allow relaxed permissions of file handles if the caller has the
> > +        * ability to mount the filesystem or create a bind-mount of the
> > +        * provided @mountdirfd.
> > +        *
> > +        * In both cases the caller may be able to get an unobstructed way to
> > +        * the encoded file handle. If the caller is only able to create a
> > +        * bind-mount we need to verify that there are no locked mounts on top
> > +        * of it that could prevent us from getting to the encoded file.
> > +        *
> > +        * In principle, locked mounts can prevent the caller from mounting the
> > +        * filesystem but that only applies to procfs and sysfs neither of which
> > +        * support decoding file handles.
> > +        *
> >          * Restrict to O_DIRECTORY to provide a deterministic API that avoids a
> >          * confusing api in the face of disconnected non-dir dentries.
> >          *
> > @@ -293,7 +299,7 @@ static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
> >         if (retval)
> >                 goto out_err;
> >
> > -       if (!capable(CAP_DAC_READ_SEARCH) && !may_decode_fh(&ctx, o_flags)) {
> > +       if (!may_decode_fh(&ctx, o_flags)) {
> >                 retval = -EPERM;
> >                 goto out_path;
> >         }
> > diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
> > index 893a1d21dc1c4abc7e52325d7a4cf0adb407f039..459508b53e77ed0597cee217ffe3d82cc7cc11a4 100644
> > --- a/include/linux/exportfs.h
> > +++ b/include/linux/exportfs.h
> > @@ -247,6 +247,9 @@ struct export_operations {
> >                                                 */
> >  #define EXPORT_OP_FLUSH_ON_CLOSE       (0x20) /* fs flushes file data on close */
> >  #define EXPORT_OP_ASYNC_LOCK           (0x40) /* fs can do async lock request */
> > +#define EXPORT_OP_UNRESTRICTED_OPEN    (0x80) /* FS allows open_by_handle_at
> > +                                                 without CAP_DAC_READ_SEARCH
> > +                                               */
> 
> Don't love the name, but I wonder, isn't SB_NOUSER already a good
> enough indication that CAP_DAC_READ_SEARCH is irrelevant?
> 
> Essentially, mnt_fd is the user's proof that they can access the mount
> and CAP_DAC_READ_SEARCH is the legacy "proof" that the user can
> reach from mount the inode by path lookup.
> 
> Which reminds me, what is the mnt_fd expected for opening a pidfd
> file by handle?

int pidfd_self = pidfd_open(getpid(), 0);
open_by_handle_at(pidfd_self, ...);

is sufficient.

Re: [PATCH v2 0/3] pidfs: implement file handle support

[Christian Brauner]

On Thu, Nov 14, 2024 at 01:48:02PM +0100, Erin Shepherd wrote:
>    
> 
> On 14/11/2024 08:02, Amir Goldstein wrote:
> > On Wed, Nov 13, 2024 at 7:01 PM Erin Shepherd <erin.shepherd@e43.eu> wrote:
> >> Since the introduction of pidfs, we have had 64-bit process identifiers
> >> that will not be reused for the entire uptime of the system. This greatly
> >> facilitates process tracking in userspace.
> >>
> >> There are two limitations at present:
> >>
> >>  * These identifiers are currently only exposed to processes on 64-bit
> >>    systems. On 32-bit systems, inode space is also limited to 32 bits and
> >>    therefore is subject to the same reuse issues.
> >>  * There is no way to go from one of these unique identifiers to a pid or
> >>    pidfd.
> >>
> >> This patch implements fh_export and fh_to_dentry which enables userspace to
> >> convert PIDs to and from PID file handles. A process can convert a pidfd into
> >> a file handle using name_to_handle_at, store it (in memory, on disk, or
> >> elsewhere) and then convert it back into a pidfd suing open_by_handle_at.
> >>
> >> To support us going from a file handle to a pidfd, we have to store a pid
> >> inside the file handle. To ensure file handles are invariant and can move
> >> between pid namespaces, we stash a pid from the initial namespace inside
> >> the file handle.
> >>
> >>   (There has been some discussion as to whether or not it is OK to include
> >>   the PID in the initial pid namespace, but so far there hasn't been any
> >>   conclusive reason given as to why this would be a bad idea)
> > IIUC, this is already exposed as st_ino on a 64bit arch?
> > If that is the case, then there is certainly no new info leak in this patch.
> 
> pid.ino is exposed, but the init-ns pid isn't exposed anywhere to my knowledge.

I see what you mean. That might be an information leak. Not a very
interesting one, I think but I need to think about it.

> 
> >> Signed-off-by: Erin Shepherd <erin.shepherd@e43.eu>
> >> ---
> >> Changes in v2:
> >> - Permit filesystems to opt out of CAP_DAC_READ_SEARCH
> >> - Inline find_pid_ns/get_pid logic; remove unnecessary put_pid
> >> - Squash fh_export & fh_to_dentry into one commit
> > Not sure why you did that.
> > It was pretty nice as separate commits if you ask me. Whatever.
> 
> I can revert that if you prefer. I squashed them because there was some churn
> when adding the init-ns-pid necessary to restore them, but I am happy to do
> things in two steps.
> 
> Do you prefer having the final handle format in the first step, or letting it
> evolve into final form over the series?
> 

Re: [PATCH v2 0/3] pidfs: implement file handle support

[Amir Goldstein]

On Thu, Nov 14, 2024 at 4:51 PM Erin Shepherd <erin.shepherd@e43.eu> wrote:
>
>
>
> On 14/11/2024 08:02, Amir Goldstein wrote:
> > On Wed, Nov 13, 2024 at 7:01 PM Erin Shepherd <erin.shepherd@e43.eu> wrote:
> >> Since the introduction of pidfs, we have had 64-bit process identifiers
> >> that will not be reused for the entire uptime of the system. This greatly
> >> facilitates process tracking in userspace.
> >>
> >> There are two limitations at present:
> >>
> >>  * These identifiers are currently only exposed to processes on 64-bit
> >>    systems. On 32-bit systems, inode space is also limited to 32 bits and
> >>    therefore is subject to the same reuse issues.
> >>  * There is no way to go from one of these unique identifiers to a pid or
> >>    pidfd.
> >>
> >> This patch implements fh_export and fh_to_dentry which enables userspace to
> >> convert PIDs to and from PID file handles. A process can convert a pidfd into
> >> a file handle using name_to_handle_at, store it (in memory, on disk, or
> >> elsewhere) and then convert it back into a pidfd suing open_by_handle_at.
> >>
> >> To support us going from a file handle to a pidfd, we have to store a pid
> >> inside the file handle. To ensure file handles are invariant and can move
> >> between pid namespaces, we stash a pid from the initial namespace inside
> >> the file handle.
> >>
> >>   (There has been some discussion as to whether or not it is OK to include
> >>   the PID in the initial pid namespace, but so far there hasn't been any
> >>   conclusive reason given as to why this would be a bad idea)
> > IIUC, this is already exposed as st_ino on a 64bit arch?
> > If that is the case, then there is certainly no new info leak in this patch.
>
> pid.ino is exposed, but the init-ns pid isn't exposed anywhere to my knowledge.
>
> >> Signed-off-by: Erin Shepherd <erin.shepherd@e43.eu>
> >> ---
> >> Changes in v2:
> >> - Permit filesystems to opt out of CAP_DAC_READ_SEARCH
> >> - Inline find_pid_ns/get_pid logic; remove unnecessary put_pid
> >> - Squash fh_export & fh_to_dentry into one commit
> > Not sure why you did that.
> > It was pretty nice as separate commits if you ask me. Whatever.
>
> I can revert that if you prefer. I squashed them because there was some churn
> when adding the init-ns-pid necessary to restore them, but I am happy to do
> things in two steps.
>
> Do you prefer having the final handle format in the first step, or letting it
> evolve into final form over the series?
>

I don't really mind. Was just wondering. Either way is fine.

Thanks,
Amir.

Re: [PATCH v2 3/3] pidfs: implement file handle support

[Erin Shepherd]

On 14/11/2024 15:13, Christian Brauner wrote:

> On Thu, Nov 14, 2024 at 02:13:06PM +0100, Erin Shepherd wrote:
>> These two concerns combined with the special flag make me wonder if pidfs
>> is so much of a special snowflake we should just special case it up front
>> and skip all of the shared handle decode logic?
> Care to try a patch and see what it looks like?

The following is a completely untested sketch on top of the existing patch series.
Some notes:

- I made heavy use of the cleanup macros. I'm happy to convert things back to
  goto out_xx style if preferred - writing things this way just made bashing out
  the code without dropping resources on the floor easier
- If you don't implement fh_to_dentry then name_to_handle_at will just return an error
  unless called with AT_HANDLE_FID. We need to decide what to do about that
- The GET_PATH_FD_IS_NORMAL/etc constants don't match (what I see as) usual kernel style
  but I'm not sure how to conventionally express something like that

Otherwise, I'm fairly happy with how it came out in the end. Maybe handle_to_path and
do_handle_to_path could be collapsed into each other at this point.

diff --git a/fs/fhandle.c b/fs/fhandle.c
index 056116e58f43..697246085b69 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -11,6 +11,7 @@
 #include <linux/personality.h>
 #include <linux/uaccess.h>
 #include <linux/compat.h>
+#include <linux/pidfs.h>
 #include "internal.h"
 #include "mount.h"

@@ -130,6 +131,11 @@ SYSCALL_DEFINE5(name_to_handle_at, int, dfd, const char __user *, name,
     return err;
 }

+enum {
+    GET_PATH_FD_IS_NORMAL = 0,
+    GET_PATH_FD_IS_PIDFD  = 1,
+};
+
 static int get_path_from_fd(int fd, struct path *root)
 {
     if (fd == AT_FDCWD) {
@@ -139,15 +145,16 @@ static int get_path_from_fd(int fd, struct path *root)
         path_get(root);
         spin_unlock(&fs->lock);
     } else {
-        struct fd f = fdget(fd);
+        CLASS(fd, f)(fd);
         if (!fd_file(f))
             return -EBADF;
+        if (pidfd_pid(fd_file(f)))
+            return GET_PATH_FD_IS_PIDFD;
         *root = fd_file(f)->f_path;
         path_get(root);
-        fdput(f);
     }

-    return 0;
+    return GET_PATH_FD_IS_NORMAL;
 }

 enum handle_to_path_flags {
@@ -287,84 +294,94 @@ static inline bool may_decode_fh(struct handle_to_path_ctx *ctx,
     return true;
 }

-static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
-           struct path *path, unsigned int o_flags)
+static int copy_handle_from_user(struct file_handle __user *ufh, struct file_handle **ret)
 {
-    int retval = 0;
-    struct file_handle f_handle;
-    struct file_handle *handle = NULL;
-    struct handle_to_path_ctx ctx = {};
-
-    retval = get_path_from_fd(mountdirfd, &ctx.root);
-    if (retval)
-        goto out_err;
-
-    if (!may_decode_fh(&ctx, o_flags)) {
-        retval = -EPERM;
-        goto out_path;
-    }
-
+    struct file_handle f_handle, *handle;
     if (copy_from_user(&f_handle, ufh, sizeof(struct file_handle))) {
-        retval = -EFAULT;
-        goto out_path;
+        return -EFAULT;
     }
     if ((f_handle.handle_bytes > MAX_HANDLE_SZ) ||
         (f_handle.handle_bytes == 0)) {
-        retval = -EINVAL;
-        goto out_path;
+        return -EINVAL;
     }
     handle = kmalloc(struct_size(handle, f_handle, f_handle.handle_bytes),
              GFP_KERNEL);
     if (!handle) {
-        retval = -ENOMEM;
-        goto out_path;
+        return -ENOMEM;
     }
+
     /* copy the full handle */
     *handle = f_handle;
     if (copy_from_user(&handle->f_handle,
                &ufh->f_handle,
                f_handle.handle_bytes)) {
-        retval = -EFAULT;
-        goto out_handle;
+        kfree(handle);
+        return -EFAULT;
     }

-    retval = do_handle_to_path(handle, path, &ctx);
+    *ret = handle;
+    return 0;
+}

-out_handle:
-    kfree(handle);
-out_path:
-    path_put(&ctx.root);
-out_err:
-    return retval;
+static int handle_to_path(struct path root, struct file_handle __user *ufh,
+           struct path *path, unsigned int o_flags)
+{
+    struct file_handle *handle = NULL;
+    struct handle_to_path_ctx ctx = {
+        .root = root,
+    };
+
+    if (!may_decode_fh(&ctx, o_flags)) {
+        return -EPERM;
+    }
+
+    return do_handle_to_path(handle, path, &ctx);
 }

 static long do_handle_open(int mountdirfd, struct file_handle __user *ufh,
                int open_flag)
 {
     long retval = 0;
-    struct path path;
-    struct file *file;
-    int fd;
+    struct file_handle *handle __free(kfree) = NULL;
+    struct path __free(path_put) root = {};
+    struct path __free(path_put) path = {};
+    struct file *file = NULL;
+    int root_type;

-    retval = handle_to_path(mountdirfd, ufh, &path, open_flag);
+    root_type = get_path_from_fd(mountdirfd, &root);
+    if (root_type < 0)
+        return root_type;
+
+    retval = copy_handle_from_user(ufh, &handle);
     if (retval)
         return retval;

-    fd = get_unused_fd_flags(open_flag);
-    if (fd < 0) {
-        path_put(&path);
+    CLASS(get_unused_fd, fd)(open_flag);
+    if (fd < 0)
         return fd;
+
+    switch (root_type) {
+    case GET_PATH_FD_IS_NORMAL:
+        retval = handle_to_path(root, handle, &path, open_flag);
+        if (retval)
+            return retval;
+        file = file_open_root(&path, "", open_flag, 0);
+        if (IS_ERR(file)) {
+            return PTR_ERR(file);
+        }
+        break;
+
+    case GET_PATH_FD_IS_PIDFD:
+        file = pidfd_open_by_handle(
+            handle->f_handle, handle->handle_bytes >> 2, handle->handle_type,
+            open_flag);
+        if (IS_ERR(file))
+            return retval;
+        break;
     }
-    file = file_open_root(&path, "", open_flag, 0);
-    if (IS_ERR(file)) {
-        put_unused_fd(fd);
-        retval =  PTR_ERR(file);
-    } else {
-        retval = fd;
-        fd_install(fd, file);
-    }
-    path_put(&path);
-    return retval;
+
+    fd_install(fd, file);
+    return take_fd(fd);
 }

 /**
diff --git a/fs/pidfs.c b/fs/pidfs.c
index 0684a9b8fe71..65b72dc05380 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -453,22 +453,53 @@ static struct file_system_type pidfs_type = {
     .kill_sb        = kill_anon_super,
 };

-struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags)
+static struct file *__pidfs_alloc_file(struct pid *pid, unsigned int flags)
 {
-
     struct file *pidfd_file;
     struct path path;
     int ret;

-    ret = path_from_stashed(&pid->stashed, pidfs_mnt, get_pid(pid), &path);
+    ret = path_from_stashed(&pid->stashed, pidfs_mnt, pid, &path);
     if (ret < 0)
         return ERR_PTR(ret);

     pidfd_file = dentry_open(&path, flags, current_cred());
+    if (!IS_ERR(pidfd_file))
+        pidfd_file->f_flags |= (flags & PIDFD_THREAD);
     path_put(&path);
     return pidfd_file;
 }

+struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags)
+{
+    return __pidfs_alloc_file(get_pid(pid), flags);
+}
+
+struct file *pidfd_open_by_handle(void *gen_fid, int fh_len, int fh_type,
+                  unsigned int flags)
+{
+    bool thread = flags & PIDFD_THREAD;
+    struct pidfd_fid *fid = (struct pidfd_fid *)gen_fid;
+    struct pid *pid;
+
+    if (fh_type != FILEID_INO64_GEN || fh_len < PIDFD_FID_LEN)
+        return ERR_PTR(-ESTALE);
+
+    scoped_guard(rcu) {
+        pid = find_pid_ns(fid->pid, &init_pid_ns);
+        if (!pid || pid->ino != fid->ino || pid_vnr(pid) == 0)
+            return ERR_PTR(-ESTALE);
+        if(!pid_has_task(pid, thread ? PIDTYPE_PID : PIDTYPE_TGID))
+            return ERR_PTR(-ESTALE);
+            /* Something better? EINVAL like pidfd_open wouldn't be
+               very obvious... */
+
+        pid = get_pid(pid);
+    }
+
+    return __pidfs_alloc_file(pid, flags);
+}
+
 void __init pidfs_init(void)
 {
     pidfs_mnt = kern_mount(&pidfs_type);
diff --git a/include/linux/pidfs.h b/include/linux/pidfs.h
index 75bdf9807802..fba2654ae60f 100644
--- a/include/linux/pidfs.h
+++ b/include/linux/pidfs.h
@@ -3,6 +3,8 @@
 #define _LINUX_PID_FS_H

 struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags);
+struct file *pidfd_open_by_handle(void *gen_fid, int fh_len, int fh_type,
+                  unsigned int flags);
 void __init pidfs_init(void);

 #endif /* _LINUX_PID_FS_H */
diff --git a/kernel/fork.c b/kernel/fork.c
index 22f43721d031..fc47c76e4ff4 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2015,11 +2015,6 @@ static int __pidfd_prepare(struct pid *pid, unsigned int flags, struct file **re
         put_unused_fd(pidfd);
         return PTR_ERR(pidfd_file);
     }
-    /*
-     * anon_inode_getfile() ignores everything outside of the
-     * O_ACCMODE | O_NONBLOCK mask, set PIDFD_THREAD manually.
-     */
-    pidfd_file->f_flags |= (flags & PIDFD_THREAD);
     *ret = pidfd_file;
     return pidfd;
 }

Re: [PATCH v2 3/3] pidfs: implement file handle support

[Amir Goldstein]

On Thu, Nov 14, 2024 at 11:51 PM Erin Shepherd <erin.shepherd@e43.eu> wrote:
>
> On 14/11/2024 15:13, Christian Brauner wrote:
>
> > On Thu, Nov 14, 2024 at 02:13:06PM +0100, Erin Shepherd wrote:
> >> These two concerns combined with the special flag make me wonder if pidfs
> >> is so much of a special snowflake we should just special case it up front
> >> and skip all of the shared handle decode logic?
> > Care to try a patch and see what it looks like?
>
> The following is a completely untested sketch on top of the existing patch series.
> Some notes:
>
> - I made heavy use of the cleanup macros. I'm happy to convert things back to
>   goto out_xx style if preferred - writing things this way just made bashing out
>   the code without dropping resources on the floor easier

Your cleanup is very welcome, just please! not in the same patch as refactoring
and logic changes. Please do these 3 different things in different commits.
This patch is unreviewable as far as I am concerned.

> - If you don't implement fh_to_dentry then name_to_handle_at will just return an error
>   unless called with AT_HANDLE_FID. We need to decide what to do about that

What's to decide? I did not understand the problem.

> - The GET_PATH_FD_IS_NORMAL/etc constants don't match (what I see as) usual kernel style
>   but I'm not sure how to conventionally express something like that

I believe the conventional way to express a custom operation is an
optional method.

For example:

static int exportfs_get_name(struct vfsmount *mnt, struct dentry *dir,
                char *name, struct dentry *child)
{
        const struct export_operations *nop = dir->d_sb->s_export_op;
        struct path path = {.mnt = mnt, .dentry = dir};

        if (nop->get_name)
                return nop->get_name(dir, name, child);
        else
                return get_name(&path, name, child);
}

There are plenty of optional custom inode, file, sb, dentry
operations with default fallback. some examples:

        if (dir_inode->i_op->atomic_open) {
                dentry = atomic_open(nd, dentry, file, open_flag, mode);

        if (!splice && file_out->f_op->copy_file_range) {
                ret = file_out->f_op->copy_file_range(file_in, pos_in,
                                                      file_out, pos_out,
                                                      len, flags);
        } else if (!splice && file_in->f_op->remap_file_range && samesb) {
                ret = file_in->f_op->remap_file_range(file_in, pos_in,

So I think the right model for you to follow is a custom optional
s_export_op->open_by_handle() operation.

Thanks,
Amir.

[PATCH RFC 0/2] pidfs: file handle preliminaries

[Christian Brauner]

Hey,

This reworks the inode number allocation for pidfs in order to support
file handles properly.

Recently we received a patchset that aims to enable file handle encoding
and decoding via name_to_handle_at(2) and open_by_handle_at(2).

A crucical step in the patch series is how to go from inode number to
struct pid without leaking information into unprivileged contexts. The
issue is that in order to find a struct pid the pid number in the
initial pid namespace must be encoded into the file handle via
name_to_handle_at(2). This can be used by containers using a separate
pid namespace to learn what the pid number of a given process in the
initial pid namespace is. While this is a weak information leak it could
be used in various exploits and in general is an ugly wart in the
design.

To solve this problem a new way is needed to lookup a struct pid based
on the inode number allocated for that struct pid. The other part is to
remove the custom inode number allocation on 32bit systems that is also
an ugly wart that should go away.

So, a new scheme is used that I was discusssing with Tejun some time
back. A cyclic ida is used for the lower 32 bits and a the high 32 bits
are used for the generation number. This gives a 64 bit inode number
that is unique on both 32 bit and 64 bit. The lower 32 bit number is
recycled slowly and can be used to lookup struct pids.

So after applying the pidfs file handle series at
https://lore.kernel.org/r/20241101135452.19359-1-erin.shepherd@e43.eu on
top of the patches here we should be able to simplify encoding and
decoding to something like:

diff --git a/fs/pidfs.c b/fs/pidfs.c
index e71294d3d607..a38b833a2d38 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -78,7 +78,7 @@ void pidfs_remove_pid(struct pid *pid)
 }
 
 /* Find a struct pid based on the inode number. */
-static __maybe_unused struct pid *pidfs_ino_get_pid(u64 ino)
+static struct pid *pidfs_ino_get_pid(u64 ino)
 {
 	ino_t pid_ino = pidfs_ino(ino);
 	u32 gen = pidfs_gen(ino);
@@ -475,49 +475,37 @@ static const struct dentry_operations pidfs_dentry_operations = {
 	.d_prune	= stashed_dentry_prune,
 };
 
-#define PIDFD_FID_LEN 3
-
-struct pidfd_fid {
-	u64 ino;
-	s32 pid;
-} __packed;
-
-static int pidfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
+static int pidfs_encode_fh(struct inode *inode, __u32 *fh, int *max_len,
 			   struct inode *parent)
 {
 	struct pid *pid = inode->i_private;
-	struct pidfd_fid *fid = (struct pidfd_fid *)fh;
 
-	if (*max_len < PIDFD_FID_LEN) {
-		*max_len = PIDFD_FID_LEN;
+	if (*max_len < 2) {
+		*max_len = 2;
 		return FILEID_INVALID;
 	}
 
-	fid->ino = pid->ino;
-	fid->pid = pid_nr(pid);
-	*max_len = PIDFD_FID_LEN;
+	*max_len = 2;
+	*(u64 *)fh = pid->ino;
 	return FILEID_INO64_GEN;
 }
 
 static struct dentry *pidfs_fh_to_dentry(struct super_block *sb,
-					 struct fid *gen_fid,
+					 struct fid *fid,
 					 int fh_len, int fh_type)
 {
 	int ret;
 	struct path path;
-	struct pidfd_fid *fid = (struct pidfd_fid *)gen_fid;
 	struct pid *pid;
+	u64 pid_ino;
 
-	if (fh_type != FILEID_INO64_GEN || fh_len < PIDFD_FID_LEN)
+	if (fh_type != FILEID_INO64_GEN || fh_len < 2)
 		return NULL;
 
-	scoped_guard(rcu) {
-		pid = find_pid_ns(fid->pid, &init_pid_ns);
-		if (!pid || pid->ino != fid->ino || pid_vnr(pid) == 0)
-			return NULL;
-
-		pid = get_pid(pid);
-	}
+	pid_ino = *(u64 *)fid;
+	pid = pidfs_ino_get_pid(pid_ino);
+	if (!pid)
+		return NULL;
 
 	ret = path_from_stashed(&pid->stashed, pidfs_mnt, pid, &path);
 	if (ret < 0)

Thanks!
Christian

---
Christian Brauner (2):
      pidfs: rework inode number allocation
      pidfs: remove 32bit inode number handling

 fs/pidfs.c            | 138 +++++++++++++++++++++++++++++++++++---------------
 include/linux/pidfs.h |   2 +
 kernel/pid.c          |  11 ++--
 3 files changed, 103 insertions(+), 48 deletions(-)
---
base-commit: b86545e02e8c22fb89218f29d381fa8e8b91d815
change-id: 20241128-work-pidfs-2bd42c7ea772

[PATCH RFC 1/2] pidfs: rework inode number allocation

[Christian Brauner]

Recently we received a patchset that aims to enable file handle encoding
and decoding via name_to_handle_at(2) and open_by_handle_at(2).

A crucical step in the patch series is how to go from inode number to
struct pid without leaking information into unprivileged contexts. The
issue is that in order to find a struct pid the pid number in the
initial pid namespace must be encoded into the file handle via
name_to_handle_at(2). This can be used by containers using a separate
pid namespace to learn what the pid number of a given process in the
initial pid namespace is. While this is a weak information leak it could
be used in various exploits and in general is an ugly wart in the design.

To solve this problem a new way is needed to lookup a struct pid based
on the inode number allocated for that struct pid. The other part is to
remove the custom inode number allocation on 32bit systems that is also
an ugly wart that should go away.

So, a new scheme is used that I was discusssing with Tejun some time
back. A cyclic ida is used for the lower 32 bits and a the high 32 bits
are used for the generation number. This gives a 64 bit inode number
that is unique on both 32 bit and 64 bit. The lower 32 bit number is
recycled slowly and can be used to lookup struct pids.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/pidfs.c            | 92 +++++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/pidfs.h |  2 ++
 kernel/pid.c          | 11 +++---
 3 files changed, 98 insertions(+), 7 deletions(-)

diff --git a/fs/pidfs.c b/fs/pidfs.c
index 618abb1fa1b84cf31282c922374e28d60cd49d00..09a0c8ac805301927a94758b3f7d1e513826daf9 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -23,6 +23,88 @@
 #include "internal.h"
 #include "mount.h"
 
+static u32 pidfs_ino_highbits;
+static u32 pidfs_ino_last_ino_lowbits;
+
+static DEFINE_IDR(pidfs_ino_idr);
+
+static inline ino_t pidfs_ino(u64 ino)
+{
+	/* On 32 bit low 32 bits are the inode. */
+	if (sizeof(ino_t) < sizeof(u64))
+		return (u32)ino;
+
+	/* On 64 bit simply return ino. */
+	return ino;
+}
+
+static inline u32 pidfs_gen(u64 ino)
+{
+	/* On 32 bit the generation number are the upper 32 bits. */
+	if (sizeof(ino_t) < sizeof(u64))
+		return ino >> 32;
+
+	/* On 64 bit the generation number is 1. */
+	return 1;
+}
+
+/*
+ * Construct an inode number for struct pid in a way that we can use the
+ * lower 32bit to lookup struct pid independent of any pid numbers that
+ * could be leaked into userspace (e.g., via file handle encoding).
+ */
+int pidfs_add_pid(struct pid *pid)
+{
+	u32 ino_highbits;
+	int ret;
+
+	ret = idr_alloc_cyclic(&pidfs_ino_idr, pid, 1, 0, GFP_ATOMIC);
+	if (ret >= 0 && ret < pidfs_ino_last_ino_lowbits)
+		pidfs_ino_highbits++;
+	ino_highbits = pidfs_ino_highbits;
+	pidfs_ino_last_ino_lowbits = ret;
+	if (ret < 0)
+		return ret;
+
+	pid->ino = (u64)ino_highbits << 32 | ret;
+	pid->stashed = NULL;
+	return 0;
+}
+
+void pidfs_remove_pid(struct pid *pid)
+{
+	idr_remove(&pidfs_ino_idr, (u32)pidfs_ino(pid->ino));
+}
+
+/* Find a struct pid based on the inode number. */
+static __maybe_unused struct pid *pidfs_ino_get_pid(u64 ino)
+{
+	ino_t pid_ino = pidfs_ino(ino);
+	u32 gen = pidfs_gen(ino);
+	struct pid *pid;
+
+	guard(rcu)();
+
+	/* Handle @pid lookup carefully so there's no risk of UAF. */
+	pid = idr_find(&pidfs_ino_idr, (u32)ino);
+	if (!pid)
+		return NULL;
+
+	if (sizeof(ino_t) < sizeof(u64)) {
+		if (gen && pidfs_gen(pid->ino) != gen)
+			pid = NULL;
+	} else {
+		if (pidfs_ino(pid->ino) != pid_ino)
+			pid = NULL;
+	}
+
+	/* Within our pid namespace hierarchy? */
+	if (pid_vnr(pid) == 0)
+		pid = NULL;
+
+	return get_pid(pid);
+}
+
 #ifdef CONFIG_PROC_FS
 /**
  * pidfd_show_fdinfo - print information about a pidfd
@@ -491,6 +573,16 @@ struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags)
 
 void __init pidfs_init(void)
 {
+	/*
+	 * On 32 bit systems the lower 32 bits are the inode number and
+	 * the higher 32 bits are the generation number. The starting
+	 * value for the inode number and the generation number is one.
+	 */
+	if (sizeof(ino_t) < sizeof(u64))
+		pidfs_ino_highbits = 1;
+	else
+		pidfs_ino_highbits = 0;
+
 	pidfs_mnt = kern_mount(&pidfs_type);
 	if (IS_ERR(pidfs_mnt))
 		panic("Failed to mount pidfs pseudo filesystem");
diff --git a/include/linux/pidfs.h b/include/linux/pidfs.h
index 75bdf9807802a5d1a9699c99aa42648c2bd34170..2958652bb108b8a2e02128e17317be4545b40a01 100644
--- a/include/linux/pidfs.h
+++ b/include/linux/pidfs.h
@@ -4,5 +4,7 @@
 
 struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags);
 void __init pidfs_init(void);
+int pidfs_add_pid(struct pid *pid);
+void pidfs_remove_pid(struct pid *pid);
 
 #endif /* _LINUX_PID_FS_H */
diff --git a/kernel/pid.c b/kernel/pid.c
index 115448e89c3e9e664d0d51c8d853e8167ba0540c..9f321c6456d24af705c28f0256ca4de771f5e681 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -64,11 +64,6 @@ int pid_max = PID_MAX_DEFAULT;
 
 int pid_max_min = RESERVED_PIDS + 1;
 int pid_max_max = PID_MAX_LIMIT;
-/*
- * Pseudo filesystems start inode numbering after one. We use Reserved
- * PIDs as a natural offset.
- */
-static u64 pidfs_ino = RESERVED_PIDS;
 
 /*
  * PID-map pages start out as NULL, they get allocated upon
@@ -157,6 +152,7 @@ void free_pid(struct pid *pid)
 		}
 
 		idr_remove(&ns->idr, upid->nr);
+		pidfs_remove_pid(pid);
 	}
 	spin_unlock_irqrestore(&pidmap_lock, flags);
 
@@ -276,8 +272,9 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 	spin_lock_irq(&pidmap_lock);
 	if (!(ns->pid_allocated & PIDNS_ADDING))
 		goto out_unlock;
-	pid->stashed = NULL;
-	pid->ino = ++pidfs_ino;
+	retval = pidfs_add_pid(pid);
+	if (retval)
+		goto out_unlock;
 	for ( ; upid >= pid->numbers; --upid) {
 		/* Make the PID visible to find_pid_ns. */
 		idr_replace(&upid->ns->idr, pid, upid->nr);

-- 
2.45.2

[PATCH RFC 2/2] pidfs: remove 32bit inode number handling

[Christian Brauner]

Now that we have a unified inode number handling model remove the custom
ida-based allocation for 32bit.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/pidfs.c | 46 +++++-----------------------------------------
 1 file changed, 5 insertions(+), 41 deletions(-)

diff --git a/fs/pidfs.c b/fs/pidfs.c
index 09a0c8ac805301927a94758b3f7d1e513826daf9..334d2fc3f01ee6f219dc3e52309bfe00726885ca 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -400,40 +400,6 @@ struct pid *pidfd_pid(const struct file *file)
 
 static struct vfsmount *pidfs_mnt __ro_after_init;
 
-#if BITS_PER_LONG == 32
-/*
- * Provide a fallback mechanism for 32-bit systems so processes remain
- * reliably comparable by inode number even on those systems.
- */
-static DEFINE_IDA(pidfd_inum_ida);
-
-static int pidfs_inum(struct pid *pid, unsigned long *ino)
-{
-	int ret;
-
-	ret = ida_alloc_range(&pidfd_inum_ida, RESERVED_PIDS + 1,
-			      UINT_MAX, GFP_ATOMIC);
-	if (ret < 0)
-		return -ENOSPC;
-
-	*ino = ret;
-	return 0;
-}
-
-static inline void pidfs_free_inum(unsigned long ino)
-{
-	if (ino > 0)
-		ida_free(&pidfd_inum_ida, ino);
-}
-#else
-static inline int pidfs_inum(struct pid *pid, unsigned long *ino)
-{
-	*ino = pid->ino;
-	return 0;
-}
-#define pidfs_free_inum(ino) ((void)(ino))
-#endif
-
 /*
  * The vfs falls back to simple_setattr() if i_op->setattr() isn't
  * implemented. Let's reject it completely until we have a clean
@@ -485,7 +451,6 @@ static void pidfs_evict_inode(struct inode *inode)
 
 	clear_inode(inode);
 	put_pid(pid);
-	pidfs_free_inum(inode->i_ino);
 }
 
 static const struct super_operations pidfs_sops = {
@@ -511,17 +476,16 @@ static const struct dentry_operations pidfs_dentry_operations = {
 
 static int pidfs_init_inode(struct inode *inode, void *data)
 {
+	struct pid *pid = data;
+
 	inode->i_private = data;
 	inode->i_flags |= S_PRIVATE;
 	inode->i_mode |= S_IRWXU;
 	inode->i_op = &pidfs_inode_operations;
 	inode->i_fop = &pidfs_file_operations;
-	/*
-	 * Inode numbering for pidfs start at RESERVED_PIDS + 1. This
-	 * avoids collisions with the root inode which is 1 for pseudo
-	 * filesystems.
-	 */
-	return pidfs_inum(data, &inode->i_ino);
+	inode->i_ino = pidfs_ino(pid->ino);
+	inode->i_generation = pidfs_gen(pid->ino);
+	return 0;
 }
 
 static void pidfs_put_data(void *data)

-- 
2.45.2

Re: [PATCH RFC 0/2] pidfs: file handle preliminaries

[Amir Goldstein]

On Thu, Nov 28, 2024 at 1:34 PM Christian Brauner <brauner@kernel.org> wrote:
>
> Hey,
>
> This reworks the inode number allocation for pidfs in order to support
> file handles properly.
>
> Recently we received a patchset that aims to enable file handle encoding
> and decoding via name_to_handle_at(2) and open_by_handle_at(2).
>
> A crucical step in the patch series is how to go from inode number to
> struct pid without leaking information into unprivileged contexts. The
> issue is that in order to find a struct pid the pid number in the
> initial pid namespace must be encoded into the file handle via
> name_to_handle_at(2). This can be used by containers using a separate
> pid namespace to learn what the pid number of a given process in the
> initial pid namespace is. While this is a weak information leak it could
> be used in various exploits and in general is an ugly wart in the
> design.
>
> To solve this problem a new way is needed to lookup a struct pid based
> on the inode number allocated for that struct pid. The other part is to
> remove the custom inode number allocation on 32bit systems that is also
> an ugly wart that should go away.
>
> So, a new scheme is used that I was discusssing with Tejun some time
> back. A cyclic ida is used for the lower 32 bits and a the high 32 bits
> are used for the generation number. This gives a 64 bit inode number
> that is unique on both 32 bit and 64 bit. The lower 32 bit number is
> recycled slowly and can be used to lookup struct pids.
>
> So after applying the pidfs file handle series at
> https://lore.kernel.org/r/20241101135452.19359-1-erin.shepherd@e43.eu on
> top of the patches here we should be able to simplify encoding and
> decoding to something like:
>
> diff --git a/fs/pidfs.c b/fs/pidfs.c
> index e71294d3d607..a38b833a2d38 100644
> --- a/fs/pidfs.c
> +++ b/fs/pidfs.c
> @@ -78,7 +78,7 @@ void pidfs_remove_pid(struct pid *pid)
>  }
>
>  /* Find a struct pid based on the inode number. */
> -static __maybe_unused struct pid *pidfs_ino_get_pid(u64 ino)
> +static struct pid *pidfs_ino_get_pid(u64 ino)
>  {
>         ino_t pid_ino = pidfs_ino(ino);
>         u32 gen = pidfs_gen(ino);
> @@ -475,49 +475,37 @@ static const struct dentry_operations pidfs_dentry_operations = {
>         .d_prune        = stashed_dentry_prune,
>  };
>
> -#define PIDFD_FID_LEN 3
> -
> -struct pidfd_fid {
> -       u64 ino;
> -       s32 pid;
> -} __packed;
> -
> -static int pidfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
> +static int pidfs_encode_fh(struct inode *inode, __u32 *fh, int *max_len,
>                            struct inode *parent)
>  {
>         struct pid *pid = inode->i_private;
> -       struct pidfd_fid *fid = (struct pidfd_fid *)fh;
>
> -       if (*max_len < PIDFD_FID_LEN) {
> -               *max_len = PIDFD_FID_LEN;
> +       if (*max_len < 2) {
> +               *max_len = 2;
>                 return FILEID_INVALID;
>         }
>
> -       fid->ino = pid->ino;
> -       fid->pid = pid_nr(pid);
> -       *max_len = PIDFD_FID_LEN;
> +       *max_len = 2;
> +       *(u64 *)fh = pid->ino;
>         return FILEID_INO64_GEN;

Semantic remark:
        /*
         * 64 bit inode number, 32 bit generation number.
         */
        FILEID_INO64_GEN = 0x81,

filesystems are free to abuse the constants and return whatever id they want
(e.g. shmem_encode_fh()), but if you want to play by the rules, this would be
either:
        /*
         * 64 bit unique kernfs id
         */
        FILEID_KERNFS = 0xfe,

or:
        /*
         * 32bit inode number, 32 bit generation number.
         */
        FILEID_INO32_GEN = 1,

which is at least sometimes correct.
or define:
        /*
         * 64 bit inode number.
         */
        FILEID_INO64 = 0x80,


Thanks,
Amir.

Re: [PATCH RFC 1/2] pidfs: rework inode number allocation

[Amir Goldstein]

On Thu, Nov 28, 2024 at 1:34 PM Christian Brauner <brauner@kernel.org> wrote:
>
> Recently we received a patchset that aims to enable file handle encoding
> and decoding via name_to_handle_at(2) and open_by_handle_at(2).
>
> A crucical step in the patch series is how to go from inode number to
> struct pid without leaking information into unprivileged contexts. The
> issue is that in order to find a struct pid the pid number in the
> initial pid namespace must be encoded into the file handle via
> name_to_handle_at(2). This can be used by containers using a separate
> pid namespace to learn what the pid number of a given process in the
> initial pid namespace is. While this is a weak information leak it could
> be used in various exploits and in general is an ugly wart in the design.
>
> To solve this problem a new way is needed to lookup a struct pid based
> on the inode number allocated for that struct pid. The other part is to
> remove the custom inode number allocation on 32bit systems that is also
> an ugly wart that should go away.
>
> So, a new scheme is used that I was discusssing with Tejun some time
> back. A cyclic ida is used for the lower 32 bits and a the high 32 bits
> are used for the generation number. This gives a 64 bit inode number
> that is unique on both 32 bit and 64 bit. The lower 32 bit number is
> recycled slowly and can be used to lookup struct pids.
>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  fs/pidfs.c            | 92 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/pidfs.h |  2 ++
>  kernel/pid.c          | 11 +++---
>  3 files changed, 98 insertions(+), 7 deletions(-)
>
> diff --git a/fs/pidfs.c b/fs/pidfs.c
> index 618abb1fa1b84cf31282c922374e28d60cd49d00..09a0c8ac805301927a94758b3f7d1e513826daf9 100644
> --- a/fs/pidfs.c
> +++ b/fs/pidfs.c
> @@ -23,6 +23,88 @@
>  #include "internal.h"
>  #include "mount.h"
>
> +static u32 pidfs_ino_highbits;
> +static u32 pidfs_ino_last_ino_lowbits;
> +
> +static DEFINE_IDR(pidfs_ino_idr);
> +
> +static inline ino_t pidfs_ino(u64 ino)
> +{
> +       /* On 32 bit low 32 bits are the inode. */
> +       if (sizeof(ino_t) < sizeof(u64))
> +               return (u32)ino;
> +
> +       /* On 64 bit simply return ino. */
> +       return ino;
> +}
> +
> +static inline u32 pidfs_gen(u64 ino)
> +{
> +       /* On 32 bit the generation number are the upper 32 bits. */
> +       if (sizeof(ino_t) < sizeof(u64))
> +               return ino >> 32;
> +
> +       /* On 64 bit the generation number is 1. */
> +       return 1;
> +}
> +
> +/*
> + * Construct an inode number for struct pid in a way that we can use the
> + * lower 32bit to lookup struct pid independent of any pid numbers that
> + * could be leaked into userspace (e.g., via file handle encoding).
> + */
> +int pidfs_add_pid(struct pid *pid)
> +{
> +       u32 ino_highbits;
> +       int ret;
> +
> +       ret = idr_alloc_cyclic(&pidfs_ino_idr, pid, 1, 0, GFP_ATOMIC);
> +       if (ret >= 0 && ret < pidfs_ino_last_ino_lowbits)
> +               pidfs_ino_highbits++;
> +       ino_highbits = pidfs_ino_highbits;
> +       pidfs_ino_last_ino_lowbits = ret;
> +       if (ret < 0)
> +               return ret;
> +

This code looks "too useful" to be in a random filesystem.
Maybe work a generation counter into idr_alloc_cyclic or
just let it let the caller know that the range has been cycled?
Only if this is not too complicated to do.

> +       pid->ino = (u64)ino_highbits << 32 | ret;
> +       pid->stashed = NULL;
> +       return 0;
> +}
> +
> +void pidfs_remove_pid(struct pid *pid)
> +{
> +       idr_remove(&pidfs_ino_idr, (u32)pidfs_ino(pid->ino));
> +}
> +
> +/* Find a struct pid based on the inode number. */
> +static __maybe_unused struct pid *pidfs_ino_get_pid(u64 ino)
> +{
> +       ino_t pid_ino = pidfs_ino(ino);
> +       u32 gen = pidfs_gen(ino);
> +       struct pid *pid;
> +
> +       guard(rcu)();
> +
> +       /* Handle @pid lookup carefully so there's no risk of UAF. */
> +       pid = idr_find(&pidfs_ino_idr, (u32)ino);
> +       if (!pid)
> +               return NULL;
> +
> +       if (sizeof(ino_t) < sizeof(u64)) {
> +               if (gen && pidfs_gen(pid->ino) != gen)
> +                       pid = NULL;
> +       } else {
> +               if (pidfs_ino(pid->ino) != pid_ino)
> +                       pid = NULL;
> +       }
> +
> +       /* Within our pid namespace hierarchy? */
> +       if (pid_vnr(pid) == 0)
> +               pid = NULL;
> +
> +       return get_pid(pid);
> +}
> +
>  #ifdef CONFIG_PROC_FS
>  /**
>   * pidfd_show_fdinfo - print information about a pidfd
> @@ -491,6 +573,16 @@ struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags)
>
>  void __init pidfs_init(void)
>  {
> +       /*
> +        * On 32 bit systems the lower 32 bits are the inode number and
> +        * the higher 32 bits are the generation number. The starting
> +        * value for the inode number and the generation number is one.
> +        */
> +       if (sizeof(ino_t) < sizeof(u64))
> +               pidfs_ino_highbits = 1;
> +       else
> +               pidfs_ino_highbits = 0;
> +
>         pidfs_mnt = kern_mount(&pidfs_type);
>         if (IS_ERR(pidfs_mnt))
>                 panic("Failed to mount pidfs pseudo filesystem");
> diff --git a/include/linux/pidfs.h b/include/linux/pidfs.h
> index 75bdf9807802a5d1a9699c99aa42648c2bd34170..2958652bb108b8a2e02128e17317be4545b40a01 100644
> --- a/include/linux/pidfs.h
> +++ b/include/linux/pidfs.h
> @@ -4,5 +4,7 @@
>
>  struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags);
>  void __init pidfs_init(void);
> +int pidfs_add_pid(struct pid *pid);
> +void pidfs_remove_pid(struct pid *pid);
>
>  #endif /* _LINUX_PID_FS_H */
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 115448e89c3e9e664d0d51c8d853e8167ba0540c..9f321c6456d24af705c28f0256ca4de771f5e681 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -64,11 +64,6 @@ int pid_max = PID_MAX_DEFAULT;
>
>  int pid_max_min = RESERVED_PIDS + 1;
>  int pid_max_max = PID_MAX_LIMIT;
> -/*
> - * Pseudo filesystems start inode numbering after one. We use Reserved
> - * PIDs as a natural offset.
> - */
> -static u64 pidfs_ino = RESERVED_PIDS;
>
>  /*
>   * PID-map pages start out as NULL, they get allocated upon
> @@ -157,6 +152,7 @@ void free_pid(struct pid *pid)
>                 }
>
>                 idr_remove(&ns->idr, upid->nr);
> +               pidfs_remove_pid(pid);
>         }
>         spin_unlock_irqrestore(&pidmap_lock, flags);
>
> @@ -276,8 +272,9 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
>         spin_lock_irq(&pidmap_lock);
>         if (!(ns->pid_allocated & PIDNS_ADDING))
>                 goto out_unlock;
> -       pid->stashed = NULL;
> -       pid->ino = ++pidfs_ino;
> +       retval = pidfs_add_pid(pid);

I hope this does not create a scalability issue compared to the prev method?

> +       if (retval)
> +               goto out_unlock;
>         for ( ; upid >= pid->numbers; --upid) {
>                 /* Make the PID visible to find_pid_ns. */
>                 idr_replace(&upid->ns->idr, pid, upid->nr);
>
> --
> 2.45.2
>

Nice idea!
If there is something wrong with it, then I could not find it ;)

Thanks,
Amir.