[PATCH v5 36/42] ext4: support idmapped mounts

Christian Brauner
2021-01-12T22:17:28Z

Enable idmapped mounts for ext4. All dedicated helpers we need for this
exist. The vfs will have already made sure that the caller's fsids are
valid when mapped according to the mount's user namespace and that they
and have a mapping in the filesystem's s_user_ns.
So map the caller's fsids according to the mount's user namespace when
initializing a new inode. The inode_init_owner() helper will already
take care of this for the common case. If the initial user namespace is
passed nothing changes so non-idmapped mounts will see identical
behavior as before.
It is also noteworthy that the idmapped mount implementation allows us
to cleanly handle ioctls() too.

Let's create simple example where we idmap an ext4 filesystem:

root@f2-vm:~# truncate -s 5G ext4.img

root@f2-vm:~# mkfs.ext4 ./ext4.img
mke2fs 1.45.5 (07-Jan-2020)
Discarding device blocks: done
Creating filesystem with 1310720 4k blocks and 327680 inodes
Filesystem UUID: 3fd91794-c6ca-4b0f-9964-289a000919cf
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736

Allocating group tables: done
Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done

root@f2-vm:~# losetup -f --show ./ext4.img
/dev/loop0

root@f2-vm:~# mount /dev/loop0 /mnt

root@f2-vm:~# ls -al /mnt/
total 24
drwxr-xr-x 3 root root 4096 Oct 28 13:34 .
drwxr-xr-x 30 root root 4096 Oct 28 13:22 ..
drwx------ 2 root root 16384 Oct 28 13:34 lost+found

# Let's create an idmapped mount at /idmapped1 where we map uid and gid
# 0 to uid and gid 1000
root@f2-vm:/# ./mount-idmapped --map-mount b:0:1000:1 /mnt/ /idmapped1/

root@f2-vm:/# ls -al /idmapped1/
total 24
drwxr-xr-x 3 ubuntu ubuntu 4096 Oct 28 13:34 .
drwxr-xr-x 30 root root 4096 Oct 28 13:22 ..
drwx------ 2 ubuntu ubuntu 16384 Oct 28 13:34 lost+found

# Let's create an idmapped mount at /idmapped2 where we map uid and gid
# 0 to uid and gid 2000
root@f2-vm:/# ./mount-idmapped --map-mount b:0:2000:1 /mnt/ /idmapped2/

root@f2-vm:/# ls -al /idmapped2/
total 24
drwxr-xr-x 3 2000 2000 4096 Oct 28 13:34 .
drwxr-xr-x 31 root root 4096 Oct 28 13:39 ..
drwx------ 2 2000 2000 16384 Oct 28 13:34 lost+found

Let's create another example where we idmap the rootfs filesystem
without a mapping for uid 0 and gid 0:

# Create an idmapped mount of for a full POSIX range of rootfs under
# /mnt but without a mapping for uid 0 to reduce attack surface

root@f2-vm:/# ./mount-idmapped --map-mount b:1:1:65536 / /mnt/

# Since we don't have a mapping for uid and gid 0 all files owned by
# uid and gid 0 should show up as uid and gid 65534:
root@f2-vm:/# ls -al /mnt/
total 664
drwxr-xr-x 31 nobody nogroup 4096 Oct 28 13:39 .
drwxr-xr-x 31 root root 4096 Oct 28 13:39 ..
lrwxrwxrwx 1 nobody nogroup 7 Aug 25 07:44 bin -> usr/bin
drwxr-xr-x 4 nobody nogroup 4096 Oct 28 13:17 boot
drwxr-xr-x 2 nobody nogroup 4096 Aug 25 07:48 dev
drwxr-xr-x 81 nobody nogroup 4096 Oct 28 04:00 etc
drwxr-xr-x 4 nobody nogroup 4096 Oct 28 04:00 home
lrwxrwxrwx 1 nobody nogroup 7 Aug 25 07:44 lib -> usr/lib
lrwxrwxrwx 1 nobody nogroup 9 Aug 25 07:44 lib32 -> usr/lib32
lrwxrwxrwx 1 nobody nogroup 9 Aug 25 07:44 lib64 -> usr/lib64
lrwxrwxrwx 1 nobody nogroup 10 Aug 25 07:44 libx32 -> usr/libx32
drwx------ 2 nobody nogroup 16384 Aug 25 07:47 lost+found
drwxr-xr-x 2 nobody nogroup 4096 Aug 25 07:44 media
drwxr-xr-x 31 nobody nogroup 4096 Oct 28 13:39 mnt
drwxr-xr-x 2 nobody nogroup 4096 Aug 25 07:44 opt
drwxr-xr-x 2 nobody nogroup 4096 Apr 15 2020 proc
drwx--x--x 6 nobody nogroup 4096 Oct 28 13:34 root
drwxr-xr-x 2 nobody nogroup 4096 Aug 25 07:46 run
lrwxrwxrwx 1 nobody nogroup 8 Aug 25 07:44 sbin -> usr/sbin
drwxr-xr-x 2 nobody nogroup 4096 Aug 25 07:44 srv
drwxr-xr-x 2 nobody nogroup 4096 Apr 15 2020 sys
drwxrwxrwt 10 nobody nogroup 4096 Oct 28 13:19 tmp
drwxr-xr-x 14 nobody nogroup 4096 Oct 20 13:00 usr
drwxr-xr-x 12 nobody nogroup 4096 Aug 25 07:45 var

# Since we do have a mapping for uid and gid 1000 all files owned by
# uid and gid 1000 should simply show up as uid and gid 1000:
root@f2-vm:/# ls -al /mnt/home/ubuntu/
total 40
drwxr-xr-x 3 ubuntu ubuntu 4096 Oct 28 00:43 .
drwxr-xr-x 4 nobody nogroup 4096 Oct 28 04:00 ..
-rw------- 1 ubuntu ubuntu 2936 Oct 28 12:26 .bash_history
-rw-r--r-- 1 ubuntu ubuntu 220 Feb 25 2020 .bash_logout
-rw-r--r-- 1 ubuntu ubuntu 3771 Feb 25 2020 .bashrc
-rw-r--r-- 1 ubuntu ubuntu 807 Feb 25 2020 .profile
-rw-r--r-- 1 ubuntu ubuntu 0 Oct 16 16:11 .sudo_as_admin_successful
-rw------- 1 ubuntu ubuntu 1144 Oct 28 00:43 .viminfo

Cc: Christoph Hellwig <hch@lst.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-ext4@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
/* v2 */
unchanged

/* v3 */
unchanged

/* v4 */
- Mauricio Vásquez Bernal <mauricio@kinvolk.io>:
- Fix spelling mistake in ext4's new idmapped mount config option.

- Christoph Hellwig <hch@lst.de>:
- Drop the config option. If ever want an administrator to disable this
feature we can simply introduce a sysctl.

- Serge Hallyn <serge@hallyn.com>:
- Use "mnt_userns" to refer to a vfsmount's userns everywhere to make
terminology consistent.

/* v5 */
base-commit: 7c53f6b671f4aba70ff15e1b05148b10d58c2837

- Christoph Hellwig <hch@lst.de>:
- Use new file_userns_helper().

- Christian Brauner <christian.brauner@ubuntu.com>:
- Improve commit message.
---
fs/ext4/acl.c | 2 +-
fs/ext4/ext4.h | 13 +++++++------
fs/ext4/ialloc.c | 7 ++++---
fs/ext4/inode.c | 11 ++++++-----
fs/ext4/ioctl.c | 19 +++++++++++--------
fs/ext4/namei.c | 30 ++++++++++++++++--------------
fs/ext4/super.c | 2 +-
7 files changed, 46 insertions(+), 38 deletions(-)

diff --git a/fs/ext4/acl.c b/fs/ext4/acl.c
index 39f453ad05a0..c5eaffccecc3 100644
--- a/fs/ext4/acl.c
+++ b/fs/ext4/acl.c
@@ -246,7 +246,7 @@ ext4_set_acl(struct user_namespace *mnt_userns, struct inode *inode,
ext4_fc_start_update(inode);

if ((type == ACL_TYPE_ACCESS) && acl) {
- error = posix_acl_update_mode(&init_user_ns, inode, &mode, &acl);
+ error = posix_acl_update_mode(mnt_userns, inode, &mode, &acl);
if (error)
goto out_stop;
if (mode != inode->i_mode)
diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 4846168249c2..58b62e08379b 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -2755,18 +2755,19 @@ extern int ext4fs_dirhash(const struct inode *dir, const char *name, int len,

/* ialloc.c */
extern int ext4_mark_inode_used(struct super_block *sb, int ino);
-extern struct inode *__ext4_new_inode(handle_t *, struct inode *, umode_t,
+extern struct inode *__ext4_new_inode(struct user_namespace *, handle_t *,
+ struct inode *, umode_t,
const struct qstr *qstr, __u32 goal,
uid_t *owner, __u32 i_flags,
int handle_type, unsigned int line_no,
int nblocks);

-#define ext4_new_inode(handle, dir, mode, qstr, goal, owner, i_flags) \
- __ext4_new_inode((handle), (dir), (mode), (qstr), (goal), (owner), \
- i_flags, 0, 0, 0)
-#define ext4_new_inode_start_handle(dir, mode, qstr, goal, owner, \
+#define ext4_new_inode(handle, dir, mode, qstr, goal, owner, i_flags) \
+ __ext4_new_inode(&init_user_ns, (handle), (dir), (mode), (qstr), \
+ (goal), (owner), i_flags, 0, 0, 0)
+#define ext4_new_inode_start_handle(mnt_userns, dir, mode, qstr, goal, owner, \
type, nblocks) \
- __ext4_new_inode(NULL, (dir), (mode), (qstr), (goal), (owner), \
+ __ext4_new_inode((mnt_userns), NULL, (dir), (mode), (qstr), (goal), (owner), \
0, (type), __LINE__, (nblocks))

diff --git a/fs/ext4/ialloc.c b/fs/ext4/ialloc.c
index 00c1ec6eee16..bf9028950a51 100644
--- a/fs/ext4/ialloc.c
+++ b/fs/ext4/ialloc.c
@@ -919,7 +919,8 @@ static int ext4_xattr_credits_for_new_inode(struct inode *dir, mode_t mode,
* For other inodes, search forward from the parent directory's block
* group to find a free inode.
*/
-struct inode *__ext4_new_inode(handle_t *handle, struct inode *dir,
+struct inode *__ext4_new_inode(struct user_namespace *mnt_userns,
+ handle_t *handle, struct inode *dir,
umode_t mode, const struct qstr *qstr,
__u32 goal, uid_t *owner, __u32 i_flags,
int handle_type, unsigned int line_no,
@@ -969,10 +970,10 @@ struct inode *__ext4_new_inode(handle_t *handle, struct inode *dir,
i_gid_write(inode, owner[1]);
} else if (test_opt(sb, GRPID)) {
inode->i_mode = mode;
- inode->i_uid = current_fsuid();
+ inode->i_uid = fsuid_into_mnt(mnt_userns);
inode->i_gid = dir->i_gid;
} else
- inode_init_owner(&init_user_ns, inode, dir, mode);
+ inode_init_owner(mnt_userns, inode, dir, mode);

if (ext4_has_feature_project(sb) &&
ext4_test_inode_flag(dir, EXT4_INODE_PROJINHERIT))
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 6e11e2d9d4a2..4139253fa37a 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -20,6 +20,7 @@
*/

#include <linux/fs.h>
+#include <linux/mount.h>
#include <linux/time.h>
#include <linux/highuid.h>
#include <linux/pagemap.h>
@@ -5334,7 +5335,7 @@ int ext4_setattr(struct user_namespace *mnt_userns, struct dentry *dentry,
ATTR_GID | ATTR_TIMES_SET))))
return -EPERM;

- error = setattr_prepare(&init_user_ns, dentry, attr);
+ error = setattr_prepare(mnt_userns, dentry, attr);
if (error)
return error;

@@ -5509,7 +5510,7 @@ int ext4_setattr(struct user_namespace *mnt_userns, struct dentry *dentry,
}

if (!error) {
- setattr_copy(&init_user_ns, inode, attr);
+ setattr_copy(mnt_userns, inode, attr);
mark_inode_dirty(inode);
}

@@ -5521,7 +5522,7 @@ int ext4_setattr(struct user_namespace *mnt_userns, struct dentry *dentry,
ext4_orphan_del(NULL, inode);

if (!error && (ia_valid & ATTR_MODE))
- rc = posix_acl_chmod(&init_user_ns, inode, inode->i_mode);
+ rc = posix_acl_chmod(mnt_userns, inode, inode->i_mode);

err_out:
if (error)
@@ -5568,7 +5569,7 @@ int ext4_getattr(struct user_namespace *mnt_userns, const struct path *path,
STATX_ATTR_NODUMP |
STATX_ATTR_VERITY);

- generic_fillattr(&init_user_ns, inode, stat);
+ generic_fillattr(mnt_userns, inode, stat);
return 0;
}

@@ -5579,7 +5580,7 @@ int ext4_file_getattr(struct user_namespace *mnt_userns,
struct inode *inode = d_inode(path->dentry);
u64 delalloc_blocks;

- ext4_getattr(&init_user_ns, path, stat, request_mask, query_flags);
+ ext4_getattr(mnt_userns, path, stat, request_mask, query_flags);

/*
* If there is inline data in the inode, the inode will normally not
diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index 42335ae5dd94..bcd1e23736c2 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -107,10 +107,12 @@ void ext4_reset_inode_seed(struct inode *inode)
* important fields of the inodes.
*
* @sb: the super block of the filesystem
+ * @mnt_userns: user namespace of the mount the inode was found from
* @inode: the inode to swap with EXT4_BOOT_LOADER_INO
*
*/
static long swap_inode_boot_loader(struct super_block *sb,
+ struct user_namespace *mnt_userns,
struct inode *inode)
{
handle_t *handle;
@@ -139,7 +141,7 @@ static long swap_inode_boot_loader(struct super_block *sb,
}

if (IS_RDONLY(inode) || IS_APPEND(inode) || IS_IMMUTABLE(inode) ||
- !inode_owner_or_capable(&init_user_ns, inode) || !capable(CAP_SYS_ADMIN)) {
+ !inode_owner_or_capable(mnt_userns, inode) || !capable(CAP_SYS_ADMIN)) {
err = -EPERM;
goto journal_err_out;
}
@@ -814,6 +816,7 @@ static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
struct inode *inode = file_inode(filp);
struct super_block *sb = inode->i_sb;
struct ext4_inode_info *ei = EXT4_I(inode);
+ struct user_namespace *mnt_userns = file_user_ns(filp);
unsigned int flags;

ext4_debug("cmd = %u, arg = %lu\n", cmd, arg);
@@ -829,7 +832,7 @@ static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
case FS_IOC_SETFLAGS: {
int err;

- if (!inode_owner_or_capable(&init_user_ns, inode))
+ if (!inode_owner_or_capable(mnt_userns, inode))
return -EACCES;

if (get_user(flags, (int __user *) arg))
@@ -871,7 +874,7 @@ static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
__u32 generation;
int err;

- if (!inode_owner_or_capable(&init_user_ns, inode))
+ if (!inode_owner_or_capable(mnt_userns, inode))
return -EPERM;

if (ext4_has_metadata_csum(inode->i_sb)) {
@@ -1010,7 +1013,7 @@ static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
case EXT4_IOC_MIGRATE:
{
int err;
- if (!inode_owner_or_capable(&init_user_ns, inode))
+ if (!inode_owner_or_capable(mnt_userns, inode))
return -EACCES;

err = mnt_want_write_file(filp);
@@ -1032,7 +1035,7 @@ static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
case EXT4_IOC_ALLOC_DA_BLKS:
{
int err;
- if (!inode_owner_or_capable(&init_user_ns, inode))
+ if (!inode_owner_or_capable(mnt_userns, inode))
return -EACCES;

err = mnt_want_write_file(filp);
@@ -1051,7 +1054,7 @@ static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
err = mnt_want_write_file(filp);
if (err)
return err;
- err = swap_inode_boot_loader(sb, inode);
+ err = swap_inode_boot_loader(sb, mnt_userns, inode);
mnt_drop_write_file(filp);
return err;
}
@@ -1214,7 +1217,7 @@ static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)

case EXT4_IOC_CLEAR_ES_CACHE:
{
- if (!inode_owner_or_capable(&init_user_ns, inode))
+ if (!inode_owner_or_capable(mnt_userns, inode))
return -EACCES;
ext4_clear_inode_es(inode);
return 0;
@@ -1260,7 +1263,7 @@ static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
return -EFAULT;

/* Make sure caller has proper permission */
- if (!inode_owner_or_capable(&init_user_ns, inode))
+ if (!inode_owner_or_capable(mnt_userns, inode))
return -EACCES;

if (fa.fsx_xflags & ~EXT4_SUPPORTED_FS_XFLAGS)
diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index e0eb5e37f65a..8272c8b1575f 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -2610,8 +2610,8 @@ static int ext4_create(struct user_namespace *mnt_userns, struct inode *dir,
credits = (EXT4_DATA_TRANS_BLOCKS(dir->i_sb) +
EXT4_INDEX_EXTRA_TRANS_BLOCKS + 3);
retry:
- inode = ext4_new_inode_start_handle(dir, mode, &dentry->d_name, 0,
- NULL, EXT4_HT_DIR, credits);
+ inode = ext4_new_inode_start_handle(mnt_userns, dir, mode, &dentry->d_name,
+ 0, NULL, EXT4_HT_DIR, credits);
handle = ext4_journal_current_handle();
err = PTR_ERR(inode);
if (!IS_ERR(inode)) {
@@ -2645,8 +2645,8 @@ static int ext4_mknod(struct user_namespace *mnt_userns, struct inode *dir,
credits = (EXT4_DATA_TRANS_BLOCKS(dir->i_sb) +
EXT4_INDEX_EXTRA_TRANS_BLOCKS + 3);
retry:
- inode = ext4_new_inode_start_handle(dir, mode, &dentry->d_name, 0,
- NULL, EXT4_HT_DIR, credits);
+ inode = ext4_new_inode_start_handle(mnt_userns, dir, mode, &dentry->d_name,
+ 0, NULL, EXT4_HT_DIR, credits);
handle = ext4_journal_current_handle();
err = PTR_ERR(inode);
if (!IS_ERR(inode)) {
@@ -2677,7 +2677,7 @@ static int ext4_tmpfile(struct user_namespace *mnt_userns, struct inode *dir,
return err;

retry:
- inode = ext4_new_inode_start_handle(dir, mode,
+ inode = ext4_new_inode_start_handle(mnt_userns, dir, mode,
NULL, 0, NULL,
EXT4_HT_DIR,
EXT4_MAXQUOTAS_INIT_BLOCKS(dir->i_sb) +
@@ -2792,7 +2792,7 @@ static int ext4_mkdir(struct user_namespace *mnt_userns, struct inode *dir,
credits = (EXT4_DATA_TRANS_BLOCKS(dir->i_sb) +
EXT4_INDEX_EXTRA_TRANS_BLOCKS + 3);
retry:
- inode = ext4_new_inode_start_handle(dir, S_IFDIR | mode,
+ inode = ext4_new_inode_start_handle(mnt_userns, dir, S_IFDIR | mode,
&dentry->d_name,
0, NULL, EXT4_HT_DIR, credits);
handle = ext4_journal_current_handle();
@@ -3329,7 +3329,7 @@ static int ext4_symlink(struct user_namespace *mnt_userns, struct inode *dir,
EXT4_INDEX_EXTRA_TRANS_BLOCKS + 3;
}

- inode = ext4_new_inode_start_handle(dir, S_IFLNK|S_IRWXUGO,
+ inode = ext4_new_inode_start_handle(mnt_userns, dir, S_IFLNK|S_IRWXUGO,
&dentry->d_name, 0, NULL,
EXT4_HT_DIR, credits);
handle = ext4_journal_current_handle();
@@ -3661,7 +3661,8 @@ static void ext4_update_dir_count(handle_t *handle, struct ext4_renament *ent)
}
}

-static struct inode *ext4_whiteout_for_rename(struct ext4_renament *ent,
+static struct inode *ext4_whiteout_for_rename(struct user_namespace *mnt_userns,
+ struct ext4_renament *ent,
int credits, handle_t **h)
{
struct inode *wh;
@@ -3675,7 +3676,8 @@ static struct inode *ext4_whiteout_for_rename(struct ext4_renament *ent,
credits += (EXT4_MAXQUOTAS_TRANS_BLOCKS(ent->dir->i_sb) +
EXT4_XATTR_TRANS_BLOCKS + 4);
retry:
- wh = ext4_new_inode_start_handle(ent->dir, S_IFCHR | WHITEOUT_MODE,
+ wh = ext4_new_inode_start_handle(mnt_userns, ent->dir,
+ S_IFCHR | WHITEOUT_MODE,
&ent->dentry->d_name, 0, NULL,
EXT4_HT_DIR, credits);

@@ -3702,9 +3704,9 @@ static struct inode *ext4_whiteout_for_rename(struct ext4_renament *ent,
* while new_{dentry,inode) refers to the destination dentry/inode
* This comes from rename(const char *oldpath, const char *newpath)
*/
-static int ext4_rename(struct inode *old_dir, struct dentry *old_dentry,
- struct inode *new_dir, struct dentry *new_dentry,
- unsigned int flags)
+static int ext4_rename(struct user_namespace *mnt_userns, struct inode *old_dir,
+ struct dentry *old_dentry, struct inode *new_dir,
+ struct dentry *new_dentry, unsigned int flags)
{
handle_t *handle = NULL;
struct ext4_renament old = {
@@ -3788,7 +3790,7 @@ static int ext4_rename(struct inode *old_dir, struct dentry *old_dentry,
goto end_rename;
}
} else {
- whiteout = ext4_whiteout_for_rename(&old, credits, &handle);
+ whiteout = ext4_whiteout_for_rename(mnt_userns, &old, credits, &handle);
if (IS_ERR(whiteout)) {
retval = PTR_ERR(whiteout);
whiteout = NULL;
@@ -4103,7 +4105,7 @@ static int ext4_rename2(struct user_namespace *mnt_userns,
new_dir, new_dentry);
}

- return ext4_rename(old_dir, old_dentry, new_dir, new_dentry, flags);
+ return ext4_rename(mnt_userns, old_dir, old_dentry, new_dir, new_dentry, flags);
}

/*
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 21121787c874..6d2f082c462d 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -6608,7 +6608,7 @@ static struct file_system_type ext4_fs_type = {
.name = "ext4",
.mount = ext4_mount,
.kill_sb = kill_block_super,
- .fs_flags = FS_REQUIRES_DEV,
+ .fs_flags = FS_REQUIRES_DEV | FS_ALLOW_IDMAP,
};
MODULE_ALIAS_FS("ext4");

--
2.30.0