[PATCH RESEND v9 2/8] proc: allow to mount many instances of proc in one pid namespace

Alexey Gladkov
2020-03-24T20:46:50Z

This patch allows to have multiple procfs instances inside the
same pid namespace. The aim here is lightweight sandboxes, and to allow
that we have to modernize procfs internals.

1) The main aim of this work is to have on embedded systems one
supervisor for apps. Right now we have some lightweight sandbox support,
however if we create pid namespacess we have to manages all the
processes inside too, where our goal is to be able to run a bunch of
apps each one inside its own mount namespace without being able to
notice each other. We only want to use mount namespaces, and we want
procfs to behave more like a real mount point.

2) Linux Security Modules have multiple ptrace paths inside some
subsystems, however inside procfs, the implementation does not guarantee
that the ptrace() check which triggers the security_ptrace_check() hook
will always run. We have the 'hidepid' mount option that can be used to
force the ptrace_may_access() check inside has_pid_permissions() to run.
The problem is that 'hidepid' is per pid namespace and not attached to
the mount point, any remount or modification of 'hidepid' will propagate
to all other procfs mounts.

This also does not allow to support Yama LSM easily in desktop and user
sessions. Yama ptrace scope which restricts ptrace and some other
syscalls to be allowed only on inferiors, can be updated to have a
per-task context, where the context will be inherited during fork(),
clone() and preserved across execve(). If we support multiple private
procfs instances, then we may force the ptrace_may_access() on
/proc/<pids>/ to always run inside that new procfs instances. This will
allow to specifiy on user sessions if we should populate procfs with
pids that the user can ptrace or not.

By using Yama ptrace scope, some restricted users will only be able to see
inferiors inside /proc, they won't even be able to see their other
processes. Some software like Chromium, Firefox's crash handler, Wine
and others are already using Yama to restrict which processes can be
ptracable. With this change this will give the possibility to restrict
/proc/<pids>/ but more importantly this will give desktop users a
generic and usuable way to specifiy which users should see all processes
and which users can not.

Side notes:
* This covers the lack of seccomp where it is not able to parse
arguments, it is easy to install a seccomp filter on direct syscalls
that operate on pids, however /proc/<pid>/ is a Linux ABI using
filesystem syscalls. With this change LSMs should be able to analyze
open/read/write/close...

In the new patchset version I removed the 'newinstance' option
as suggested by Eric W. Biederman.

Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
---
fs/proc/base.c | 13 +++++++++----
fs/proc/inode.c | 4 ++--
fs/proc/root.c | 42 +++++++++++++++++++++++++----------------
fs/proc/self.c | 6 +++---
fs/proc/thread_self.c | 6 +++---
include/linux/proc_fs.h | 12 ++++++++++++
6 files changed, 55 insertions(+), 28 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index e7efe9d6f3d6..9d2e425338c8 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -3301,6 +3301,7 @@ struct dentry *proc_pid_lookup(struct dentry *dentry, unsigned int flags)
{
struct task_struct *task;
unsigned tgid;
+ struct proc_fs_info *fs_info;
struct pid_namespace *ns;
struct dentry *result = ERR_PTR(-ENOENT);

@@ -3308,7 +3309,8 @@ struct dentry *proc_pid_lookup(struct dentry *dentry, unsigned int flags)
if (tgid == ~0U)
goto out;

- ns = dentry->d_sb->s_fs_info;
+ fs_info = proc_sb_info(dentry->d_sb);
+ ns = fs_info->pid_ns;
rcu_read_lock();
task = find_task_by_pid_ns(tgid, ns);
if (task)
@@ -3372,6 +3374,7 @@ static struct tgid_iter next_tgid(struct pid_namespace *ns, struct tgid_iter ite
int proc_pid_readdir(struct file *file, struct dir_context *ctx)
{
struct tgid_iter iter;
+ struct proc_fs_info *fs_info = proc_sb_info(file_inode(file)->i_sb);
struct pid_namespace *ns = proc_pid_ns(file_inode(file));
loff_t pos = ctx->pos;

@@ -3379,13 +3382,13 @@ int proc_pid_readdir(struct file *file, struct dir_context *ctx)
return 0;

if (pos == TGID_OFFSET - 2) {
- struct inode *inode = d_inode(ns->proc_self);
+ struct inode *inode = d_inode(fs_info->proc_self);
if (!dir_emit(ctx, "self", 4, inode->i_ino, DT_LNK))
return 0;
ctx->pos = pos = pos + 1;
}
if (pos == TGID_OFFSET - 1) {
- struct inode *inode = d_inode(ns->proc_thread_self);
+ struct inode *inode = d_inode(fs_info->proc_thread_self);
if (!dir_emit(ctx, "thread-self", 11, inode->i_ino, DT_LNK))
return 0;
ctx->pos = pos = pos + 1;
@@ -3599,6 +3602,7 @@ static struct dentry *proc_task_lookup(struct inode *dir, struct dentry * dentry
struct task_struct *task;
struct task_struct *leader = get_proc_task(dir);
unsigned tid;
+ struct proc_fs_info *fs_info;
struct pid_namespace *ns;
struct dentry *result = ERR_PTR(-ENOENT);

@@ -3609,7 +3613,8 @@ static struct dentry *proc_task_lookup(struct inode *dir, struct dentry * dentry
if (tid == ~0U)
goto out;

- ns = dentry->d_sb->s_fs_info;
+ fs_info = proc_sb_info(dentry->d_sb);
+ ns = fs_info->pid_ns;
rcu_read_lock();
task = find_task_by_pid_ns(tid, ns);
if (task)
diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index 1e730ea1dcd6..6e4c6728338b 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -167,8 +167,8 @@ void proc_invalidate_siblings_dcache(struct hlist_head *inodes, spinlock_t *lock

static int proc_show_options(struct seq_file *seq, struct dentry *root)
{
- struct super_block *sb = root->d_sb;
- struct pid_namespace *pid = sb->s_fs_info;
+ struct proc_fs_info *fs_info = proc_sb_info(root->d_sb);
+ struct pid_namespace *pid = fs_info->pid_ns;

if (!gid_eq(pid->pid_gid, GLOBAL_ROOT_GID))
seq_printf(seq, ",gid=%u", from_kgid_munged(&init_user_ns, pid->pid_gid));
diff --git a/fs/proc/root.c b/fs/proc/root.c
index 2633f10446c3..b28adbb0b937 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -30,7 +30,7 @@
#include "internal.h"

struct proc_fs_context {
- struct pid_namespace *pid_ns;
+ struct proc_fs_info *fs_info;
unsigned int mask;
int hidepid;
int gid;
@@ -92,7 +92,8 @@ static void proc_apply_options(struct super_block *s,

static int proc_fill_super(struct super_block *s, struct fs_context *fc)
{
- struct pid_namespace *pid_ns = get_pid_ns(s->s_fs_info);
+ struct proc_fs_context *ctx = fc->fs_private;
+ struct pid_namespace *pid_ns = get_pid_ns(ctx->fs_info->pid_ns);
struct inode *root_inode;
int ret;

@@ -106,6 +107,7 @@ static int proc_fill_super(struct super_block *s, struct fs_context *fc)
s->s_magic = PROC_SUPER_MAGIC;
s->s_op = &proc_sops;
s->s_time_gran = 1;
+ s->s_fs_info = ctx->fs_info;

/*
* procfs isn't actually a stacking filesystem; however, there is
@@ -113,7 +115,7 @@ static int proc_fill_super(struct super_block *s, struct fs_context *fc)
* top of it
*/
s->s_stack_depth = FILESYSTEM_MAX_STACK_DEPTH;
-
+
/* procfs dentries and inodes don't require IO to create */
s->s_shrink.seeks = 0;

@@ -140,7 +142,8 @@ static int proc_fill_super(struct super_block *s, struct fs_context *fc)
static int proc_reconfigure(struct fs_context *fc)
{
struct super_block *sb = fc->root->d_sb;
- struct pid_namespace *pid = sb->s_fs_info;
+ struct proc_fs_info *fs_info = proc_sb_info(sb);
+ struct pid_namespace *pid = fs_info->pid_ns;

sync_filesystem(sb);

@@ -150,16 +153,14 @@ static int proc_reconfigure(struct fs_context *fc)

static int proc_get_tree(struct fs_context *fc)
{
- struct proc_fs_context *ctx = fc->fs_private;
-
- return get_tree_keyed(fc, proc_fill_super, ctx->pid_ns);
+ return get_tree_nodev(fc, proc_fill_super);
}

static void proc_fs_context_free(struct fs_context *fc)
{
struct proc_fs_context *ctx = fc->fs_private;

- put_pid_ns(ctx->pid_ns);
+ put_pid_ns(ctx->fs_info->pid_ns);
kfree(ctx);
}

@@ -178,9 +179,15 @@ static int proc_init_fs_context(struct fs_context *fc)
if (!ctx)
return -ENOMEM;

- ctx->pid_ns = get_pid_ns(task_active_pid_ns(current));
+ ctx->fs_info = kzalloc(sizeof(struct proc_fs_info), GFP_KERNEL);
+ if (!ctx->fs_info) {
+ kfree(ctx);
+ return -ENOMEM;
+ }
+
+ ctx->fs_info->pid_ns = get_pid_ns(task_active_pid_ns(current));
put_user_ns(fc->user_ns);
- fc->user_ns = get_user_ns(ctx->pid_ns->user_ns);
+ fc->user_ns = get_user_ns(ctx->fs_info->pid_ns->user_ns);
fc->fs_private = ctx;
fc->ops = &proc_fs_context_ops;
return 0;
@@ -188,15 +195,18 @@ static int proc_init_fs_context(struct fs_context *fc)

static void proc_kill_sb(struct super_block *sb)
{
- struct pid_namespace *ns;
+ struct proc_fs_info *fs_info = proc_sb_info(sb);
+ struct pid_namespace *ns = fs_info->pid_ns;
+
+ if (fs_info->proc_self)
+ dput(fs_info->proc_self);
+
+ if (fs_info->proc_thread_self)
+ dput(fs_info->proc_thread_self);

- ns = (struct pid_namespace *)sb->s_fs_info;
- if (ns->proc_self)
- dput(ns->proc_self);
- if (ns->proc_thread_self)
- dput(ns->proc_thread_self);
kill_anon_super(sb);
put_pid_ns(ns);
+ kfree(fs_info);
}

static struct file_system_type proc_fs_type = {
diff --git a/fs/proc/self.c b/fs/proc/self.c
index 57c0a1047250..309301ac0136 100644
--- a/fs/proc/self.c
+++ b/fs/proc/self.c
@@ -36,10 +36,10 @@ static unsigned self_inum __ro_after_init;
int proc_setup_self(struct super_block *s)
{
struct inode *root_inode = d_inode(s->s_root);
- struct pid_namespace *ns = proc_pid_ns(root_inode);
+ struct proc_fs_info *fs_info = proc_sb_info(s);
struct dentry *self;
int ret = -ENOMEM;
-
+
inode_lock(root_inode);
self = d_alloc_name(s->s_root, "self");
if (self) {
@@ -62,7 +62,7 @@ int proc_setup_self(struct super_block *s)
if (ret)
pr_err("proc_fill_super: can't allocate /proc/self\n");
else
- ns->proc_self = self;
+ fs_info->proc_self = self;

return ret;
}
diff --git a/fs/proc/thread_self.c b/fs/proc/thread_self.c
index f61ae53533f5..2493cbbdfa6f 100644
--- a/fs/proc/thread_self.c
+++ b/fs/proc/thread_self.c
@@ -36,7 +36,7 @@ static unsigned thread_self_inum __ro_after_init;
int proc_setup_thread_self(struct super_block *s)
{
struct inode *root_inode = d_inode(s->s_root);
- struct pid_namespace *ns = proc_pid_ns(root_inode);
+ struct proc_fs_info *fs_info = proc_sb_info(s);
struct dentry *thread_self;
int ret = -ENOMEM;

@@ -60,9 +60,9 @@ int proc_setup_thread_self(struct super_block *s)
inode_unlock(root_inode);

if (ret)
- pr_err("proc_fill_super: can't allocate /proc/thread_self\n");
+ pr_err("proc_fill_super: can't allocate /proc/thread-self\n");
else
- ns->proc_thread_self = thread_self;
+ fs_info->proc_thread_self = thread_self;

return ret;
}
diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
index 40a7982b7285..5920a4ecd71b 100644
--- a/include/linux/proc_fs.h
+++ b/include/linux/proc_fs.h
@@ -27,6 +27,17 @@ struct proc_ops {
unsigned long (*proc_get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
};

+struct proc_fs_info {
+ struct pid_namespace *pid_ns;
+ struct dentry *proc_self; /* For /proc/self */
+ struct dentry *proc_thread_self; /* For /proc/thread-self */
+};
+
+static inline struct proc_fs_info *proc_sb_info(struct super_block *sb)
+{
+ return sb->s_fs_info;
+}
+
#ifdef CONFIG_PROC_FS

typedef int (*proc_write_t)(struct file *, char *, size_t);
@@ -161,6 +172,7 @@ int open_related_ns(struct ns_common *ns,
/* get the associated pid namespace for a file in procfs */
static inline struct pid_namespace *proc_pid_ns(const struct inode *inode)
{
+ return proc_sb_info(inode->i_sb)->pid_ns;
return inode->i_sb->s_fs_info;
}

--
2.25.2