proc: prevent stacking filesystems on top

This prevents stacking filesystems (ecryptfs and overlayfs) from using
procfs as lower filesystem.  There is too much magic going on inside
procfs, and there is no good reason to stack stuff on top of procfs.

(For example, procfs does access checks in VFS open handlers, and
ecryptfs by design calls open handlers from a kernel thread that doesn't
drop privileges or so.)

Signed-off-by: Jann Horn <jannh@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/fs/proc/root.c b/fs/proc/root.c
index 361ab4ee42fc..ec649c92d270 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -121,6 +121,13 @@ static struct dentry *proc_mount(struct file_system_type *fs_type,
         if (IS_ERR(sb))
                 return ERR_CAST(sb);
 
+        /*
+         * procfs isn't actually a stacking filesystem; however, there is
+         * too much magic going on inside it to permit stacking things on
+         * top of it
+         */
+        sb->s_stack_depth = FILESYSTEM_MAX_STACK_DEPTH;
+
         if (!proc_parse_options(options, ns)) {
                 deactivate_locked_super(sb);
                 return ERR_PTR(-EINVAL);