proc: prevent stacking filesystems on top

commit e54ad7f1ee263ffa5a2de9c609d58dfa27b21cd9 upstream.

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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/fs/proc/root.c b/fs/proc/root.c
index dd77e14831962f31ff984a2492f3beb28426c73a..98992cb44c354490a7bf176fa03a2486bb778b80 100644 (file)
--- 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);