[PATCH 3.16 275/305] proc: prevent stacking filesystems on top

[Ben Hutchings]

3.16.37-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jann Horn <jannh@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/proc/root.c | 7 +++++++
 1 file changed, 7 insertions(+)

--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -121,6 +121,13 @@ static struct dentry *proc_mount(struct
 	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);