Patch "proc: prevent stacking filesystems on top" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

    proc: prevent stacking filesystems on top

to the 4.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     proc-prevent-stacking-filesystems-on-top.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let [off-list ref] know about it.

From e54ad7f1ee263ffa5a2de9c609d58dfa27b21cd9 Mon Sep 17 00:00:00 2001

From: Jann Horn <jannh@google.com>
Date: Wed, 1 Jun 2016 11:55:05 +0200
Subject: proc: prevent stacking filesystems on top

From: Jann Horn <jannh@google.com>

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 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);

Patches currently in stable-queue which might be from jannh@google.com are

queue-4.4/ecryptfs-forbid-opening-files-without-mmap-handler.patch
queue-4.4/sched-panic-on-corrupted-stack-end.patch
queue-4.4/proc-prevent-stacking-filesystems-on-top.patch