Patch "vfs: don't do RCU lookup of empty pathnames" has been added to the 4.4-stable tree

Patch "vfs: don't do RCU lookup of empty pathnames" has been added to the 4.4-stable tree

[gregkh]

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

    vfs: don't do RCU lookup of empty pathnames

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:
     vfs-don-t-do-rcu-lookup-of-empty-pathnames.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 <stable@vger.kernel.org> know about it.


>From c0eb027e5aef70b71e5a38ee3e264dc0b497f343 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Sun, 2 Apr 2017 17:10:08 -0700
Subject: vfs: don't do RCU lookup of empty pathnames

From: Linus Torvalds <torvalds@linux-foundation.org>

commit c0eb027e5aef70b71e5a38ee3e264dc0b497f343 upstream.

Normal pathname lookup doesn't allow empty pathnames, but using
AT_EMPTY_PATH (with name_to_handle_at() or fstatat(), for example) you
can trigger an empty pathname lookup.

And not only is the RCU lookup in that case entirely unnecessary
(because we'll obviously immediately finalize the end result), it is
actively wrong.

Why? An empth path is a special case that will return the original
'dirfd' dentry - and that dentry may not actually be RCU-free'd,
resulting in a potential use-after-free if we were to initialize the
path lazily under the RCU read lock and depend on complete_walk()
finalizing the dentry.

Found by syzkaller and KASAN.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Reported-by: Vegard Nossum <vegard.nossum@gmail.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Eric Biggers <ebiggers3@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/namei.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2000,6 +2000,9 @@ static const char *path_init(struct name
 	int retval = 0;
 	const char *s = nd->name->name;
 
+	if (!*s)
+		flags &= ~LOOKUP_RCU;
+
 	nd->last_type = LAST_ROOT; /* if there are only slashes... */
 	nd->flags = flags | LOOKUP_JUMPED | LOOKUP_PARENT;
 	nd->depth = 0;


Patches currently in stable-queue which might be from torvalds@linux-foundation.org are

queue-4.4/x86-cpu-change-type-of-x86_cache_size-variable-to-unsigned-int.patch
queue-4.4/mm-hide-a-warning-for-compile_test.patch
queue-4.4/vfs-don-t-do-rcu-lookup-of-empty-pathnames.patch
queue-4.4/kvm-x86-reduce-retpoline-performance-impact-in-slot_handle_level_range-by-always-inlining-iterator-helper-methods.patch