[4.2.y-ckt stable] Patch "net: possible use after free in dst_release" has been added to the 4.2.y-ckt tree

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    net: possible use after free in dst_release

to the linux-4.2.y-queue branch of the 4.2.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-4.2.y-queue

This patch is scheduled to be released in version 4.2.8-ckt2.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 4.2.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From 2baf9413e771d340e253af64685506d6522d09f2 Mon Sep 17 00:00:00 2001
From: Francesco Ruggeri <fruggeri at aristanetworks.com>
Date: Wed, 6 Jan 2016 00:18:48 -0800
Subject: net: possible use after free in dst_release

commit 07a5d38453599052aff0877b16bb9c1585f08609 upstream.

dst_release should not access dst->flags after decrementing
__refcnt to 0. The dst_entry may be in dst_busy_list and
dst_gc_task may dst_destroy it before dst_release gets a chance
to access dst->flags.

Fixes: d69bbf88c8d0 ("net: fix a race in dst_release()")
Fixes: 27b75c95f10d ("net: avoid RCU for NOCACHE dst")
Signed-off-by: Francesco Ruggeri <fruggeri at arista.com>
Acked-by: Eric Dumazet <edumazet at google.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 net/core/dst.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/core/dst.c b/net/core/dst.c
index cc4a086..27e030f 100644
--- a/net/core/dst.c
+++ b/net/core/dst.c
@@ -282,12 +282,13 @@ void dst_release(struct dst_entry *dst)
 {
 	if (dst) {
 		int newrefcnt;
+		unsigned short nocache = dst->flags & DST_NOCACHE;

 		newrefcnt = atomic_dec_return(&dst->__refcnt);
 		if (unlikely(newrefcnt < 0))
 			net_warn_ratelimited("%s: dst:%p refcnt:%d\n",
 					     __func__, dst, newrefcnt);
-		if (!newrefcnt && unlikely(dst->flags & DST_NOCACHE))
+		if (!newrefcnt && unlikely(nocache))
 			call_rcu(&dst->rcu_head, dst_destroy_rcu);
 	}
 }
--
1.9.1