linux-kernel - [ 16/38] neighbour: Fixed race condition at tbl->nht

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Message-Id: <20120316233448.700578666@linuxfoundation.org>
Date:	Fri, 16 Mar 2012 16:35:03 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, Michel Machado <michel@...irati.com.br>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: [ 16/38] neighbour: Fixed race condition at tbl->nht

3.0-stable review patch.  If anyone has any objections, please let me know.

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

From: Michel Machado <michel@...irati.com.br>

[ Upstream commit 84338a6c9dbb6ff3de4749864020f8f25d86fc81 ]

When the fixed race condition happens:

1. While function neigh_periodic_work scans the neighbor hash table
pointed by field tbl->nht, it unlocks and locks tbl->lock between
buckets in order to call cond_resched.

2. Assume that function neigh_periodic_work calls cond_resched, that is,
the lock tbl->lock is available, and function neigh_hash_grow runs.

3. Once function neigh_hash_grow finishes, and RCU calls
neigh_hash_free_rcu, the original struct neigh_hash_table that function
neigh_periodic_work was using doesn't exist anymore.

4. Once back at neigh_periodic_work, whenever the old struct
neigh_hash_table is accessed, things can go badly.

Signed-off-by: Michel Machado <michel@...irati.com.br>
CC: "David S. Miller" <davem@...emloft.net>
CC: Eric Dumazet <eric.dumazet@...il.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 net/core/neighbour.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -823,6 +823,8 @@ next_elt:
 		write_unlock_bh(&tbl->lock);
 		cond_resched();
 		write_lock_bh(&tbl->lock);
+		nht = rcu_dereference_protected(tbl->nht,
+						lockdep_is_held(&tbl->lock));
 	}
 	/* Cycle through all hash buckets every base_reachable_time/2 ticks.
 	 * ARP entry timeouts range from 1/2 base_reachable_time to 3/2

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