[PATCH] neighbour: Prevent a dead entry from updating gc_list

[Chinmay Agarwal <chinagar@codeaurora.org>]

Following race condition was detected:
<CPU A, t0> - neigh_flush_dev() is under execution and calls neigh_mark_dead(n),
marking the neighbour entry 'n' as dead.

<CPU B, t1> - Executing: __netif_receive_skb() -> __netif_receive_skb_core()
-> arp_rcv() -> arp_process().arp_process() calls __neigh_lookup() which takes
a reference on neighbour entry 'n'.

<CPU A, t2> - Moves further along neigh_flush_dev() and calls
neigh_cleanup_and_release(n), but since reference count increased in t2,
'n' couldn't be destroyed.

<CPU B, t3> - Moves further along, arp_process() and calls
neigh_update()-> __neigh_update() -> neigh_update_gc_list(), which adds
the neighbour entry back in gc_list(neigh_mark_dead(), removed it
earlier in t0 from gc_list)

<CPU B, t4> - arp_process() finally calls neigh_release(n), destroying
the neighbour entry.

This leads to 'n' still being part of gc_list, but the actual
neighbour structure has been freed.

The situation can be prevented from happening if we disallow a dead
entry to have any possibility of updating gc_list. This is what the
patch intends to achieve.

Signed-off-by: Chinmay Agarwal <chinagar@codeaurora.org>
---
 net/core/neighbour.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index ff07358..cf8e3076 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -1244,13 +1244,14 @@ static int __neigh_update(struct neighbour *neigh, const u8 *lladdr,
 	old    = neigh->nud_state;
 	err    = -EPERM;
 
-	if (!(flags & NEIGH_UPDATE_F_ADMIN) &&
-	    (old & (NUD_NOARP | NUD_PERMANENT)))
-		goto out;
 	if (neigh->dead) {
 		NL_SET_ERR_MSG(extack, "Neighbor entry is now dead");
+		new=old;
 		goto out;
 	}
+	if (!(flags & NEIGH_UPDATE_F_ADMIN) &&
+	    (old & (NUD_NOARP | NUD_PERMANENT)))
+		goto out;
 
 	ext_learn_change = neigh_update_ext_learned(neigh, flags, &notify);
 
--