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Subject[PATCH 5.4 61/65] neighbour: Prevent a dead entry from updating gc_list
From: Chinmay Agarwal <>

commit eb4e8fac00d1e01ada5e57c05d24739156086677 upstream.

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.

Fixes: 9c29a2f55ec0 ("neighbor: Fix locking order for gc_list changes")
Signed-off-by: Chinmay Agarwal <>
Reviewed-by: Cong Wang <>
Reviewed-by: David Ahern <>
Signed-off-by: Jakub Kicinski <>
Signed-off-by: Greg Kroah-Hartman <>
net/core/neighbour.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -1244,13 +1244,14 @@ static int __neigh_update(struct neighbo
old = neigh->nud_state;
err = -EPERM;

- if (!(flags & NEIGH_UPDATE_F_ADMIN) &&
- 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) &&
+ goto out;

ext_learn_change = neigh_update_ext_learned(neigh, flags, &notify);

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