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neighbour: Prevent a dead entry from updating gc_list

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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.

Change-Id: Ic8d4870586e09a8f6b6ebd190e815a18f49a269e
Fixes: 9c29a2f55e ("neighbor: Fix locking order for gc_list changes")
Signed-off-by: Chinmay Agarwal <chinagar@codeaurora.org>
This commit is contained in:
Chinmay Agarwal 2021-01-26 00:14:27 +05:30
commit eff30f44eb
1 changed files with 4 additions and 3 deletions
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9
net/core/neighbour.c
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@ -1251,13 +1251,14 @@ static int __neigh_update(struct neighbour *neigh, const u8 *lladdr,
old = neigh->nud_state;
err = -EPERM;
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;
if (neigh->dead) {
NL_SET_ERR_MSG(extack, "Neighbor entry is now dead");
goto out;
}
ext_learn_change = neigh_update_ext_learned(neigh, flags, &notify);