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

On Mon, Jan 25, 2021 at 11:59 AM Chinmay Agarwal
[off-list ref] wrote:

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 <redacted>

Please add a Fixes tag for bug fixes, in this case it is probably:

Fixes: 9c29a2f55ec0 ("neighbor: Fix locking order for gc_list changes")

And, make sure you run checkpatch.pl before sending out. For your
patch, it will definitely complain about the missing spaces around the
assignment "new=old;".

Thanks.