netdev - [PATCH net] net: fix possible store tearing in neigh_periodic

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Message-ID: <20230921084626.865912-1-edumazet@google.com>
Date: Thu, 21 Sep 2023 08:46:26 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, eric.dumazet@...il.com, 
	Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net] net: fix possible store tearing in neigh_periodic_work()

While looking at a related syzbot report involving neigh_periodic_work(),
I found that I forgot to add an annotation when deleting an
RCU protected item from a list.

Readers use rcu_deference(*np), we need to use either
rcu_assign_pointer() or WRITE_ONCE() on writer side
to prevent store tearing.

I use rcu_assign_pointer() to have lockdep support,
this was the choice made in neigh_flush_dev().

Fixes: 767e97e1e0db ("neigh: RCU conversion of struct neighbour")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/core/neighbour.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 6b76cd103195374f73786b7bb94fb2f4051a3c73..7212c7e521ef6388f450f3882077e26088bb3644 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -988,7 +988,9 @@ static void neigh_periodic_work(struct work_struct *work)
 			    (state == NUD_FAILED ||
 			     !time_in_range_open(jiffies, n->used,
 						 n->used + NEIGH_VAR(n->parms, GC_STALETIME)))) {
-				*np = n->next;
+				rcu_assign_pointer(*np,
+					rcu_dereference_protected(n->next,
+						lockdep_is_held(&tbl->lock)));
 				neigh_mark_dead(n);
 				write_unlock(&n->lock);
 				neigh_cleanup_and_release(n);
-- 
2.42.0.459.ge4e396fd5e-goog