netdev - [PATCH net] net: gro_cells: fix lock imbalance in gro_cells

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Message-ID: <20251020161114.1891141-1-edumazet@google.com>
Date: Mon, 20 Oct 2025 16:11:14 +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: Simon Horman <horms@...nel.org>, Kuniyuki Iwashima <kuniyu@...gle.com>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com, Eric Dumazet <edumazet@...gle.com>, 
	syzbot+f9651b9a8212e1c8906f@...kaller.appspotmail.com, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH net] net: gro_cells: fix lock imbalance in gro_cells_receive()

syzbot found that the local_unlock_nested_bh() call was
missing in some cases.

WARNING: possible recursive locking detected
syzkaller #0 Not tainted
--------------------------------------------
syz.2.329/7421 is trying to acquire lock:
 ffffe8ffffd48888 ((&cell->bh_lock)){+...}-{3:3}, at: spin_lock include/linux/spinlock_rt.h:44 [inline]
 ffffe8ffffd48888 ((&cell->bh_lock)){+...}-{3:3}, at: gro_cells_receive+0x404/0x790 net/core/gro_cells.c:30

but task is already holding lock:
 ffffe8ffffd48888 ((&cell->bh_lock)){+...}-{3:3}, at: spin_lock include/linux/spinlock_rt.h:44 [inline]
 ffffe8ffffd48888 ((&cell->bh_lock)){+...}-{3:3}, at: gro_cells_receive+0x404/0x790 net/core/gro_cells.c:30

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock((&cell->bh_lock));
  lock((&cell->bh_lock));

 *** DEADLOCK ***

Given the introduction of @have_bh_lock variable, it seems the author
intent was to have the local_unlock_nested_bh() after the @unlock label.

Fixes: 25718fdcbdd2 ("net: gro_cells: Use nested-BH locking for gro_cell")
Reported-by: syzbot+f9651b9a8212e1c8906f@...kaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/68f65eb9.a70a0220.205af.0034.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 net/core/gro_cells.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/net/core/gro_cells.c b/net/core/gro_cells.c
index b43911562f4d10aa3d05c60f343ff89c5d9ed58d..fd57b845de333ff0e397eeb95aa67926d4e4a730 100644
--- a/net/core/gro_cells.c
+++ b/net/core/gro_cells.c
@@ -43,12 +43,11 @@ int gro_cells_receive(struct gro_cells *gcells, struct sk_buff *skb)
 	if (skb_queue_len(&cell->napi_skbs) == 1)
 		napi_schedule(&cell->napi);
 
-	if (have_bh_lock)
-		local_unlock_nested_bh(&gcells->cells->bh_lock);
-
 	res = NET_RX_SUCCESS;
 
 unlock:
+	if (have_bh_lock)
+		local_unlock_nested_bh(&gcells->cells->bh_lock);
 	rcu_read_unlock();
 	return res;
 }
-- 
2.51.0.858.gf9c4a03a3a-goog