[PATCH net 01/14] batman-adv: fix tp_meter counter underflow during shutdown

[Simon Wunderlich <sw@simonwunderlich.de>]

From: Luxiao Xu <rakukuip@gmail.com>

batadv_tp_sender_shutdown() unconditionally decrements the "sending"
atomic counter. If multiple paths (e.g. timeout, user cancel, and
normal finish) call this function, the counter can underflow to -1.

Since the sender logic treats any non-zero value as "still sending",
a negative value causes the sender kthread to loop indefinitely.
This leads to a use-after-free when the interface is removed while
the zombie thread is still active.

Fix this by using atomic_xchg() to ensure the counter only transitions
from 1 to 0 once.

Fixes: 33a3bb4a3345 ("batman-adv: throughput meter implementation")
Cc: stable@kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Signed-off-by: Luxiao Xu <rakukuip@gmail.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
[sven: added missing change in batadv_tp_send]
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
---
 net/batman-adv/tp_meter.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/batman-adv/tp_meter.c b/net/batman-adv/tp_meter.c
index 066c76113fc43..a4397aa881dd4 100644
--- a/net/batman-adv/tp_meter.c
+++ b/net/batman-adv/tp_meter.c
@@ -451,7 +451,7 @@ static void batadv_tp_sender_end(struct batadv_priv *bat_priv,
 static void batadv_tp_sender_shutdown(struct batadv_tp_vars *tp_vars,
 				      enum batadv_tp_meter_reason reason)
 {
-	if (!atomic_dec_and_test(&tp_vars->sending))
+	if (atomic_xchg(&tp_vars->sending, 0) != 1)
 		return;
 
 	tp_vars->reason = reason;
@@ -885,7 +885,7 @@ static int batadv_tp_send(void *arg)
 				   "Meter: %s() cannot send packets (%d)\n",
 				   __func__, err);
 			/* ensure nobody else tries to stop the thread now */
-			if (atomic_dec_and_test(&tp_vars->sending))
+			if (atomic_xchg(&tp_vars->sending, 0) == 1)
 				tp_vars->reason = err;
 			break;
 		}
-- 
2.47.3