[PATCH 5.16 0740/1017] net/x25: Fix null-ptr-deref caused by x25_disconnect

Greg Kroah-Hartman
2022-04-05T10:08:03Z

From: Duoming Zhou <duoming@zju.edu.cn>

[ Upstream commit 7781607938c8371d4c2b243527430241c62e39c2 ]

When the link layer is terminating, x25->neighbour will be set to NULL
in x25_disconnect(). As a result, it could cause null-ptr-deref bugs in
x25_sendmsg(),x25_recvmsg() and x25_connect(). One of the bugs is
shown below.

(Thread 1) | (Thread 2)
x25_link_terminated() | x25_recvmsg()
x25_kill_by_neigh() | ...
x25_disconnect() | lock_sock(sk)
... | ...
x25->neighbour = NULL //(1) |
... | x25->neighbour->extended //(2)

The code sets NULL to x25->neighbour in position (1) and dereferences
x25->neighbour in position (2), which could cause null-ptr-deref bug.

This patch adds lock_sock() in x25_kill_by_neigh() in order to synchronize
with x25_sendmsg(), x25_recvmsg() and x25_connect(). What`s more, the
sock held by lock_sock() is not NULL, because it is extracted from x25_list
and uses x25_list_lock to synchronize.

Fixes: 4becb7ee5b3d ("net/x25: Fix x25_neigh refcnt leak when x25 disconnect")
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Reviewed-by: Lin Ma <linma@zju.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/x25/af_x25.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/net/x25/af_x25.c b/net/x25/af_x25.c
index 3583354a7d7f..3a171828638b 100644
--- a/net/x25/af_x25.c
+++ b/net/x25/af_x25.c
@@ -1765,10 +1765,15 @@ void x25_kill_by_neigh(struct x25_neigh *nb)

write_lock_bh(&x25_list_lock);

- sk_for_each(s, &x25_list)
- if (x25_sk(s)->neighbour == nb)
+ sk_for_each(s, &x25_list) {
+ if (x25_sk(s)->neighbour == nb) {
+ write_unlock_bh(&x25_list_lock);
+ lock_sock(s);
x25_disconnect(s, ENETUNREACH, 0, 0);
-
+ release_sock(s);
+ write_lock_bh(&x25_list_lock);
+ }
+ }
write_unlock_bh(&x25_list_lock);

/* Remove any related forwards */
--
2.34.1