[PATCH] net: do not release sk in sk_wait_event

[sunyiqi <sunyiqixm@gmail.com>]

When investigating the kcm socket UAF which is also found by syzbot,
I found that the root cause of this problem is actually in
sk_wait_event.

In sk_wait_event, sk is released and relocked and called by
sk_stream_wait_memory. Protocols like tcp, kcm, etc., called it in some
ops function like *sendmsg which will lock the sk at the beginning.
But sk_stream_wait_memory releases sk unexpectedly and destroy
the thread safety. Finally it causes the kcm sk UAF.

If at the time when a thread(thread A) calls sk_stream_wait_memory
and the other thread(thread B) is waiting for lock in lock_sock,
thread B will successfully get the sk lock as thread A release sk lock
in sk_wait_event.

The thread B may change the sk which is not thread A expecting.

As a result, it will lead kernel to the unexpected behavior. Just like
the kcm sk UAF, which is actually cause by sk_wait_event in
sk_stream_wait_memory.

Previous commit d9dc8b0f8b4e ("net: fix sleeping for sk_wait_event()")
in 2016 seems do not solved this problem. Is it necessary to release
sock in sk_wait_event? Or just delete it to make the protocol ops
thread-secure.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Link: https://syzkaller.appspot.com/bug?extid=b72d86aa5df17ce74c60
Signed-off-by: sunyiqi <sunyiqixm@gmail.com>
---
 include/net/sock.h | 2 --
 1 file changed, 2 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index cce23ac4d514..08d3b204b019 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1145,7 +1145,6 @@ static inline void sock_rps_reset_rxhash(struct sock *sk)
 
 #define sk_wait_event(__sk, __timeo, __condition, __wait)		\
 	({	int __rc, __dis = __sk->sk_disconnects;			\
-		release_sock(__sk);					\
 		__rc = __condition;					\
 		if (!__rc) {						\
 			*(__timeo) = wait_woken(__wait,			\
@@ -1153,7 +1152,6 @@ static inline void sock_rps_reset_rxhash(struct sock *sk)
 						*(__timeo));		\
 		}							\
 		sched_annotate_sleep();					\
-		lock_sock(__sk);					\
 		__rc = __dis == __sk->sk_disconnects ? __condition : -EPIPE; \
 		__rc;							\
 	})
-- 
2.34.1