[PATCH] udp6: Fix race condition in udp6_sendmsg & connect

[Vladislav Efanov]

Syzkaller got the following report:
BUG: KASAN: use-after-free in sk_setup_caps+0x621/0x690 net/core/sock.c:2018
Read of size 8 at addr ffff888027f82780 by task syz-executor276/3255

The function sk_setup_caps (called by ip6_sk_dst_store_flow->
ip6_dst_store) referenced already freed memory as this memory was
freed by parallel task in udpv6_sendmsg->ip6_sk_dst_lookup_flow->
sk_dst_check.

          task1 (connect)              task2 (udp6_sendmsg)
        sk_setup_caps->sk_dst_set |
                                  |  sk_dst_check->
                                  |      sk_dst_set
                                  |      dst_release
        sk_setup_caps references  |
        to already freed dst_entry|

The reason for this race condition is: udp6_sendmsg() calls
ip6_sk_dst_lookup() without lock for sock structure and tries to
allocate/add dst_entry structure to sock structure in parallel with
"connect" task.

Found by Linux Verification Center (linuxtesting.org) with syzkaller.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Vladislav Efanov <VEfanov@xxxxxxxxx>
---
 net/ipv6/udp.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index e5a337e6b970..a5ecd5d93b0a 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -1563,12 +1563,15 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 
 	fl6->flowlabel = ip6_make_flowinfo(ipc6.tclass, fl6->flowlabel);
 
+	lock_sock(sk);
 	dst = ip6_sk_dst_lookup_flow(sk, fl6, final_p, connected);
 	if (IS_ERR(dst)) {
 		err = PTR_ERR(dst);
 		dst = NULL;
+		release_sock(sk);
 		goto out;
 	}
+	release_sock(sk);
 
 	if (ipc6.hlimit < 0)
 		ipc6.hlimit = ip6_sk_dst_hoplimit(np, fl6, dst);
-- 
2.34.1