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

From: Vlad Efanov
Date: Fri May 26 2023 - 12:10:14 EST


udp6_sendmsg() currently still locks the socket (on line 1595).

Best regards,


On 26.05.2023 18:29, Eric Dumazet wrote:
On Fri, May 26, 2023 at 5:08 PM Vladislav Efanov <VEfanov@xxxxxxxxx> wrote:
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
Please include a full report.

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->

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 ( with syzkaller.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
This is a bogus Fixes: tag

In old times, UDP sendmsg() was using the socket lock.

Then, in linux-4.0 Vlad Yasevich made UDP v6 sendmsg() lockless (and
racy in many points)

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);

There must be another way really.
You just killed UDP performance.