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

Eric Dumazet
2023-05-26T15:30:00Z

On Fri, May 26, 2023 at 5:08 PM Vladislav Efanov <VEfanov@ispras.ru> 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->
> 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")

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@ispras.ru>
> ---
> 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
>

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