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

[Vlad Efanov]

Eric,


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


Best regards,

Vlad.


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