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

[Ефанов Владислав Александрович]

Paolo,


I don't think that we can just move sk_dst_set() call.

I think we can destroy dst of sendmsg task in this case.


Best regards,

Vlad.


On 26.05.2023 18:33, Paolo Abeni wrote:
> On Fri, 2023-05-26 at 18:08 +0300, Vladislav Efanov 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
> >
> > 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>
> Thank you for the detailed report!
>
> > ---
> >   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);
> Acquiring the socket lock in this fast-path is going to kill the xmit
> performances, I think we can't do that.
>
> What about something like the following instead? Does that addresses
> the UaF? (completely untested, not even built ;) If so, feel free to
> take it over.
>
> Thanks.
>
> Paolo
> ---
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 5440e67bcfe3..24f2761bdb1d 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -2381,7 +2381,6 @@ void sk_setup_caps(struct sock *sk, struct dst_entry *dst)
>   {
>   	u32 max_segs = 1;
>   
> -	sk_dst_set(sk, dst);
>   	sk->sk_route_caps = dst->dev->features;
>   	if (sk_is_tcp(sk))
>   		sk->sk_route_caps |= NETIF_F_GSO;
> @@ -2400,6 +2399,7 @@ void sk_setup_caps(struct sock *sk, struct dst_entry *dst)
>   		}
>   	}
>   	sk->sk_gso_max_segs = max_segs;
> +	sk_dst_set(sk, dst);
>   }
>   EXPORT_SYMBOL_GPL(sk_setup_caps);
>