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

Ефанов Владислав Александрович
2023-05-26T15:58:55Z

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@ispras.ru>
> 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);
>
>