linux-kernel - Re: [PATCH] udp6: Fix race condition in udp6

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Message-ID: <027d28a0-b31b-ab42-9eb6-2826c04c9364@ispras.ru>
Date:   Fri, 26 May 2023 18:58:48 +0300
From:   Ефанов Владислав Александрович <vefanov@...ras.ru>
To:     Paolo Abeni <pabeni@...hat.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        David Ahern <dsahern@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, lvc-project@...uxtesting.org
Subject: Re: [PATCH] udp6: Fix race condition in udp6_sendmsg & connect

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@...ras.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);
>   
>