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

[Vlad Efanov <vefanov@ispras.ru>]

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@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.