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

Vlad Efanov
2023-05-26T16:57:55Z

Yes.

There is no lock for this lines and my patch does not broken this logic.

I sugessted to set lock only for lines 1566-1571
(ip6_sk_dst_lookup_flow() call).

Best regards,

Vlad.

On 26.05.2023 19:46, Eric Dumazet wrote:
> On Fri, May 26, 2023 at 6:09 PM Vlad Efanov <vefanov@ispras.ru> wrote:
>> Eric,
>>
>>
>> udp6_sendmsg() currently still locks the socket (on line 1595).
>>
> Not really, look more closely at lines 1580 -> 1594
>
>
>> 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.