Re: [PATCH bpf] bpf, sockmap: Fix af_unix null-ptr-deref in proto update

[Martin KaFai Lau <martin.lau@linux.dev>]

On 1/31/26 2:06 AM, Kuniyuki Iwashima wrote:
> On Fri, Jan 30, 2026 at 1:30 PM Martin KaFai Lau <martin.lau@linux.dev> wrote:
>>
>> On 1/30/26 3:00 AM, Michal Luczaj wrote:
>>>>> Follow-up to discussion at
>>>>> https://lore.kernel.org/netdev/20240610174906.32921-1-kuniyu@amazon.com/.
>>>>
>>>> It is a long thread to dig. Please summarize the discussion in the
>>>> commit message.
>>>
>>> OK, there we go:
>>>
>>> The root cause of the null-ptr-deref is that unix_stream_connect() sets
>>> sk_state (`WRITE_ONCE(sk->sk_state, TCP_ESTABLISHED)`) _before_ it assigns
>>> a peer (`unix_peer(sk) = newsk`). sk_state == TCP_ESTABLISHED makes
>>> sock_map_sk_state_allowed() believe that socket is properly set up, which
>>> would include having a defined peer.
>>>
>>> In other words, there's a window when you can call
>>> unix_stream_bpf_update_proto() on socket which still has unix_peer(sk) == NULL.
>>>
>>> My initial idea was to simply move peer assignment _before_ the sk_state
>>> update, but the maintainer wasn't interested in changing the
>>> unix_stream_connect() hot path. He suggested taking care of it in the
>>> sockmap code.
> 
> Yes, we already have a memory barrier for unix_peer(sk) there
> (to save sock_hold()/sock_put() in sendmsg(), see 830a1e5c212fb)
> and another one just for sk->sk_state is not worth the unlikely
> case in sockmap by a buggy user.
> 
> 
>>>
>>> My understanding is that users are not supposed to put sockets in a sockmap
>>> when said socket is only half-way through connect() call. Hence `return
>>> -EINVAL` on a missing peer. Now, if users should be allowed to legally race
>>> connect() vs. sockmap update, then I guess we can wait for connect() to
>>> "finalize" e.g. by taking the unix_state_lock(), as discussed below.
> 
> If a user hit the issue, the user must have updated sockmap while the
> user knows connect() had not returned.  Such a user must prepare
> for failures since it could occur before sock_map_sk_state_allowed() too.
> 
> This is a subtle timing issue and I don't think the kernel should be
> friendly to such buggy users by waiting for connect() etc.

I don't have a use case for parallel connect and map update either. 
Also, I have no concern about returning -EINVAL in map_update for the 
not-yet-completed unix_stream_connect().

However, TCP/UDP (and probably vsock also?) do not have this racing 
issue because sock_map follows the lock usage in TCP/UDP as in other 
parts of the kernel. Why AF_UNIX is an exception and unix_state_lock() 
is not used in sock_map.

John and Jakub Stinicki, could this be an oversight?

>>>
>>>>    From looking at this commit message, if the existing lock_sock held by
>>>> update_elem is not useful for af_unix,
>>>
>>> Right, the existing lock_sock is not useful. update's lock_sock holds
>>> sock::sk_lock, while unix_state_lock() holds unix_sock::lock.
>>
>> It sounds like lock_sock is the incorrect lock to hold for af_unix. Is
>> taking lock_sock in sock_map doing anything useful for af_unix? Should
>> sock_map hold the unix_state_lock instead of lock_sock?
> 
> If sockmap code does not sleep, unix_state_lock can be used there.

afaik, bpf prog using the sockmap cannot sleep. Search for "if 
(prog->sleepable)" in "check_map_prog_compatibility()". The bpf prog 
attached to sock_map cannot sleep either, there is "can_be_sleepable()" 
check in the verifier.

>>>
>>>           if (!psock->sk_pair) {
>>> +               unix_state_lock(sk);
>>> +               unix_state_unlock(sk);
> 
> I don't like this... we had a similar one in recvmsg(MSG_PEEK) path
> for GC with a biiiiiig comment, which I removed in 118f457da9ed .

Me neither, for both empty critical section and big fat comment. This 
usually suggests something else is incorrect to begin with. I believe 
the wrong lock is taken in this case. Unless there is something 
prohibiting taking unix_state_lock in sock_map, fix it properly instead.

> 
> 
>>>                   sk_pair = unix_peer(sk);
>>>                   sock_hold(sk_pair);
>>
>> I don't have a strong opinion on waiting or checking NULL. imo, both are
>> not easy to understand. One is sk_state had already been checked earlier
>> under a lock_sock but still needs to check NULL on unix_peer(). Another
>> one is an empty unix_state_[un]lock(). If taking unix_state_lock, may as
>> well just use the existing unix_peer_get(sk).
> 
> Yes, unix_peer_get() can be safely used there (with extra sock_put()).

The sock_map needs to sock_hold(sk_pair) anyway and stores it in 
psock->sk_pair, so I don't think it needs an extra sock_put().

> 
> 
>> If its return value cannot
>> (?) be NULL, WARN_ON_ONCE() instead of having a special empty
> 
> I suggested WARN_ON_ONCE() because Michal reproduced it with
> mdelay() and I did not think it could occur in real life, but considering
> PREEMPT_RT, it could be real.  So, the current form in this patch looks
> good to me.

hmm... If unix_peer_get(sk) is used and it takes (and waits for) the 
unix_state_lock, it shouldn't be NULL? The above empty [un]lock idea 
does not check for NULL on unix_peer() either. Or am I missing something?

Regardless, if the proper lock is held, all this complication and 
reasoning will go away.