Re: [PATCH bpf v3 3/5] bpf, sockmap: Fix af_unix iter deadlock

[Michal Luczaj <mhal@rbox.co>]

On 3/6/26 07:04, Jiayuan Chen wrote:
> On 3/6/26 7:30 AM, Michal Luczaj wrote:
>> @@ -3729,15 +3729,14 @@ static int bpf_iter_unix_seq_show(struct seq_file *seq, void *v)
>>   	struct bpf_prog *prog;
>>   	struct sock *sk = v;
>>   	uid_t uid;
>> -	bool slow;
>>   	int ret;
>>   
>>   	if (v == SEQ_START_TOKEN)
>>   		return 0;
>>   
>> -	slow = lock_sock_fast(sk);
>> +	lock_sock(sk);
>>   
>> -	if (unlikely(sk_unhashed(sk))) {
>> +	if (unlikely(sock_flag(sk, SOCK_DEAD))) {
>>   		ret = SEQ_SKIP;
>>   		goto unlock;
>>   	}
> 
> 
> Switching to lock_sock() fixes the deadlock, but it does not provide mutual
> exclusion with unix_release_sock(), which uses unix_state_lock() exclusively
> and does not touch lock_sock() at all. So a dying socket can still reach the
> BPF prog concurrently with unix_release_sock() running on another CPU.

That's right. Note that although the socket is dying, iter holds a
reference to it, so the socket is far from being freed (as in: memory
released).

> Both SOCK_DEAD and the clearing of unix_peer(sk) happen under
> unix_state_lock() in unix_release_sock(). Without taking unix_state_lock()
> before the SOCK_DEAD check, there is a window:
>
> iter                              unix_release_sock()
> ---  lock_sock(sk)
> SOCK_DEAD == 0  (check passes)
>                                   unix_state_lock(sk)
>                                   unix_peer(sk) = NULL
>                                   sock_set_flag(sk, SOCK_DEAD)
>                                   unix_state_unlock(sk)
> BPF prog runs
> → accesses unix_peer(sk) == NULL → crash
>
> This was not raised in the v2 discussion.

It was raised in v1[1]. Conclusion was that bpf prog bytecode directly
accessing unix_peer(sk) is not an issue; bpf machinery will handle any
faults. That said, should a "bad" value of unix_peer(sk) end up as a
parameter of a bpf helper, yes, that is a well known[2] problem (that have
a solution unrelated to this series).

[1]:
https://lore.kernel.org/bpf/6de6f1bf-c8ee-4dfb-9b8c-f89185946630@linux.dev/
[2]:
https://lore.kernel.org/bpf/CAADnVQK_93g_KkNFYXSr8ZvA1fYh4hoFRJCJFPS-zs4ox0HhAA@mail.gmail.com/