Re: [PATCH] sctp: check transport existence before processing a send primitive

["Ricardo Cañuelo Navarro" <rcn@igalia.com>]

On Fri, Apr 04 2025 at 10:22:38, Xin Long <lucien.xin@gmail.com> wrote:

>> Something like this:
>>
>> @@ -9225,7 +9227,9 @@ static int sctp_wait_for_sndbuf(struct sctp_association *asoc, long *timeo_p,
>>         pr_debug("%s: asoc:%p, timeo:%ld, msg_len:%zu\n", __func__, asoc,
>>                  *timeo_p, msg_len);
>>
>> -       /* Increment the association's refcnt.  */
>> +       /* Increment the transport and association's refcnt. */
>> +       if (transport)
>> +               sctp_transport_hold(transport);
>>         sctp_association_hold(asoc);
>>
>>         /* Wait on the association specific sndbuf space. */
>> @@ -9252,6 +9256,8 @@ static int sctp_wait_for_sndbuf(struct sctp_association *asoc, long *timeo_p,
>>                 lock_sock(sk);
>>                 if (sk != asoc->base.sk)
>>                         goto do_error;
>> +               if (transport && transport->dead)
>> +                       goto do_nonblock;
>>
>>                 *timeo_p = current_timeo;
>>         }
>> @@ -9259,7 +9265,9 @@ static int sctp_wait_for_sndbuf(struct sctp_association *asoc, long *timeo_p,
>>  out:
>>         finish_wait(&asoc->wait, &wait);
>>
>> -       /* Release the association's refcnt.  */
>> +       /* Release the transport and association's refcnt. */
>> +       if (transport)
>> +               sctp_transport_put(transport);
>>         sctp_association_put(asoc);
>>
>>         return err;
>>
>>
>> So by the time the sending thread re-claims the socket lock it can tell
>> whether someone else removed the transport by checking transport->dead
>> (set in sctp_transport_free()) and there's a guarantee that the
>> transport hasn't been freed yet because we hold a reference to it.
>>
>> If the whole receive path through sctp_assoc_rm_peer() is protected by
>> the same socket lock, as you said, this should be safe. The tests I ran
>> seem to work fine. If you're ok with it I'll send another patch to
>> supersede this one.
>>
> LGTM.

Good, thanks! I submitted a patch that supersedes this one:
https://lore.kernel.org/linux-sctp/20250404-kasan_slab-use-after-free_read_in_sctp_outq_select_transport__20250404-v1-1-5ce4a0b78ef2@igalia.com
so we can drop this.

Cheers,
Ricardo