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

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Message-ID: <87o6x8e81j.fsf@igalia.com>
Date: Mon, 07 Apr 2025 09:37:44 +0200
From: Ricardo Cañuelo Navarro <rcn@...lia.com>
To: Xin Long <lucien.xin@...il.com>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
 <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman
 <horms@...nel.org>, kernel-dev@...lia.com, linux-sctp@...r.kernel.org,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org
Subject: Re: [PATCH] sctp: check transport existence before processing a
 send primitive

On Fri, Apr 04 2025 at 10:22:38, Xin Long <lucien.xin@...il.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