Subject: Re: [PATCH net-next 1/4] net: sctp: sctp_seq_dump_local_addrs: throw BUG if primary_path is NULL

From: Daniel Borkmann <[email-masked]>
Date: Fri, 14 Jun 2013 16:51:18 +0200

On 06/14/2013 04:33 PM, Vlad Yasevich wrote:
> On 06/13/2013 12:04 PM, Daniel Borkmann wrote:
>> This clearly states a BUG somewhere in the SCTP code as e.g. fixed once
>> in f28156335 ("sctp: Use correct sideffect command in duplicate cookie
>> handling"). If this ever comes up again, throw a BUG and add a comment
>> why this is the case since it is not too obvious when primary != NULL
>> test passes and at a later point in time triggering a NULL ptr dereference
>> caused by primary. While at it, also fix up the white space.
>>
>> Signed-off-by: Daniel Borkmann <[email-masked]>
>> ---
>>   net/sctp/proc.c | 15 ++++++++++++---
>>   1 file changed, 12 insertions(+), 3 deletions(-)
>>
>> diff --git a/net/sctp/proc.c b/net/sctp/proc.c
>> index 4e45ee3..f171366 100644
>> --- a/net/sctp/proc.c
>> +++ b/net/sctp/proc.c
>> @@ -134,9 +134,18 @@ static void sctp_seq_dump_local_addrs(struct seq_file *seq, struct sctp_ep_commo
>>       struct sctp_af *af;
>>
>>       if (epb->type == SCTP_EP_TYPE_ASSOCIATION) {
>> -        asoc = sctp_assoc(epb);
>> -        peer = asoc->peer.primary_path;
>> -        primary = &peer->saddr;
>> +        asoc = sctp_assoc(epb);
>> +        peer = asoc->peer.primary_path;
>> +
>> +        /* There must be no such case where an association is linked
>> +         * into sctp_assoc_hashtable that does not have a primary
>> +         * path! This means either sctp_association_free() was called
>> +         * without sctp_unhash_established(), or somewhere in the
>> +         * interpreter SCTP_CMD_ASOC_NEW was called on a non-fully
>> +         * set up association. So do hara-kiri until this is fixed.
>> +         */
>> +        BUG_ON(peer == NULL);
>> +        primary = &peer->saddr;
>
> I am still trying to convince myself whether this BUG_ON() is the right thing to do...
>
> The fact that we reached this association may not necessarily help in diagnosing how we managed that and what might be going on.  Also crashing the system just because someone read /proc is a bit of an
> overkill, especially considering that the system might have stayed up
> if the user did not read /proc.

Well, but this patch actually makes no difference at all. Even if this BUG_ON would
not be there, then the next thing that happens is a NULL ptr dereference in cmp_addr()
on the primary pointer, right as in the stack trace I've sent with the recent patch.

So we might as well tell the user why this is happening before he debugs on this code
for quite a while.

> One thought I had was to change the above into something like this:
>      if (peer == NULL) {
>          WARN(1, "Association %p with NULL primary path", asoc);
>          return;
>      }

Ok, this could be an alternative. It would suck to just have a crash because we want
to print out an asterisk character. :-)

I'm not sure about the side effects if we leave that association in the list and just
warn, maybe it will also trigger a crash sooner or later, but sure, we could do this
like that.

> And add the following to handler for SCTP_CMD_NEW_ASOC and may be also to sctp_cmd_delete_tcb()
>
>      BUG_ON(asoc->peer.primary_path == NULL);
>
> This way, we would bug on additional and removal paths which have the
> possibility of giving us a lot more information about why the condition
> occurred in the first place.

Agreed, sounds good to me. Then let me resubmit the set with the proposed change.

Cheers,

Daniel