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Date:	Fri, 11 Dec 2015 11:51:21 -0200
From:	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	Vlad Yasevich <vyasevich@...il.com>,
	netdev <netdev@...r.kernel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	syzkaller <syzkaller@...glegroups.com>,
	linux-sctp@...r.kernel.org, Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: use-after-free in sctp_do_sm

Em 11-12-2015 11:35, Dmitry Vyukov escreveu:
> On Wed, Dec 9, 2015 at 5:41 PM, Marcelo Ricardo Leitner
> <marcelo.leitner@...il.com> wrote:
>> On Wed, Dec 09, 2015 at 01:03:56PM -0200, Marcelo Ricardo Leitner wrote:
>>> On Wed, Dec 09, 2015 at 03:41:29PM +0100, Dmitry Vyukov wrote:
>>>> On Tue, Dec 8, 2015 at 8:22 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
>>>>> On Tue, Dec 8, 2015 at 6:40 PM, Marcelo Ricardo Leitner
>>>>> <marcelo.leitner@...il.com> wrote:
>>> ...
>>>>>> The patches were combined already, but this last pick by Vlad is just
>>>>>> not yet patched. It's not necessary for your testing and I didn't want
>>>>>> to interrupt it in case you were already testing it.
>>>>>>
>>>>>> You can use my last patch here, from 2 emails ago, the one which
>>>>>> contains this line:
>>>>>> -       case SCTP_DISPOSITION_ABORT:
>>>>>
>>>>>
>>>>> You are right. I missed that they are combined. Testing with it now.
>>>>
>>>>
>>>>
>>>>
>>>> Use-after-free still happens.
>>>> I am on commit aa53685549a2cfb5f175b0c4a20bc9aa1e5a1b85 (Dec 8) plus
>>>> the following sctp-related changes:
>>>
>>> Changes are fine.  Ugh. Ok, I'll try your new reproducer here.
>>
>> Heh I wasn't going to reproduce this by myself anytime soon, I think.
>> It's using the same socket to connect to itself, and only happens if the
>> connect() gets there before the listen() call. Figured this out because
>> I could only reproduce it under strace at first.
>>
>> Please give this other patch a try. A state command
>> (sctp_sf_cookie_wait_prm_abort) was issuing SCTP_CMD_INIT_FAILED, which
>> leads to SCTP_CMD_DELETE_TCB, but returning SCTP_DISPOSITION_CONSUME,
>> which fooled the patch.
>>
>> ---8<---
>> commit 9f84d50e36cee0ce66e4ce9b3b1665e0a1dbcdd3
>> Author: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
>> Date:   Fri Dec 4 15:30:23 2015 -0200
>>
>>      sctp: fix use-after-free in pr_debug statement
>>
>>      Dmitry Vyukov reported a use-after-free in the code expanded by the
>>      macro debug_post_sfx, which is caused by the use of the asoc pointer
>>      after it was freed within sctp_side_effect() scope.
>>
>>      This patch fixes it by allowing sctp_side_effect to clear that asoc
>>      pointer when the TCB is freed.
>>
>>      As Vlad explained, we also have to cover the SCTP_DISPOSITION_ABORT case
>>      because it will trigger DELETE_TCB too on that same loop.
>>
>>      Also, there was a place issuing SCTP_CMD_INIT_FAILED but returning
>>      SCTP_DISPOSITION_CONSUME, which would fool the scheme above. Fix it by
>>      returning SCTP_DISPOSITION_ABORT instead.
>>
>>      The macro is already prepared to handle such NULL pointer.
>>
>>      Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
>>
>> diff --git a/net/sctp/sm_sideeffect.c b/net/sctp/sm_sideeffect.c
>> index 6098d4c42fa9..be23d5c2074f 100644
>> --- a/net/sctp/sm_sideeffect.c
>> +++ b/net/sctp/sm_sideeffect.c
>> @@ -63,7 +63,7 @@ static int sctp_cmd_interpreter(sctp_event_t event_type,
>>   static int sctp_side_effects(sctp_event_t event_type, sctp_subtype_t subtype,
>>                               sctp_state_t state,
>>                               struct sctp_endpoint *ep,
>> -                            struct sctp_association *asoc,
>> +                            struct sctp_association **asoc,
>>                               void *event_arg,
>>                               sctp_disposition_t status,
>>                               sctp_cmd_seq_t *commands,
>> @@ -1123,7 +1123,7 @@ int sctp_do_sm(struct net *net, sctp_event_t event_type, sctp_subtype_t subtype,
>>          debug_post_sfn();
>>
>>          error = sctp_side_effects(event_type, subtype, state,
>> -                                 ep, asoc, event_arg, status,
>> +                                 ep, &asoc, event_arg, status,
>>                                    &commands, gfp);
>>          debug_post_sfx();
>>
>> @@ -1136,7 +1136,7 @@ int sctp_do_sm(struct net *net, sctp_event_t event_type, sctp_subtype_t subtype,
>>   static int sctp_side_effects(sctp_event_t event_type, sctp_subtype_t subtype,
>>                               sctp_state_t state,
>>                               struct sctp_endpoint *ep,
>> -                            struct sctp_association *asoc,
>> +                            struct sctp_association **asoc,
>>                               void *event_arg,
>>                               sctp_disposition_t status,
>>                               sctp_cmd_seq_t *commands,
>> @@ -1151,7 +1151,7 @@ static int sctp_side_effects(sctp_event_t event_type, sctp_subtype_t subtype,
>>           * disposition SCTP_DISPOSITION_CONSUME.
>>           */
>>          if (0 != (error = sctp_cmd_interpreter(event_type, subtype, state,
>> -                                              ep, asoc,
>> +                                              ep, *asoc,
>>                                                 event_arg, status,
>>                                                 commands, gfp)))
>>                  goto bail;
>> @@ -1174,11 +1174,12 @@ static int sctp_side_effects(sctp_event_t event_type, sctp_subtype_t subtype,
>>                  break;
>>
>>          case SCTP_DISPOSITION_DELETE_TCB:
>> +       case SCTP_DISPOSITION_ABORT:
>>                  /* This should now be a command. */
>> +               *asoc = NULL;
>>                  break;
>>
>>          case SCTP_DISPOSITION_CONSUME:
>> -       case SCTP_DISPOSITION_ABORT:
>>                  /*
>>                   * We should no longer have much work to do here as the
>>                   * real work has been done as explicit commands above.
>> diff --git a/net/sctp/sm_statefuns.c b/net/sctp/sm_statefuns.c
>> index 6f46aa16cb76..d801e151498a 100644
>> --- a/net/sctp/sm_statefuns.c
>> +++ b/net/sctp/sm_statefuns.c
>> @@ -4959,12 +4959,10 @@ sctp_disposition_t sctp_sf_cookie_wait_prm_abort(
>>          sctp_cmd_seq_t *commands)
>>   {
>>          struct sctp_chunk *abort = arg;
>> -       sctp_disposition_t retval;
>>
>>          /* Stop T1-init timer */
>>          sctp_add_cmd_sf(commands, SCTP_CMD_TIMER_STOP,
>>                          SCTP_TO(SCTP_EVENT_TIMEOUT_T1_INIT));
>> -       retval = SCTP_DISPOSITION_CONSUME;
>>
>>          sctp_add_cmd_sf(commands, SCTP_CMD_REPLY, SCTP_CHUNK(abort));
>>
>> @@ -4983,7 +4981,7 @@ sctp_disposition_t sctp_sf_cookie_wait_prm_abort(
>>          sctp_add_cmd_sf(commands, SCTP_CMD_INIT_FAILED,
>>                          SCTP_PERR(SCTP_ERROR_USER_ABORT));
>>
>> -       return retval;
>> +       return SCTP_DISPOSITION_ABORT;
>>   }
>>
>>   /*
>
>
> Still happens...
> I am on commit aa53685549a2cfb5f175b0c4a20bc9aa1e5a1b85 with your
> latest patch applied.
> Can you figure out what happens now from the report below? If not I
> can create a repro, it's just somewhat time consuming.

I can imagine. I don't know how this fuzzer works, but it would be nice 
if this reproducer extractor could be executed easier. So far, we have 
identified 3 different issues already leading to this bug:
- 1st, the handling on DELETE_TCB
- 2nd, the handling on DISPOSITION_ABORT
- 3rd, the bad combination on internal state-machine command to a return 
value

I can and will review it again, but it's doing nasty stuff like using 
the same socket to connect to itself. It's hard to imagine all those 
combinations in mind that might lead to that use-after-free.

Keep you posted.. thanks.

   Marcelo

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