linux-kernel - Re: [PATCH] Selinux/hooks.c: Fix a NULL pointer dereference caused by semop()

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Date:	Fri, 23 Jan 2015 10:19:14 +0800
From:	ethan zhao <ethan.zhao@...cle.com>
To:	Stephen Smalley <stephen.smalley@...il.com>
CC:	Ethan Zhao <ethan.kernel@...il.com>,
	Manfred Spraul <manfred@...orfullife.com>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	James Morris <james.l.morris@...cle.com>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Eric Paris <eparis@...isplace.org>,
	Paul Moore <paul@...l-moore.com>,
	selinux <selinux@...ho.nsa.gov>,
	linux-security-module@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>, ethan.kernel@...il.conm
Subject: Re: [PATCH] Selinux/hooks.c: Fix a NULL pointer dereference caused
 by semop()

Stephen,

On 2015/1/23 3:05, Stephen Smalley wrote:
> On Wed, Jan 21, 2015 at 9:44 PM, Ethan Zhao <ethan.kernel@...il.com> wrote:
>> On Wed, Jan 21, 2015 at 1:30 PM, Manfred Spraul
>> <manfred@...orfullife.com> wrote:
>>> On 01/21/2015 04:53 AM, Ethan Zhao wrote:
>>>> On Tue, Jan 20, 2015 at 10:10 PM, Stephen Smalley <sds@...ho.nsa.gov>
>>>> wrote:
>>>>> On 01/20/2015 04:18 AM, Ethan Zhao wrote:
>>>>>>        sys_semget()
>>>>>>        ->newary()
>>>>>>            ->security_sem_alloc()
>>>>>>              ->sem_alloc_security()
>>>>>>                    selinux_sem_alloc_security()
>>>>>>                    ->ipc_alloc_security() {
>>>>>>                      ->rc = avc_has_perm()
>>>>>>                                 if (rc) {
>>>>>>
>>>>>> ipc_free_security(&sma->sem_perm);
>>>>>>                                         return rc;
>>>>> We free the security structure here to avoid a memory leak on a
>>>>> failed/denied semaphore set creation.  In this situation, we return an
>>>>> error to the caller (ultimately to newary), it does an
>>>>> ipc_rcu_putref(sma, ipc_rcu_free), and it returns an error to the
>>>>> caller.  Thus, it never calls ipc_addid() and the semaphore set is not
>>>>> created.  So how then can you call semtimedop() on it?
>>>> Seems it wouldn't happen after commit
>>>> e8577d1f0329d4842e8302e289fb2c22156abef4 ?
>>> That was my first guess when I read the bug report - but it can't be the
>>> fix, because security_sem_alloc() is before the ipc_addid(), with or without
>>> the patch.
>>>
>>> thread A:
>>>              thread B:
>>>
>>> semtimedop()
>>> -> sem_obtain_object_check()
>>>              semctl(IPC_RMID)
>>>              -> freeary()
>>>              -> ipc_rcu_putref()
>>>              -> call_rcu()
>>> -> somehow a grace period
>>>              -> sem_rcu_free()
>>>              -> security_sem_free()
>>>
>>> Perhaps: modify ipc_free_security() to hexdump perm and a few more bytes if
>>> the pointer is NULL?
>> I tried to ask for vmcore and do more analysis, basically, the race condition
>> still exists and open a hole to be DoS.
> You said the patch was tested with 3.19-rc5.
  I just threw the 3.19-rc5 with my test patch to the 'user', he said he
  doesn't hit. maybe he didn't hit or occasionally failed to reproduce it.
>   But did you reproduce
> the bug on that kernel version before the patch?
  Good news is not hit yet.

>   If not, what kernel
> version were you running when you triggered the bug?
  To be honest, a kernel from distro, but not released, but before we 
get it clear, we wouldn't public more.

  Thanks,
  Ethan


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