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

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Date:	Thu, 22 Jan 2015 19:15:55 +0100
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Ethan Zhao <ethan.kernel@...il.com>
CC:	Stephen Smalley <sds@...ho.nsa.gov>,
	Ethan Zhao <ethan.zhao@...cle.com>, james.l.morris@...cle.com,
	serge@...lyn.com, Eric Paris <eparis@...isplace.org>,
	Paul Moore <paul@...l-moore.com>, selinux@...ho.nsa.gov,
	linux-security-module@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Selinux/hooks.c: Fix a NULL pointer dereference caused
 by semop()

On 01/22/2015 03:44 AM, Ethan Zhao 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.
Is the issue reproducable?
If yes, can you try something like the attached patch?
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -5129,6 +5129,8 @@ static int ipc_has_perm(struct kern_ipc_perm
> *ipc_perms,
>         u32 sid = current_sid();
>
>         isec = ipc_perms->security;
> +     if (!isec)
> +             return -EACCES;
>
>         ad.type = LSM_AUDIT_DATA_IPC;
>         ad.u.ipc_id = ipc_perms->key;
ipc_has_perm() runs without any spinlocks/semaphores, only rcu_read_lock().
Testing for ipc_perms->security!=NULL does solve the issue that 
ipc_perm->key could be an access to kfree'd memory: Nothing prevents 
that the kfree could happen just after the test.

I.e.: The patch can't be the right solution.

--
     Manfred

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