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

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Date:	Wed, 21 Jan 2015 06:30:25 +0100
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Ethan Zhao <ethan.kernel@...il.com>,
	Stephen Smalley <sds@...ho.nsa.gov>
CC:	Ethan Zhao <ethan.zhao@...cle.com>, james.l.morris@...cle.com,
	serge@...lyn.com, eparis@...isplace.org, paul@...l-moore.com,
	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()

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?

--
     Manfred

> Thanks,
> Ethan
>>>       So ipc_perms->security was NULL, then semtimedop() was called as
>>>       following:
>>>
>>>          sys_semtimedop() / semop()
>>>          ->selinux_sem_semop()
>>>           ->ipc_has_perm()
>>>             ->avc_has_perm(sid, isec->sid, isec->sclass, perms, &ad);
>>>                                            ^- NULL pointer dereference happens
>>>
>>> The test kernel was running on VMware.
>>> This patch use to fix this serious security issue could be triggered by user space.
>>> This patch was tested with v3.19-rc5.
>>>
>>> Signed-off-by: Ethan Zhao <ethan.zhao@...cle.com>
>>> ---
>>>   security/selinux/hooks.c | 2 ++
>>>   1 file changed, 2 insertions(+)
>>>
>>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
>>> index 6da7532..bbe76f5 100644
>>> --- 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;
>>>
>> That is not the correct fix; it just hides a bug.  If we reach
>> ipc_has_perm() with a NULL isec, it is a bug in the ipc code.
>>
>> --
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