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

Stephen Smalley
2015-01-22T19:05:47Z

On Wed, Jan 21, 2015 at 9:44 PM, Ethan Zhao <ethan.kernel@gmail.com> wrote:
> On Wed, Jan 21, 2015 at 1:30 PM, Manfred Spraul
> <manfred@colorfullife.com> wrote:
>> On 01/21/2015 04:53 AM, Ethan Zhao wrote:
>>>
>>> On Tue, Jan 20, 2015 at 10:10 PM, Stephen Smalley <sds@tycho.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. But did you reproduce
the bug on that kernel version before the patch? If not, what kernel
version were you running when you triggered the bug?