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

ethan zhao
2015-01-23T02:19:45Z

Stephen,

On 2015/1/23 3:05, Stephen Smalley wrote:
> 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.
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