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

ethan zhao
2015-01-23T02:00:41Z

Manfred,

On 2015/1/23 2:15, Manfred Spraul wrote:
> On 01/22/2015 03:44 AM, Ethan Zhao 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.
> Is the issue reproducable?
It was hit on an user's VMware while running a process named "opcmon"
maybe ported from HP_UX. I asked for the vmwcore, but not get it yet.

> If yes, can you try something like the attached patch?
Yes, will.

Thanks,
Ethan
>
>> --- 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