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

[ethan zhao]

Stephen,

On 2015/1/23 3:05, Stephen Smalley wrote:
> On Wed, Jan 21, 2015 at 9:44 PM, Ethan Zhao <ethan.kernel@xxxxxxxxx> wrote:
> > On Wed, Jan 21, 2015 at 1:30 PM, Manfred Spraul
> > <manfred@xxxxxxxxxxxxxxxx> wrote:
> > > On 01/21/2015 04:53 AM, Ethan Zhao wrote:
> > > > On Tue, Jan 20, 2015 at 10:10 PM, Stephen Smalley <sds@xxxxxxxxxxxxx>
> > > > 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


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