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

Stephen Smalley
2015-01-20T21:03:11Z

On 01/20/2015 01:49 PM, Manfred Spraul wrote:
> Hi,
>
> On 01/20/2015 03:10 PM, Stephen Smalley wrote:
>> On 01/20/2015 04:18 AM, Ethan Zhao wrote:
>>> A NULL pointer dereference was observed as following panic:
>>>
>>> BUG: unable to handle kernel NULL pointer dereference at (null)
>>> IP: [<ffffffff812735eb>] ipc_has_perm+0x4b/0x60
>>> ...
>>> Process opcmon (pid: 30712, threadinfo ffff880237f2a000,
>>> task ffff88022ac70e40)
>>> Stack:
>>> ffff880237f2bc04 ffffffff01020953 ffff880237f2bce8
>>> ffffffff8125818e
>>> 0000000000000001 0000000037f78004 ffff880237f2bcd8
>>> ffffffff81273619
>>> ffff880237f2bce8 ffffffff8126e3e6 ffff880237f2bf68
>>> ffffffff8125c206
>>> Call Trace:
>>> [<ffffffff8125818e>] ? ipcperms+0xae/0x110
>>> [<ffffffff81273619>] selinux_sem_semop+0x19/0x20
>>> [<ffffffff8126e3e6>] security_sem_semop+0x16/0x20
>>> [<ffffffff8125c206>] sys_semtimedop+0x346/0x750
>>> [<ffffffff81188c0c>] ? handle_pte_fault+0x1dc/0x200
>>> [<ffffffff8161d830>] ? __do_page_fault+0x280/0x500
>>> [<ffffffff810d97d0>] ? __lock_release+0x90/0x1b0
>>> [<ffffffff8161d830>] ? __do_page_fault+0x280/0x500
>>> [<ffffffff8109a763>] ? up_read+0x23/0x40
>>> [<ffffffff8161d830>] ? __do_page_fault+0x280/0x500
>>> [<ffffffff81182f1c>] ? might_fault+0x5c/0xb0
>>> [<ffffffff81081f96>] ? sys_newuname+0x66/0xf0
>>> [<ffffffff810d97d0>] ? __lock_release+0x90/0x1b0
>>> [<ffffffff81081f96>] ? sys_newuname+0x66/0xf0
>>> [<ffffffff81622f45>] ? sysret_check+0x22/0x5d
>>> [<ffffffff8125c620>] sys_semop+0x10/0x20
>>> [<ffffffff81622f19>] system_call_fastpath+0x16/0x1b
>>> Code: b8 00 00 48 8b 80 48 06 00 00 41 8b 54 24 40 4c 8d
>>> 45 d0 89 d9 45 31 c9 48 8b 40 70 8b 78 04 49 8b 44 24 60 c6 45 d0 04
>>> 89 55 d8
>>> <0f> b7 10 8b 70 04 e8 0a dc ff ff 48 83 c4 20 5b 41 5c c9 c3 90
>>> RIP [<ffffffff812735eb>] ipc_has_perm+0x4b/0x60
>>> RSP <ffff880237f2bc98>
>>> CR2: 0000000000000000
>>>
>>> The root cause is semtimedop() was called after semget() without
>>> checking its
>>> return value in process opcmon. and semget() failed to check
>>> permission in
>>> function avc_has_perm() then sem_perm->security was freed shown as
>>> following:
>>>
>>> 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?
> My only idea would be a race of semtimedop() with IPC_RMID:
> If a rcu grace period happens between sem_obtain_object_check() and the
> ipc_has_perm() call, the the observed NULL pointer assignment would happen.

We only free and clear the ipc_perms->security field on a failure during
newary() -> security_sem_alloc(), in which case we fail with an error
before the ipc_addid() call has occurred, or during sem_rcu_free() ->
security_sem_free() just prior to calling ipc_rcu_free(). So I don't
see how ipc_perms->security can be NULL in ipc_has_perm(). We could rcu
free the ipc_perms->security field but I don't see why that would be
correct/necessary.