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

[Manfred Spraul <manfred@colorfullife.com>]

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.

>>       So ipc_perms->security was NULL, then semtimedop() was called as
>>       following:
>>
>> 	  sys_semtimedop() / semop()
>> 	  ->selinux_sem_semop()
>> 	   ->ipc_has_perm()
>> 	     ->avc_has_perm(sid, isec->sid, isec->sclass, perms, &ad);
>>    	                                    ^- NULL pointer dereference happens
>>
>> The test kernel was running on VMware.
Can you give some more details?
- which RCU type?
- PREEMPT?

--
     Manfred