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

Stephen Smalley
2015-01-20T21:10:26Z

On 01/20/2015 04:06 PM, Eric Paris wrote:
> What kernel version was this? Didn't we have this problem and solve it
> upstream some time ago? IPC could be allocated with a valid security
> context, the ipc would be freed. the isec was free'd syncronously, but
> then the ipc could stick around until some rcu period or some usage flag
> got to 0, then it got freed...
>
> this seems so familiar, but it was a while ago

Don't know the kernel version.

Are you thinking of the inode security bug? Because in that case the
vfs changed underneath us to rcu free the inode but left the
security_inode_free() call outside of the rcu callback (due to the rcu
callback only being used in the default case where the filesystem does
not implement its own destroy_inode() method). In this case, we don't
seem to have that situation AFAICS.

>
> On Tue, 2015-01-20 at 16:01 -0500, Stephen Smalley wrote:
>> 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.
>>
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