Re: BUG: unable to handle kernel NULL pointer dereference in sidtab_search_core

[Paul Moore]

On Fri, Dec 22, 2017 at 4:14 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> On Thu, Dec 21, 2017 at 9:40 PM, Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
>> On Wed, Dec 20, 2017 at 2:48 AM, syzbot
>> <bot+904436b33e141b4f4c57c1ddc94199ffd2e34b9d@xxxxxxxxxxxxxxxxxxxxxxxxx>
>> wrote:
>>> Hello,
>>>
>>> syzkaller hit the following crash on
>>> 6084b576dca2e898f5c101baef151f7bfdbb606d
>>> git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/master
>>> compiler: gcc (GCC) 7.1.1 20170620
>>> .config is attached
>>> Raw console output is attached.
>>> C reproducer is attached
>>> syzkaller reproducer is attached. See https://goo.gl/kgGztJ
>>> for information about syzkaller reproducers
>>>
>>>
>>> SELinux: security_compute_sid:  unrecognized SID 1
>>> SELinux: security_compute_sid:  unrecognized SID 1
>>> SELinux: security_compute_sid:  unrecognized SID 1
>>> SELinux: security_compute_sid:  unrecognized SID 1
>>> SELinux: security_compute_sid:  unrecognized SID 1
>>> BUG: unable to handle kernel NULL pointer dereference at 0000000000000001
>>> IP: sidtab_search_core+0x88/0x110 security/selinux/ss/sidtab.c:100
>>> PGD 0 P4D 0
>>> Oops: 0000 [#1] SMP
>>> Dumping ftrace buffer:
>>>    (ftrace buffer empty)
>>> Modules linked in:
>>> CPU: 1 PID: 4252 Comm: kworker/u4:1 Not tainted 4.15.0-rc3-next-20171214+
>>> #67
>>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
>>> Google 01/01/2011
>>> RIP: 0010:sidtab_search_core+0x88/0x110 security/selinux/ss/sidtab.c:100
>>> RSP: 0018:ffffc900028abc18 EFLAGS: 00010293
>>> RAX: ffff8802131a87c0 RBX: 0000000000000001 RCX: ffffffff8165d978
>>> RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffffffff83fd17a0
>>> RBP: ffffc900028abc40 R08: 0000000000000001 R09: 0000000000000001
>>> R10: ffffc900028abbe0 R11: 0000000000000000 R12: 0000000000000001
>>> R13: 0000000000000001 R14: 0000000000000000 R15: ffff880214d93800
>>> FS:  0000000000000000(0000) GS:ffff88021fd00000(0000) knlGS:0000000000000000
>>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>> CR2: 0000000000000001 CR3: 0000000214e31000 CR4: 00000000001406e0
>>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>>> Call Trace:
>>>  sidtab_search+0x1f/0x30 security/selinux/ss/sidtab.c:111
>>>  security_compute_sid.part.11+0xe2/0x710 security/selinux/ss/services.c:1618
>>>  security_compute_sid+0x92/0xa0 security/selinux/ss/services.c:1598
>>>  security_transition_sid+0x57/0x70 security/selinux/ss/services.c:1764
>>>  selinux_bprm_set_creds+0x215/0x2f0 security/selinux/hooks.c:2423
>>>  security_bprm_set_creds+0x41/0x60 security/security.c:332
>>>  prepare_binprm+0xae/0x1f0 fs/exec.c:1561
>>>  do_execveat_common.isra.30+0x6f7/0xb90 fs/exec.c:1784
>>>  do_execve+0x31/0x40 fs/exec.c:1848
>>>  call_usermodehelper_exec_async+0x104/0x190 kernel/umh.c:100
>>>  ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:524
>>> Code: 8b 5b 50 48 85 db 75 e5 e8 e6 c9 c5 ff 49 8b 5f 18 48 85 db 75 10 eb
>>> 43 e8 d6 c9 c5 ff 48 8b 5b 50 48 85 db 74 35 e8 c8 c9 c5 ff <44> 8b 23 41 83
>>> fc 02 76 e4 e8 ba c9 c5 ff 41 83 fc 03 75 1c 48
>>> RIP: sidtab_search_core+0x88/0x110 security/selinux/ss/sidtab.c:100 RSP:
>>> ffffc900028abc18
>>> CR2: 0000000000000001
>>> ---[ end trace 571c0ea6c6959387 ]---
>>> Kernel panic - not syncing: Fatal exception
>>> Dumping ftrace buffer:
>>>    (ftrace buffer empty)
>>> Kernel Offset: disabled
>>> Rebooting in 86400 seconds..
>>
>> Based on the reproducer and the stack trace, I'm guessing the system
>> is attempting to load a kernel module for a a defined, but unloaded,
>> protocol.  Looking quickly at the SELinux bprm and sidtab code,
>> nothing obvious is jumping out at me.  Considering the number of false
>> positives I've been seeing from syzbot lately, I'm assuming this is
>> more of the same.
>
> Hi Paul,
>
> What are these false positives? Please elaborate.
> There is no single false positive that I am aware of. All the ones
> that were debugged are real kernel bugs.

I've replied to several of the syzbot automated reports with the
"invalid" response.  I haven't been keeping track, but it seems like
approximately 50% of the SELinux related reports don't make sense upon
inspection.

-- 
paul moore
www.paul-moore.com