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

[Dmitry Vyukov]

On Fri, Dec 22, 2017 at 5:03 PM, Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
>>>>> 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.
>>
>> Can you please point me to some of these bugs? I don't see anything
>> like this in my inbox, in google group nor in database.
>
> Not easily, no.  I don't keep track of these reports once I've
> responded to the syzbot mail.


There must be traces of this in database and on mailing lists (even if
you drop syzkaller-bugs@ syzbot will re-add it). So far I did not find
any traces...