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

From: Paul Moore <paul@paul-moore.com>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: syzbot 
	<bot+904436b33e141b4f4c57c1ddc94199ffd2e34b9d@syzkaller.appspotmail.com>,
	elfring@users.sourceforge.net, Eric Paris <eparis@parisplace.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	James Morris <james.l.morris@oracle.com>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-security-module@vger.kernel.org,
	Philippe Ombredanne <pombredanne@nexb.com>,
	Stephen Smalley <sds@tycho.nsa.gov>,
	selinux@tycho.nsa.gov, serge@hallyn.com,
	syzkaller-bugs@googlegroups.com,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: BUG: unable to handle kernel NULL pointer dereference in sidtab_search_core
Date: Fri, 22 Dec 2017 11:03:29 -0500	[thread overview]
Message-ID: <CAHC9VhSB74i5Teczp-yMk7rWkOBGGFXPQM455iyLcFu33ar5iA@mail.gmail.com> (raw)
In-Reply-To: <CACT4Y+bq3DLE1Pv9cP5cM4+m8UO6aovRMbqZ-SdvLv7b=AFmyg@mail.gmail.com>

On Fri, Dec 22, 2017 at 10:48 AM, Dmitry Vyukov <dvyukov@google.com> wrote:
> On Fri, Dec 22, 2017 at 3:03 PM, Paul Moore <paul@paul-moore.com> wrote:
>> On Fri, Dec 22, 2017 at 4:14 AM, Dmitry Vyukov <dvyukov@google.com> wrote:
>>> On Thu, Dec 21, 2017 at 9:40 PM, Paul Moore <paul@paul-moore.com> wrote:
>>>> On Wed, Dec 20, 2017 at 2:48 AM, syzbot
>>>> <bot+904436b33e141b4f4c57c1ddc94199ffd2e34b9d@syzkaller.appspotmail.com>
>>>> 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.

-- 
paul moore
www.paul-moore.com