Re: [PATCH] general protection fault in sock_has_perm

[Paul Moore]

On Fri, Jan 19, 2018 at 10:49 AM, Mark Salyzyn <salyzyn@xxxxxxxxxxx> wrote:
> On 01/18/2018 02:36 PM, Paul Moore wrote:
>> On Thu, Jan 18, 2018 at 4:58 PM, Mark Salyzyn <salyzyn@xxxxxxxxxxx> wrote:
>>> general protection fault: 0000 [#1] PREEMPT SMP KASAN
>>> CPU: 1 PID: 14233 Comm: syz-executor2 Not tainted 4.4.112-g5f6325b #28
>>> . . .
>>> [<ffffffff81b6a19d>] selinux_socket_setsockopt+0x4d/0x80
>>> security/selinux/hooks.c:4338
>>> [<ffffffff81b4873d>] security_socket_setsockopt+0x7d/0xb0
>>> security/security.c:1257
>>> [<ffffffff82df1ac8>] SYSC_setsockopt net/socket.c:1757 [inline]
>>> [<ffffffff82df1ac8>] SyS_setsockopt+0xe8/0x250 net/socket.c:1746
>>> [<ffffffff83776499>] entry_SYSCALL_64_fastpath+0x16/0x92
>>> Code: c2 42 9b b6 81 be 01 00 00 00 48 c7 c7 a0 cb 2b 84 e8
>>> f7 2f 6d ff 49 8d 7d 10 48 b8 00 00 00 00 00 fc ff df 48 89
>>> fa 48 c1 ea 03 <0f> b6 04 02 84 c0 74 08 3c 03 0f 8e 83 01 00
>>> 00 41 8b 75 10 31
>>> RIP  [<ffffffff81b69b7e>] sock_has_perm+0x1fe/0x3e0
>>> security/selinux/hooks.c:4069
>>> RSP <ffff8800b5957ce0>
>>> ---[ end trace 7b5aaf788fef6174 ]---
>>>
>>> In the absence of commit a4298e4522d6 ("net: add SOCK_RCU_FREE socket
>>> flag") and all the associated infrastructure changes to take advantage
>>> of a RCU grace period before freeing, there is a heightened
>>> possibility that a security check is performed while an ill-timed
>>> setsockopt call races in from user space.  It then is prudent to null
>>> check sk_security, and if the case, reject the permissions.
>>>
>>> This adjustment is orthogonal to infrastructure improvements that may
>>> nullify the needed check, but should be added as good code hygiene.
>>
>> I'm skeptical that this is the full solution for systems that lack the
>> SOCK_RCU_FREE protection.  Is this really limited to just
>> setsockopt()?
>
> Maybe overstepped in my analysis and assumptions?
>
> This is a result of a fuzzer hitting an android 4.4 KASAN kernel. We (so
> far) have _not_ seen this with an android 4.9 KASAN kernel (which has the
> SOCK_RCU_FREE adjustments). There is no standalone duplication or PoC
> _except_ via the fuzzer. The rest of the statements stands based on this
> tidbit (statements of general good code hygiene, not 100% sure SOCK_RCU_FREE
> usage is completely covered, KISS solution etc).
>
> To be honest, yes, this may be a layer in the onion (swat this NULL check
> does not by itself solve the _problem_), I'd prefer kernel continuing on in
> a rational manner rather than panic ... and I have a gut feeling this could
> be a gratuitous NULL check if all the bugs in the network layer have been
> solved <that may be sarcasm, I can not tell>. Programming to solve a problem
> with one's gut is not a good practice, but hygiene is. This is 10
> characters, and an estimated 1.2ns of added hygiene.
>
> No, I do not think this is limited to setsockopt, but would be willing to
> believe a multithreaded attack of any socket functions or ioctl would drop
> down to the check with sock_has_perm at possibly the wrong time in socket
> teardown.

I'm not necessarily opposed to adding additional safety checks, if
warranted, but I am opposed to adding a single check and declaring
mission accomplished when there is a suspicion that additional checks
may be needed.

Perhaps in this particular case it really is only setsockopt(), but
from what I can tell from your comments and the SOCK_RCU_FREE commit
message it would appear that there is a race condition here between a
socket's lifetime and its visibility to userspace.  Assuming this is
the core problem you are trying to workaround with this patch, I
suspect that more than just the SELinux/LSM hook for setsockopt() is
affected; before merging this patch I would like to see a better
investigation into all the socket related SELinux/LSM hooks to see if
they suffer from the same problem.

For example, if we stick with the setsockopt() syscall and the
SELinux/LSM hook we can see that there are two functions which access
the socket struct: sock_has_perm() and
selinux_netlbl_socket_setsockopt().  While you did add the safety
check to sock_has_perm() you neglected to add a check to
selinux_netlbl_socket_setsockopt(), I'm guessing because the Android
kernels probably do not enable CONFIG_NETLABEL.  Looking beyond
setsockopt() into other socket related syscalls I see a number of
hooks which should have similar protections.

When you see problems like this, please do the investigation to make
sure you are fixing everything like it and not just the one instance
that blew up.

-- 
paul moore
www.paul-moore.com