çå: [BUG]kernel softlockup due to sidtab_search_context run for long time because of too many sidtab context node

[yangjihong]

On 12/15/2017 10:31 PM, yangjihong wrote:
>On 12/14/2017 12:42 PM, Casey Schaufler wrote:
>> On 12/14/2017 9:15 AM, Stephen Smalley wrote:
>>> On Thu, 2017-12-14 at 09:00 -0800, Casey Schaufler wrote:
>>>> On 12/14/2017 8:42 AM, Stephen Smalley wrote:
>>>>> On Thu, 2017-12-14 at 08:18 -0800, Casey Schaufler wrote:
>>>>>> On 12/13/2017 7:18 AM, Stephen Smalley wrote:
>>>>>>> On Wed, 2017-12-13 at 09:25 +0000, yangjihong wrote:
>>>>>>>> Hello,
>>>>>>>>
>>>>>>>> I am doing stressing testing on 3.10 kernel(centos 7.4), to 
>>>>>>>> constantly starting numbers of docker ontainers with selinux 
>>>>>>>> enabled, and after about 2 days, the kernel softlockup panic:
>>>>>>>>   <IRQ>  [<ffffffff810bb778>] sched_show_task+0xb8/0x120
>>>>>>>>   [<ffffffff8116133f>] show_lock_info+0x20f/0x3a0
>>>>>>>>   [<ffffffff811226aa>] watchdog_timer_fn+0x1da/0x2f0
>>>>>>>>   [<ffffffff811224d0>] ?
>>>>>>>> watchdog_enable_all_cpus.part.4+0x40/0x40
>>>>>>>>   [<ffffffff810abf82>] __hrtimer_run_queues+0xd2/0x260
>>>>>>>>   [<ffffffff810ac520>] hrtimer_interrupt+0xb0/0x1e0
>>>>>>>>   [<ffffffff8104a477>] local_apic_timer_interrupt+0x37/0x60
>>>>>>>>   [<ffffffff8166fd90>] smp_apic_timer_interrupt+0x50/0x140
>>>>>>>>   [<ffffffff8166e1dd>] apic_timer_interrupt+0x6d/0x80
>>>>>>>>   <EOI>  [<ffffffff812b4193>] ?
>>>>>>>> sidtab_context_to_sid+0xb3/0x480
>>>>>>>>   [<ffffffff812b41f0>] ? sidtab_context_to_sid+0x110/0x480
>>>>>>>>   [<ffffffff812c0d15>] ? mls_setup_user_range+0x145/0x250
>>>>>>>>   [<ffffffff812bd477>] security_get_user_sids+0x3f7/0x550
>>>>>>>>   [<ffffffff812b1a8b>] sel_write_user+0x12b/0x210
>>>>>>>>   [<ffffffff812b1960>] ? sel_write_member+0x200/0x200
>>>>>>>>   [<ffffffff812b01d8>] selinux_transaction_write+0x48/0x80
>>>>>>>>   [<ffffffff811f444d>] vfs_write+0xbd/0x1e0
>>>>>>>>   [<ffffffff811f4eef>] SyS_write+0x7f/0xe0
>>>>>>>>   [<ffffffff8166d433>] system_call_fastpath+0x16/0x1b
>>>>>>>>
>>>>>>>> My opinion:
>>>>>>>> when the docker container starts, it would mount overlay 
>>>>>>>> filesystem with different selinux context, mount point such as:
>>>>>>>> overlay on
>>>>>>>> /var/lib/docker/overlay2/be3ef517730d92fc4530e0e952eae4f6cb0f
>>>>>>>> 07b4
>>>>>>>> bc32
>>>>>>>> 6cb07495ca08fc9ddb66/merged type overlay
>>>>>>>> (rw,relatime,context="system_u:object_r:svirt_sandbox_file_t:
>>>>>>>> s0:c
>>>>>>>> 414,
>>>>>>>> c873",lowerdir=/var/lib/docker/overlay2/l/Z4U7WY6ASNV5CFWLADP
>>>>>>>> ARHH
>>>>>>>> WY7:
>>>>>>>> /var/lib/docker/overlay2/l/V2S3HOKEFEOQLHBVAL5WLA3YLS:/var/li
>>>>>>>> b/do
>>>>>>>> cker
>>>>>>>> /overlay2/l/46YGYO474KLOULZGDSZDW2JPRI,upperdir=/var/lib/dock
>>>>>>>> er/o
>>>>>>>> verl
>>>>>>>> ay2/be3ef517730d92fc4530e0e952eae4f6cb0f07b4bc326cb07495ca08f
>>>>>>>> c9dd
>>>>>>>> b66/
>>>>>>>> diff,workdir=/var/lib/docker/overlay2/be3ef517730d92fc4530e0e
>>>>>>>> 952e
>>>>>>>> ae4f
>>>>>>>> 6cb0f07b4bc326cb07495ca08fc9ddb66/work)
>>>>>>>> shm on
>>>>>>>> /var/lib/docker/containers/9fd65e177d2132011d7b422755793449c9
>>>>>>>> 1327
>>>>>>>> ca57
>>>>>>>> 7b8f5d9d6a4adf218d4876/shm type tmpfs 
>>>>>>>> (rw,nosuid,nodev,noexec,relatime,context="system_u:object_r:s
>>>>>>>> virt
>>>>>>>> _san
>>>>>>>> dbox_file_t:s0:c414,c873",size=65536k)
>>>>>>>> overlay on
>>>>>>>> /var/lib/docker/overlay2/38d1544d080145c7d76150530d0255991dfb
>>>>>>>> 7258
>>>>>>>> cbca
>>>>>>>> 14ff6d165b94353eefab/merged type overlay
>>>>>>>> (rw,relatime,context="system_u:object_r:svirt_sandbox_file_t:
>>>>>>>> s0:c
>>>>>>>> 431,
>>>>>>>> c651",lowerdir=/var/lib/docker/overlay2/l/3MQQXB4UCLFB7ANVRHP
>>>>>>>> AVRC
>>>>>>>> RSS:
>>>>>>>> /var/lib/docker/overlay2/l/46YGYO474KLOULZGDSZDW2JPRI,upperdi
>>>>>>>> r=/v
>>>>>>>> ar/l
>>>>>>>> ib/docker/overlay2/38d1544d080145c7d76150530d0255991dfb7258cb
>>>>>>>> ca14
>>>>>>>> ff6d
>>>>>>>> 165b94353eefab/diff,workdir=/var/lib/docker/overlay2/38d1544d
>>>>>>>> 0801
>>>>>>>> 45c7
>>>>>>>> d76150530d0255991dfb7258cbca14ff6d165b94353eefab/work)
>>>>>>>> shm on
>>>>>>>> /var/lib/docker/containers/662e7f798fc08b09eae0f0f944537a4bce
>>>>>>>> dc1d
>>>>>>>> cf05
>>>>>>>> a65866458523ffd4a71614/shm type tmpfs 
>>>>>>>> (rw,nosuid,nodev,noexec,relatime,context="system_u:object_r:s
>>>>>>>> virt
>>>>>>>> _san
>>>>>>>> dbox_file_t:s0:c431,c651",size=65536k)
>>>>>>>>
>>>>>>>> sidtab_search_context check the context whether is in the sidtab 
>>>>>>>> list, If not found, a new node is generated and insert into the 
>>>>>>>> list, As the number of containers is increasing,  context nodes 
>>>>>>>> are also more and more, we tested the final number of nodes 
>>>>>>>> reached
>>>>>>>> 300,000 +,
>>>>>>>> sidtab_context_to_sid runtime needs 100-200ms, which will lead 
>>>>>>>> to the system softlockup.
>>>>>>>>
>>>>>>>> Is this a selinux bug? When filesystem umount, why context node 
>>>>>>>> is not deleted?  I cannot find the relevant function to delete 
>>>>>>>> the node in sidtab.c
>>>>>>>>
>>>>>>>> Thanks for reading and looking forward to your reply.
>>>>>>> So, does docker just keep allocating a unique category set for 
>>>>>>> every new container, never reusing them even if the container is 
>>>>>>> destroyed?
>>>>>>> That would be a bug in docker IMHO.  Or are you creating an 
>>>>>>> unbounded number of containers and never destroying the older 
>>>>>>> ones?
>>>>>> You can't reuse the security context. A process in ContainerA 
>>>>>> sends a labeled packet to MachineB. ContainerA goes away and its 
>>>>>> context is recycled in ContainerC. MachineB responds some time 
>>>>>> later, again with a labeled packet. ContainerC gets information 
>>>>>> intended for ContainerA, and uses the information to take over the 
>>>>>> Elbonian government.
>>>>> Docker isn't using labeled networking (nor is anything else by 
>>>>> default; it is only enabled if explicitly configured).
>>>> If labeled networking weren't an issue we'd have full security 
>>>> module stacking by now. Yes, it's an edge case. If you want to use 
>>>> labeled NFS or a local filesystem that gets mounted in each 
>>>> container (don't tell me that nobody would do that) you've got the 
>>>> same problem.
>>> Even if someone were to configure labeled networking, Docker is not 
>>> presently relying on that or SELinux network enforcement for any 
>>> security properties, so it really doesn't matter.
>> True enough. I can imagine a use case, but as you point out, it would 
>> be a very complex configuration and coordination exercise using 
>> SELinux.
>>
>>> And if they wanted
>>> to do that, they'd have to coordinate category assignments across all 
>>> systems involved, for which no facility exists AFAIK.  If you have 
>>> two docker instances running on different hosts, I'd wager that they 
>>> can hand out the same category sets today to different containers.
>>>
>>> With respect to labeled NFS, that's also not the default for nfs 
>>> mounts, so again it is a custom configuration and Docker isn't 
>>> relying on it for any guarantees today.  For local filesystems, they 
>>> would normally be context-mounted or using genfscon rather than 
>>> xattrs in order to be accessible to the container, thus no persistent 
>>> storage of the category sets.
>Well Kubernetes and OpenShift do set the labels to be the same within a project, and they can manage across nodes.  But yes we are not using labeled networking at this point.
>> I know that is the intended configuration, but I see people do all 
>> sorts of stoopid things for what they believe are good reasons.
>> Unfortunately, lots of people count on containers to provide 
>> isolation, but create "solutions" for data sharing that defeat it.
>>
>>> Certainly docker could provide an option to not reuse category sets, 
>>> but making that the default is not sane and just guarantees 
>>> exhaustion of the SID and context space (just create and tear down 
>>> lots of containers every day or more frequently).
>> It seems that Docker might have a similar issue with UIDs, but it 
>> takes longer to run out of UIDs than sidtab entries.
>>
>>>>>>> On the selinux userspace side, we'd also like to eliminate the 
>>>>>>> use of /sys/fs/selinux/user (sel_write_user -> 
>>>>>>> security_get_user_sids) entirely, which is what triggered this 
>>>>>>> for you.
>>>>>>>
>>>>>>> We cannot currently delete a sidtab node because we have no way 
>>>>>>> of knowing if there are any lingering references to the SID.  
>>>>>>> Fixing that would require reference-counted SIDs, which goes 
>>>>>>> beyond just SELinux since SIDs/secids are returned by LSM hooks 
>>>>>>> and cached in other kernel data structures.
>>>>>> You could delete a sidtab node. The code already deals with 
>>>>>> unfindable SIDs. The issue is that eventually you run out of SIDs. 
>>>>>> Then you are forced to recycle SIDs, which leads to the overthrow 
>>>>>> of the Elbonian government.
>>>>> We don't know when we can safely delete a sidtab node since SIDs 
>>>>> aren't reference counted and we can't know whether it is still in 
>>>>> use somewhere in the kernel.  Doing so prematurely would lead to 
>>>>> the SID being remapped to the unlabeled context, and then likely to 
>>>>> undesired denials.
>>>> I would suggest that if you delete a sidtab node and someone comes 
>>>> along later and tries to use it that denial is exactly what you 
>>>> would desire. I don't see any other rational action.
>>> Yes, if we know that the SID wasn't in use at the time we tore it down.
>>>   But if we're just randomly deleting sidtab entries based on age or 
>>> something (since we have no reference count), we'll almost certainly 
>>> encounter situations where a SID hasn't been accessed in a long time 
>>> but is still being legitimately cached somewhere.  Just a file that 
>>> hasn't been accessed in a while might have that SID still cached in 
>>> its inode security blob, or anywhere else.
>>>
>>>>>>> sidtab_search_context() could no doubt be optimized for the 
>>>>>>> negative case; there was an earlier optimization for the positive 
>>>>>>> case by adding a cache to sidtab_context_to_sid() prior to 
>>>>>>> calling it.  It's a reverse lookup in the sidtab.
>>>>>> This seems like a bad idea.
>>>>> Not sure what you mean, but it can certainly be changed to at least 
>>>>> use a hash table for these reverse lookups.
>>>>>
>>>>>
>>
>>
>>
Thanks for reply and discussion.
I think docker container is only a case, Is it possible there is a similar way, through some means of attack, triggered a constantly increasing of  SIDs list, eventually leading to the system panic?

I think the issue is that is takes too long to search SID node when SIDs list too large, 
If can optimize the node's data structure(ie : tree structure) or search algorithm to ensure that traversing all nodes can be very short time even in many nodes, maybe it can solve the problem.
Or, in sidtab.c provides "delete_sidtab_node" interface, when umount fs, delete the SID node. Because when fs is umounted, the SID is useless, could delete it to control the size of SIDs list.

Thanks for reading and looking forward to your reply.

Best wishes!