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

[Casey Schaufler <casey@schaufler-ca.com>]

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/be3ef517730d92fc4530e0e952eae4f6cb0f07b4bc32
>> 6cb07495ca08fc9ddb66/merged type overlay
>> (rw,relatime,context="system_u:object_r:svirt_sandbox_file_t:s0:c414,
>> c873",lowerdir=/var/lib/docker/overlay2/l/Z4U7WY6ASNV5CFWLADPARHHWY7:
>> /var/lib/docker/overlay2/l/V2S3HOKEFEOQLHBVAL5WLA3YLS:/var/lib/docker
>> /overlay2/l/46YGYO474KLOULZGDSZDW2JPRI,upperdir=/var/lib/docker/overl
>> ay2/be3ef517730d92fc4530e0e952eae4f6cb0f07b4bc326cb07495ca08fc9ddb66/
>> diff,workdir=/var/lib/docker/overlay2/be3ef517730d92fc4530e0e952eae4f
>> 6cb0f07b4bc326cb07495ca08fc9ddb66/work)
>> shm on
>> /var/lib/docker/containers/9fd65e177d2132011d7b422755793449c91327ca57
>> 7b8f5d9d6a4adf218d4876/shm type tmpfs
>> (rw,nosuid,nodev,noexec,relatime,context="system_u:object_r:svirt_san
>> dbox_file_t:s0:c414,c873",size=65536k)
>> overlay on
>> /var/lib/docker/overlay2/38d1544d080145c7d76150530d0255991dfb7258cbca
>> 14ff6d165b94353eefab/merged type overlay
>> (rw,relatime,context="system_u:object_r:svirt_sandbox_file_t:s0:c431,
>> c651",lowerdir=/var/lib/docker/overlay2/l/3MQQXB4UCLFB7ANVRHPAVRCRSS:
>> /var/lib/docker/overlay2/l/46YGYO474KLOULZGDSZDW2JPRI,upperdir=/var/l
>> ib/docker/overlay2/38d1544d080145c7d76150530d0255991dfb7258cbca14ff6d
>> 165b94353eefab/diff,workdir=/var/lib/docker/overlay2/38d1544d080145c7
>> d76150530d0255991dfb7258cbca14ff6d165b94353eefab/work)
>> shm on
>> /var/lib/docker/containers/662e7f798fc08b09eae0f0f944537a4bcedc1dcf05
>> a65866458523ffd4a71614/shm type tmpfs
>> (rw,nosuid,nodev,noexec,relatime,context="system_u:object_r:svirt_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.

> 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.

> 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.