Re: [BUG]kernel softlockup due to sidtab_search_context run for long time because of too many sidtab context node
From: Daniel Walsh
Date: Fri Dec 15 2017 - 09:50:56 EST
On 12/15/2017 08:56 AM, Stephen Smalley wrote:
On Fri, 2017-12-15 at 03:09 +0000, yangjihong wrote:
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/be3ef517730d92fc4530e0e952ea
e4f6cb0f
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/Z4U7WY6ASNV
5CFWLADP
ARHH
WY7:
/var/lib/docker/overlay2/l/V2S3HOKEFEOQLHBVAL5WLA3YLS
:/var/li
b/do
cker
/overlay2/l/46YGYO474KLOULZGDSZDW2JPRI,upperdir=/var/
lib/dock
er/o
verl
ay2/be3ef517730d92fc4530e0e952eae4f6cb0f07b4bc326cb07
495ca08f
c9dd
b66/
diff,workdir=/var/lib/docker/overlay2/be3ef517730d92f
c4530e0e
952e
ae4f
6cb0f07b4bc326cb07495ca08fc9ddb66/work)
shm on
/var/lib/docker/containers/9fd65e177d2132011d7b422755
793449c9
1327
ca57
7b8f5d9d6a4adf218d4876/shm type tmpfs
(rw,nosuid,nodev,noexec,relatime,context="system_u:ob
ject_r:s
virt
_san
dbox_file_t:s0:c414,c873",size=65536k)
overlay on
/var/lib/docker/overlay2/38d1544d080145c7d76150530d02
55991dfb
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/3MQQXB4UCLF
B7ANVRHP
AVRC
RSS:
/var/lib/docker/overlay2/l/46YGYO474KLOULZGDSZDW2JPRI
,upperdi
r=/v
ar/l
ib/docker/overlay2/38d1544d080145c7d76150530d0255991d
fb7258cb
ca14
ff6d
165b94353eefab/diff,workdir=/var/lib/docker/overlay2/
38d1544d
0801
45c7
d76150530d0255991dfb7258cbca14ff6d165b94353eefab/work
)
shm on
/var/lib/docker/containers/662e7f798fc08b09eae0f0f944
537a4bce
dc1d
cf05
a65866458523ffd4a71614/shm type tmpfs
(rw,nosuid,nodev,noexec,relatime,context="system_u:ob
ject_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.
We cannot safely delete entries in the sidtab without first adding
reference counting of SIDs, which goes beyond just SELinux since they
are cached in other kernel data structures and returned by LSM hooks.
That's a non-trivial undertaking.
Far more practical in the near term would be to introduce a hash table
or other mechanism for efficient reverse lookups in the sidtab. Are
you offering to implement that or just requesting it?
Independent of that, docker should support reuse of category sets when
containers are deleted, at least as an option and probably as the
default.
Docker does reuse categories of containers that are removed, by default.