[PATCH v2] lockdown,selinux: avoid bogus SELinux lockdown permission checks

Fri May 28 17:09:57 AEST 2021

On 5/28/21 3:37 AM, Paul Moore wrote:
> On Mon, May 17, 2021 at 5:22 AM Ondrej Mosnacek <omosnace at redhat.com> wrote:
>>
>> Commit 59438b46471a ("security,lockdown,selinux: implement SELinux
>> lockdown") added an implementation of the locked_down LSM hook to
>> SELinux, with the aim to restrict which domains are allowed to perform
>> operations that would breach lockdown.
>>
>> However, in several places the security_locked_down() hook is called in
>> situations where the current task isn't doing any action that would
>> directly breach lockdown, leading to SELinux checks that are basically
>> bogus.
>>
>> Since in most of these situations converting the callers such that
>> security_locked_down() is called in a context where the current task
>> would be meaningful for SELinux is impossible or very non-trivial (and
>> could lead to TOCTOU issues for the classic Lockdown LSM
>> implementation), fix this by modifying the hook to accept a struct cred
>> pointer as argument, where NULL will be interpreted as a request for a
>> "global", task-independent lockdown decision only. Then modify SELinux
>> to ignore calls with cred == NULL.
> 
> I'm not overly excited about skipping the access check when cred is
> NULL.  Based on the description and the little bit that I've dug into
> thus far it looks like using SECINITSID_KERNEL as the subject would be
> much more appropriate.  *Something* (the kernel in most of the
> relevant cases it looks like) is requesting that a potentially
> sensitive disclosure be made, and ignoring it seems like the wrong
> thing to do.  Leaving the access control intact also provides a nice
> avenue to audit these requests should users want to do that.

I think the rationale/workaround for ignoring calls with cred == NULL (or the previous
patch with the unimplemented hook) from Ondrej was two-fold, at least speaking for his
seen tracing cases:

   i) The audit events that are triggered due to calls to security_locked_down()
      can OOM kill a machine, see below details [0].

  ii) It seems to be causing a deadlock via slow_avc_audit() -> audit_log_end()
      when presumingly trying to wake up kauditd [1].

How would your suggestion above solve both i) and ii)?

[0] https://bugzilla.redhat.com/show_bug.cgi?id=1955585 :

   I starting seeing this with F-34. When I run a container that is traced with eBPF
   to record the syscalls it is doing, auditd is flooded with messages like:

   type=AVC msg=audit(1619784520.593:282387): avc:  denied  { confidentiality } for
    pid=476 comm="auditd" lockdown_reason="use of bpf to read kernel RAM"
     scontext=system_u:system_r:auditd_t:s0 tcontext=system_u:system_r:auditd_t:s0 tclass=lockdown permissive=0

   This seems to be leading to auditd running out of space in the backlog buffer and
   eventually OOMs the machine.

   auditd running at 99% CPU presumably processing all the messages, eventually I get:
   Apr 30 12:20:42 fedora kernel: audit: backlog limit exceeded
   Apr 30 12:20:42 fedora kernel: audit: backlog limit exceeded
   Apr 30 12:20:42 fedora kernel: audit: audit_backlog=2152579 > audit_backlog_limit=64
   Apr 30 12:20:42 fedora kernel: audit: audit_backlog=2152626 > audit_backlog_limit=64
   Apr 30 12:20:42 fedora kernel: audit: audit_backlog=2152694 > audit_backlog_limit=64
   Apr 30 12:20:42 fedora kernel: audit: audit_lost=6878426 audit_rate_limit=0 audit_backlog_limit=64
   Apr 30 12:20:45 fedora kernel: oci-seccomp-bpf invoked oom-killer: gfp_mask=0x100cca(GFP_HIGHUSER_MOVABLE), order=0, oom_score_adj=-1000
   Apr 30 12:20:45 fedora kernel: CPU: 0 PID: 13284 Comm: oci-seccomp-bpf Not tainted 5.11.12-300.fc34.x86_64 #1
   Apr 30 12:20:45 fedora kernel: Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-2.fc32 04/01/2014

[1] https://lore.kernel.org/linux-audit/CANYvDQN7H5tVp47fbYcRasv4XF07eUbsDwT_eDCHXJUj43J7jQ@mail.gmail.com/ :

   Upstream kernel 5.11.0-rc7 and later was found to deadlock during a bpf_probe_read_compat()
   call within a sched_switch tracepoint. The problem is reproducible with the reg_alloc3
   testcase from SystemTap's BPF backend testsuite on x86_64 as well as the runqlat,runqslower
   tools from bcc on ppc64le. Example stack trace from [1]:

   [  730.868702] stack backtrace:
   [  730.869590] CPU: 1 PID: 701 Comm: in:imjournal Not tainted, 5.12.0-0.rc2.20210309git144c79ef3353.166.fc35.x86_64 #1
   [  730.871605] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014
   [  730.873278] Call Trace:
   [  730.873770]  dump_stack+0x7f/0xa1
   [  730.874433]  check_noncircular+0xdf/0x100
   [  730.875232]  __lock_acquire+0x1202/0x1e10
   [  730.876031]  ? __lock_acquire+0xfc0/0x1e10
   [  730.876844]  lock_acquire+0xc2/0x3a0
   [  730.877551]  ? __wake_up_common_lock+0x52/0x90
   [  730.878434]  ? lock_acquire+0xc2/0x3a0
   [  730.879186]  ? lock_is_held_type+0xa7/0x120
   [  730.880044]  ? skb_queue_tail+0x1b/0x50
   [  730.880800]  _raw_spin_lock_irqsave+0x4d/0x90
   [  730.881656]  ? __wake_up_common_lock+0x52/0x90
   [  730.882532]  __wake_up_common_lock+0x52/0x90
   [  730.883375]  audit_log_end+0x5b/0x100
   [  730.884104]  slow_avc_audit+0x69/0x90
   [  730.884836]  avc_has_perm+0x8b/0xb0
   [  730.885532]  selinux_lockdown+0xa5/0xd0
   [  730.886297]  security_locked_down+0x20/0x40
   [  730.887133]  bpf_probe_read_compat+0x66/0xd0
   [  730.887983]  bpf_prog_250599c5469ac7b5+0x10f/0x820
   [  730.888917]  trace_call_bpf+0xe9/0x240
   [  730.889672]  perf_trace_run_bpf_submit+0x4d/0xc0
   [  730.890579]  perf_trace_sched_switch+0x142/0x180
   [  730.891485]  ? __schedule+0x6d8/0xb20
   [  730.892209]  __schedule+0x6d8/0xb20
   [  730.892899]  schedule+0x5b/0xc0
   [  730.893522]  exit_to_user_mode_prepare+0x11d/0x240
   [  730.894457]  syscall_exit_to_user_mode+0x27/0x70
   [  730.895361]  entry_SYSCALL_64_after_hwframe+0x44/0xae

>> Since most callers will just want to pass current_cred() as the cred
>> parameter, rename the hook to security_cred_locked_down() and provide
>> the original security_locked_down() function as a simple wrapper around
>> the new hook.
[...]
> 
>> 3. kernel/trace/bpf_trace.c:bpf_probe_read_kernel{,_str}_common()
>>       Called when a BPF program calls a helper that could leak kernel
>>       memory. The task context is not relevant here, since the program
>>       may very well be run in the context of a different task than the
>>       consumer of the data.
>>       See: https://bugzilla.redhat.com/show_bug.cgi?id=1955585
> 
> The access control check isn't so much who is consuming the data, but
> who is requesting a potential violation of a "lockdown", yes?  For
> example, the SELinux policy rule for the current lockdown check looks
> something like this:
> 
>    allow <who> <who> : lockdown { <reason> };
> 
> It seems to me that the task context is relevant here and performing
> the access control check based on the task's domain is correct.
This doesn't make much sense to me, it's /not/ the task 'requesting a potential
violation of a "lockdown"', but rather the running tracing program which is e.g.
inspecting kernel data structures around the triggered event. If I understood
you correctly, having an 'allow' check on, say, httpd would be rather odd since
things like perf/bcc/bpftrace/systemtap/etc is installing the tracing probe instead.

Meaning, if we would /not/ trace such events (like in the prior mentioned syscall
example), then there is also no call to the security_locked_down() from that same/
unmodified application.

Thanks,
Daniel