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

[Jiri Olsa <jolsa@redhat.com>]

On Fri, May 28, 2021 at 09:09:57AM +0200, Daniel Borkmann wrote:
> On 5/28/21 3:37 AM, Paul Moore wrote:
> > On Mon, May 17, 2021 at 5:22 AM Ondrej Mosnacek <omosnace@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].

hi,
I saw the same deadlock, ended up with this sequence:

  rq_lock(rq) -> trace_sched_switch -> bpf_prog -> selinux_lockdown -> audit_log_end -> wake_up_interruptible -> try_to_wake_up -> rq_lock(rq)

problem is that trace_sched_switch already holds rq_lock

I had powerpc server where I could reproduce this easily,
but now for some reason I can't hit the issue anymore

jirka

> 
> 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
>