[PATCH v2] lockdown,selinux: avoid bogus SELinux lockdown permission checks
Jiri Olsa
jolsa at redhat.com
Fri May 28 09:53:08 UTC 2021
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 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].
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
>
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