Re: [PATCH v2] lockdown,selinux: avoid bogus SELinux lockdown permission checks
From: Daniel Borkmann
Date: Fri May 28 2021 - 03:10:08 EST
On 5/28/21 3:37 AM, Paul Moore wrote:
On Mon, May 17, 2021 at 5:22 AM Ondrej Mosnacek <omosnace@xxxxxxxxxx> 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@xxxxxxxxxxxxxx/ :
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