From: Daniel Borkmann daniel@iogearbox.net
[ Upstream commit ff40e51043af63715ab413995ff46996ecf9583f ]
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. This is indirectly also getting audit subsystem involved to report events. The latter is problematic, as reported by Ondrej and Serhei, since it can bring down the whole system via audit:
1) The audit events that are triggered due to calls to security_locked_down() can OOM kill a machine, see below details [0].
2) It also seems to be causing a deadlock via avc_has_perm()/slow_avc_audit() when trying to wake up kauditd, for example, when using trace_sched_switch() tracepoint, see details in [1]. Triggering this was not via some hypothetical corner case, but with existing tools like runqlat & runqslower from bcc, for example, which make use of this tracepoint. Rough call sequence goes like:
rq_lock(rq) -> -------------------------+ trace_sched_switch() -> | bpf_prog_xyz() -> +-> deadlock selinux_lockdown() -> | audit_log_end() -> | wake_up_interruptible() -> | try_to_wake_up() -> | rq_lock(rq) --------------+
What's worse is that the intention of 59438b46471a to further restrict lockdown settings for specific applications in respect to the global lockdown policy is completely broken for BPF. The SELinux policy rule for the current lockdown check looks something like this:
allow <who> <who> : lockdown { <reason> };
However, this doesn't match with the 'current' task where the security_locked_down() is executed, example: httpd does a syscall. There is a tracing program attached to the syscall which triggers a BPF program to run, which ends up doing a bpf_probe_read_kernel{,_str}() helper call. The selinux_lockdown() hook does the permission check against 'current', that is, httpd in this example. httpd has literally zero relation to this tracing program, and it would be nonsensical having to write an SELinux policy rule against httpd to let the tracing helper pass. The policy in this case needs to be against the entity that is installing the BPF program. For example, if bpftrace would generate a histogram of syscall counts by user space application:
bpftrace -e 'tracepoint:raw_syscalls:sys_enter { @[comm] = count(); }'
bpftrace would then go and generate a BPF program from this internally. One way of doing it [for the sake of the example] could be to call bpf_get_current_task() helper and then access current->comm via one of bpf_probe_read_kernel{,_str}() helpers. So the program itself has nothing to do with httpd or any other random app doing a syscall here. The BPF program _explicitly initiated_ the lockdown check. The allow/deny policy belongs in the context of bpftrace: meaning, you want to grant bpftrace access to use these helpers, but other tracers on the system like my_random_tracer _not_.
Therefore fix all three issues at the same time by taking a completely different approach for the security_locked_down() hook, that is, move the check into the program verification phase where we actually retrieve the BPF func proto. This also reliably gets the task (current) that is trying to install the BPF tracing program, e.g. bpftrace/bcc/perf/systemtap/etc, and it also fixes the OOM since we're moving this out of the BPF helper's fast-path which can be called several millions of times per second.
The check is then also in line with other security_locked_down() hooks in the system where the enforcement is performed at open/load time, for example, open_kcore() for /proc/kcore access or module_sig_check() for module signatures just to pick few random ones. What's out of scope in the fix as well as in other security_locked_down() hook locations /outside/ of BPF subsystem is that if the lockdown policy changes on the fly there is no retrospective action. This requires a different discussion, potentially complex infrastructure, and it's also not clear whether this can be solved generically. Either way, it is out of scope for a suitable stable fix which this one is targeting. Note that the breakage is specifically on 59438b46471a where it started to rely on 'current' as UAPI behavior, and _not_ earlier infrastructure such as 9d1f8be5cf42 ("bpf: Restrict bpf when kernel lockdown is in confidentiality mode").
[0] https://bugzilla.redhat.com/show_bug.cgi?id=1955585, Jakub Hrozek says:
I starting seeing this with F-34. When I run a container that is traced with BPF 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_eDC..., Serhei Makarov says:
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:
[...] [ 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 [...]
Fixes: 59438b46471a ("security,lockdown,selinux: implement SELinux lockdown") Reported-by: Ondrej Mosnacek omosnace@redhat.com Reported-by: Jakub Hrozek jhrozek@redhat.com Reported-by: Serhei Makarov smakarov@redhat.com Reported-by: Jiri Olsa jolsa@redhat.com Signed-off-by: Daniel Borkmann daniel@iogearbox.net Acked-by: Alexei Starovoitov ast@kernel.org Tested-by: Jiri Olsa jolsa@redhat.com Cc: Paul Moore paul@paul-moore.com Cc: James Morris jamorris@linux.microsoft.com Cc: Jerome Marchand jmarchan@redhat.com Cc: Frank Eigler fche@redhat.com Cc: Linus Torvalds torvalds@linux-foundation.org Link: https://lore.kernel.org/bpf/01135120-8bf7-df2e-cff0-1d73f1f841c3@iogearbox.n... Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/bpf/helpers.c | 7 +++++-- kernel/trace/bpf_trace.c | 32 ++++++++++++-------------------- 2 files changed, 17 insertions(+), 22 deletions(-)
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c index d0fc091d2ab4..f7e99bb8c3b6 100644 --- a/kernel/bpf/helpers.c +++ b/kernel/bpf/helpers.c @@ -14,6 +14,7 @@ #include <linux/jiffies.h> #include <linux/pid_namespace.h> #include <linux/proc_ns.h> +#include <linux/security.h>
#include "../../lib/kstrtox.h"
@@ -728,11 +729,13 @@ bpf_base_func_proto(enum bpf_func_id func_id) case BPF_FUNC_probe_read_user: return &bpf_probe_read_user_proto; case BPF_FUNC_probe_read_kernel: - return &bpf_probe_read_kernel_proto; + return security_locked_down(LOCKDOWN_BPF_READ) < 0 ? + NULL : &bpf_probe_read_kernel_proto; case BPF_FUNC_probe_read_user_str: return &bpf_probe_read_user_str_proto; case BPF_FUNC_probe_read_kernel_str: - return &bpf_probe_read_kernel_str_proto; + return security_locked_down(LOCKDOWN_BPF_READ) < 0 ? + NULL : &bpf_probe_read_kernel_str_proto; case BPF_FUNC_snprintf_btf: return &bpf_snprintf_btf_proto; default: diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c index fcbfc9564996..01710831fd02 100644 --- a/kernel/trace/bpf_trace.c +++ b/kernel/trace/bpf_trace.c @@ -212,16 +212,11 @@ const struct bpf_func_proto bpf_probe_read_user_str_proto = { static __always_inline int bpf_probe_read_kernel_common(void *dst, u32 size, const void *unsafe_ptr) { - int ret = security_locked_down(LOCKDOWN_BPF_READ); + int ret;
- if (unlikely(ret < 0)) - goto fail; ret = copy_from_kernel_nofault(dst, unsafe_ptr, size); if (unlikely(ret < 0)) - goto fail; - return ret; -fail: - memset(dst, 0, size); + memset(dst, 0, size); return ret; }
@@ -243,10 +238,7 @@ const struct bpf_func_proto bpf_probe_read_kernel_proto = { static __always_inline int bpf_probe_read_kernel_str_common(void *dst, u32 size, const void *unsafe_ptr) { - int ret = security_locked_down(LOCKDOWN_BPF_READ); - - if (unlikely(ret < 0)) - goto fail; + int ret;
/* * The strncpy_from_kernel_nofault() call will likely not fill the @@ -259,11 +251,7 @@ bpf_probe_read_kernel_str_common(void *dst, u32 size, const void *unsafe_ptr) */ ret = strncpy_from_kernel_nofault(dst, unsafe_ptr, size); if (unlikely(ret < 0)) - goto fail; - - return ret; -fail: - memset(dst, 0, size); + memset(dst, 0, size); return ret; }
@@ -1293,16 +1281,20 @@ bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog) case BPF_FUNC_probe_read_user: return &bpf_probe_read_user_proto; case BPF_FUNC_probe_read_kernel: - return &bpf_probe_read_kernel_proto; + return security_locked_down(LOCKDOWN_BPF_READ) < 0 ? + NULL : &bpf_probe_read_kernel_proto; case BPF_FUNC_probe_read_user_str: return &bpf_probe_read_user_str_proto; case BPF_FUNC_probe_read_kernel_str: - return &bpf_probe_read_kernel_str_proto; + return security_locked_down(LOCKDOWN_BPF_READ) < 0 ? + NULL : &bpf_probe_read_kernel_str_proto; #ifdef CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE case BPF_FUNC_probe_read: - return &bpf_probe_read_compat_proto; + return security_locked_down(LOCKDOWN_BPF_READ) < 0 ? + NULL : &bpf_probe_read_compat_proto; case BPF_FUNC_probe_read_str: - return &bpf_probe_read_compat_str_proto; + return security_locked_down(LOCKDOWN_BPF_READ) < 0 ? + NULL : &bpf_probe_read_compat_str_proto; #endif #ifdef CONFIG_CGROUPS case BPF_FUNC_get_current_cgroup_id: