selinux: add tracepoint on audited events

The audit data currently captures which process and which target
is responsible for a denial. There is no data on where exactly in the
process that call occurred. Debugging can be made easier by being able to
reconstruct the unified kernel and userland stack traces [1]. Add a
tracepoint on the SELinux denials which can then be used by userland
(i.e. perf).

Although this patch could manually be added by each OS developer to
trouble shoot a denial, adding it to the kernel streamlines the
developers workflow.

It is possible to use perf for monitoring the event:
  # perf record -e avc:selinux_audited -g -a
  ^C
  # perf report -g
  [...]
      6.40%     6.40%  audited=800000 tclass=4
               |
                  __libc_start_main
                  |
                  |--4.60%--__GI___ioctl
                  |          entry_SYSCALL_64
                  |          do_syscall_64
                  |          __x64_sys_ioctl
                  |          ksys_ioctl
                  |          binder_ioctl
                  |          binder_set_nice
                  |          can_nice
                  |          capable
                  |          security_capable
                  |          cred_has_capability.isra.0
                  |          slow_avc_audit
                  |          common_lsm_audit
                  |          avc_audit_post_callback
                  |          avc_audit_post_callback
                  |

It is also possible to use the ftrace interface:
  # echo 1 > /sys/kernel/debug/tracing/events/avc/selinux_audited/enable
  # cat /sys/kernel/debug/tracing/trace
  tracer: nop
  entries-in-buffer/entries-written: 1/1   #P:8
  [...]
  dmesg-3624  [001] 13072.325358: selinux_denied: audited=800000 tclass=4

The tclass value can be mapped to a class by searching
security/selinux/flask.h. The audited value is a bit field of the
permissions described in security/selinux/av_permissions.h for the
corresponding class.

[1] https://source.android.com/devices/tech/debug/native_stack_dump

Signed-off-by: Thiébaud Weksteen <tweek@google.com>
Suggested-by: Joel Fernandes <joelaf@google.com>
Reviewed-by: Peter Enderborg <peter.enderborg@sony.com>
Acked-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

diff --git a/MAINTAINERS b/MAINTAINERS
index deaafb6..ed19f56 100644 (file)
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -15569,6 +15569,7 @@ T:      git git://git.kernel.org/pub/scm/linux/kernel/git/pcmoore/selinux.git
 F:     Documentation/ABI/obsolete/sysfs-selinux-checkreqprot
 F:     Documentation/ABI/obsolete/sysfs-selinux-disable
 F:     Documentation/admin-guide/LSM/SELinux.rst
+F:     include/trace/events/avc.h
 F:     include/uapi/linux/selinux_netlink.h
 F:     scripts/selinux/
 F:     security/selinux/

diff --git a/include/trace/events/avc.h b/include/trace/events/avc.h
new file mode 100644 (file)
index 0000000..07c058a
--- /dev/null
+++ b/include/trace/events/avc.h
@@ -0,0 +1,37 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Author: Thiébaud Weksteen <tweek@google.com>
+ */
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM avc
+
+#if !defined(_TRACE_SELINUX_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_SELINUX_H
+
+#include <linux/tracepoint.h>
+
+TRACE_EVENT(selinux_audited,
+
+       TP_PROTO(struct selinux_audit_data *sad),
+
+       TP_ARGS(sad),
+
+       TP_STRUCT__entry(
+               __field(unsigned int, tclass)
+               __field(unsigned int, audited)
+       ),
+
+       TP_fast_assign(
+               __entry->tclass = sad->tclass;
+               __entry->audited = sad->audited;
+       ),
+
+       TP_printk("tclass=%u audited=%x",
+               __entry->tclass,
+               __entry->audited)
+);
+
+#endif
+
+/* This part must be outside protection */
+#include <trace/define_trace.h>

diff --git a/security/selinux/avc.c b/security/selinux/avc.c
index d18cb32..b0a0af7 100644 (file)
--- a/security/selinux/avc.c
+++ b/security/selinux/avc.c
@@ -31,6 +31,9 @@
 #include "avc_ss.h"
 #include "classmap.h"
 
+#define CREATE_TRACE_POINTS
+#include <trace/events/avc.h>
+
 #define AVC_CACHE_SLOTS                        512
 #define AVC_DEF_CACHE_THRESHOLD                512
 #define AVC_CACHE_RECLAIM              16
@@ -706,6 +709,8 @@ static void avc_audit_post_callback(struct audit_buffer *ab, void *a)
        u32 scontext_len;
        int rc;
 
+       trace_selinux_audited(sad);
+
        rc = security_sid_to_context(sad->state, sad->ssid, &scontext,
                                     &scontext_len);
        if (rc)