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selinux: add tracepoint on audited events

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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>
This commit is contained in:
Thiébaud Weksteen 2020-08-21 16:08:21 +02:00 committed by Paul Moore
parent 0eea609153
commit dd8166212d
3 changed files with 43 additions and 0 deletions
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1
MAINTAINERS
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@ -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/

37
include/trace/events/avc.h Normal file
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@ -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>

5
security/selinux/avc.c
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@ -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)