Re: [PATCH v2 1/2] selinux: add tracepoint on denials

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On 8/13/20 10:48 AM, Thiébaud Weksteen wrote:

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

An explanation here of how one might go about decoding audited and tclass would be helpful to users (even better would be a script to do it for them).  Again, I know how to do that but not everyone using perf/ftrace will.





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