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

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Date:   Thu, 13 Aug 2020 11:41:48 -0400
From:   Stephen Smalley <stephen.smalley.work@...il.com>
To:     Thiébaud Weksteen <tweek@...gle.com>,
        Paul Moore <paul@...l-moore.com>
Cc:     Nick Kralevich <nnk@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Peter Enderborg <peter.enderborg@...y.com>,
        Eric Paris <eparis@...isplace.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Rob Herring <robh@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        linux-kernel@...r.kernel.org, selinux@...r.kernel.org
Subject: Re: [PATCH v2 1/2] selinux: add tracepoint on denials

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.