[PATCH] selinux: log raw contexts as untrusted strings

[Paul Moore]

On Tue, Jun 11, 2019 at 4:07 AM Ondrej Mosnacek <omosnace at redhat.com> wrote:
> These strings may come from untrusted sources (e.g. file xattrs) so they
> need to be properly escaped.
>
> Reproducer:
>     # setenforce 0
>     # touch /tmp/test
>     # setfattr -n security.selinux -v 'kuřecí řízek' /tmp/test
>     # runcon system_u:system_r:sshd_t:s0 cat /tmp/test
>     (look at the generated AVCs)
>
> Actual result:
>     type=AVC [...] trawcon=kuřecí řízek
>
> Expected result:
>     type=AVC [...] trawcon=6B75C5996563C3AD20C599C3AD7A656B
>
> Fixes: fede148324c3 ("selinux: log invalid contexts in AVCs")
> Cc: stable at vger.kernel.org # v5.1+
> Signed-off-by: Ondrej Mosnacek <omosnace at redhat.com>
> ---
>  security/selinux/avc.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)

Thanks, the patch looks fine to me, but it is borderline -stable
material in my opinion.  I'll add it to the stable-5.2 branch, but in
the future I would prefer if you left the stable marking off patches
and sent a reply discussing *why* this should go to stable so we can
discuss it.  I realize Greg likes to pull a lot of stuff into stable,
but I try to be a bit more conservative about what gets marked.  Even
the simplest fix can still break things :)

I'm going to start building a test kernel now with this fix, but I
might hold off on sending this up to Linus for a couple of days to see
if I can catch Gen Zhang's patches in the same PR.

> diff --git a/security/selinux/avc.c b/security/selinux/avc.c
> index 8346a4f7c5d7..a99be508f93d 100644
> --- a/security/selinux/avc.c
> +++ b/security/selinux/avc.c
> @@ -739,14 +739,20 @@ static void avc_audit_post_callback(struct audit_buffer *ab, void *a)
>         rc = security_sid_to_context_inval(sad->state, sad->ssid, &scontext,
>                                            &scontext_len);
>         if (!rc && scontext) {
> -               audit_log_format(ab, " srawcon=%s", scontext);
> +               if (scontext_len && scontext[scontext_len - 1] == '\0')
> +                       scontext_len--;
> +               audit_log_format(ab, " srawcon=");
> +               audit_log_n_untrustedstring(ab, scontext, scontext_len);
>                 kfree(scontext);
>         }
>
>         rc = security_sid_to_context_inval(sad->state, sad->tsid, &scontext,
>                                            &scontext_len);
>         if (!rc && scontext) {
> -               audit_log_format(ab, " trawcon=%s", scontext);
> +               if (scontext_len && scontext[scontext_len - 1] == '\0')
> +                       scontext_len--;
> +               audit_log_format(ab, " trawcon=");
> +               audit_log_n_untrustedstring(ab, scontext, scontext_len);
>                 kfree(scontext);
>         }
>  }
> --
> 2.20.1

-- 
paul moore
www.paul-moore.com