linux-kernel - Re: kernel panic: audit: backlog limit exceeded

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Message-ID: <CAHC9VhTsKCJf8bjOT+cxWZEX1y4c57KcVz0Y2c3vRGnJJQA4pA@mail.gmail.com>
Date:   Mon, 2 Mar 2020 08:43:28 -0500
From:   Paul Moore <paul@...l-moore.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        syzbot <syzbot+9a5e789e4725b9ef1316@...kaller.appspotmail.com>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: kernel panic: audit: backlog limit exceeded

On Mon, Mar 2, 2020 at 3:47 AM Dmitry Vyukov <dvyukov@...gle.com> wrote:
> On Fri, Feb 28, 2020 at 2:09 PM Paul Moore <paul@...l-moore.com> wrote:
> > On Fri, Feb 28, 2020 at 5:03 AM Tetsuo Handa
> > <penguin-kernel@...ove.sakura.ne.jp> wrote:
> > > On 2020/02/28 9:14, Paul Moore wrote:
> > > > We could consider adding a fuzz-friendly build time config which would
> > > > disable the panic failsafe, but it probably isn't worth it at the
> > > > moment considering the syzbot's pid namespace limitations.
> > >
> > > I think adding a fuzz-friendly build time config does worth. For example,
> > > we have locations where printk() emits "BUG:" or "WARNING:" and fuzzer
> > > misunderstands that a crash occurred. PID namespace is irrelevant.
> > > I proposed one at
> > > https://lkml.kernel.org/r/20191216095955.9886-1-penguin-kernel@I-love.SAKURA.ne.jp .
> > > I appreciate your response.
> >
> > To be clear, I was talking specifically about the intentional panic in
> > audit_panic().  It is different from every other panic I've ever seen
> > (perhaps there are others?) in that it doesn't indicate a serious
> > error condition in the kernel, it indicates that audit records were
> > dropped.  It seems extreme to most people, but some use cases require
> > that the system panic rather than lose audit records.
> >
> > My suggestion was that we could introduce a Kconfig build flag that
> > syzbot (and other fuzzers) could use to make the AUDIT_FAIL_PANIC case
> > in audit_panic() less panicky.  However, as syzbot isn't currently
> > able to test the kernel's audit code due to it's pid namespace
> > restrictions, it doesn't make much sense to add this capability.  If
> > syzbot removes that restriction, or when we get to the point that we
> > support multiple audit daemons, we can revisit this.
>
> Yes, we need some story for both panic and pid ns.
>
> We also use a separate net ns, but allow fuzzer to create some sockets
> in the init net ns to overcome similar limitations. This is done using
> a pseudo-syscall hack:
> https://github.com/google/syzkaller/blob/4a4e0509de520c7139ca2b5606712cbadc550db2/executor/common_linux.h#L1546-L1562
>
> But the pid ns is different and looks a bit harder as we need it
> during send of netlink messages.
>
> As a strawman proposal: the comment there says "for now":
>
> /* Only support auditd and auditctl in initial pid namespace
>  * for now. */
> if (task_active_pid_ns(current) != &init_pid_ns)
>   return -EPERM;
>
> What does that mean? Is it a kind of TODO? I mean if removing that
> limitation is useful for other reasons, then maybe we could kill 2
> birds with 1 stone.

Long story made short - the audit subsystem doesn't handle namespaces
or containers as well as it should.  Work is ongoing to add the
necessary support, but it isn't there yet and I don't want us to just
start removing restrictions until we have the proper support in place
(this what I alluded to with my "... when we get to the point that we
support multiple audit daemons, we can revisit this").

-- 
paul moore
www.paul-moore.com