linux-kernel - Re: [RFC PATCH 5/6] security/fbfam: Detect a fork brute force attack

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Date:   Fri, 11 Sep 2020 02:01:56 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        John Wood <john.wood@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Jonathan Corbet <corbet@....net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, linux-doc@...r.kernel.org,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>
Subject: Re: [RFC PATCH 5/6] security/fbfam: Detect a fork brute force attack

On Fri, Sep 11, 2020 at 1:49 AM Kees Cook <keescook@...omium.org> wrote:
> On Thu, Sep 10, 2020 at 01:21:06PM -0700, Kees Cook wrote:
> > From: John Wood <john.wood@....com>
> >
> > To detect a fork brute force attack it is necessary to compute the
> > crashing rate of the application. This calculation is performed in each
> > fatal fail of a task, or in other words, when a core dump is triggered.
> > If this rate shows that the application is crashing quickly, there is a
> > clear signal that an attack is happening.
> >
> > Since the crashing rate is computed in milliseconds per fault, if this
> > rate goes under a certain threshold a warning is triggered.
> >
> > Signed-off-by: John Wood <john.wood@....com>
> > ---
> >  fs/coredump.c          |  2 ++
> >  include/fbfam/fbfam.h  |  2 ++
> >  security/fbfam/fbfam.c | 39 +++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 43 insertions(+)
> >
> > diff --git a/fs/coredump.c b/fs/coredump.c
> > index 76e7c10edfc0..d4ba4e1828d5 100644
> > --- a/fs/coredump.c
> > +++ b/fs/coredump.c
> > @@ -51,6 +51,7 @@
> >  #include "internal.h"
> >
> >  #include <trace/events/sched.h>
> > +#include <fbfam/fbfam.h>
> >
> >  int core_uses_pid;
> >  unsigned int core_pipe_limit;
> > @@ -825,6 +826,7 @@ void do_coredump(const kernel_siginfo_t *siginfo)
> >  fail_creds:
> >       put_cred(cred);
> >  fail:
> > +     fbfam_handle_attack(siginfo->si_signo);
>
> I don't think this is the right place for detecting a crash -- isn't
> this only for the "dumping core" condition? In other words, don't you
> want to do this in get_signal()'s "fatal" block? (i.e. very close to the
> do_coredump, but without the "should I dump?" check?)
>
> Hmm, but maybe I'm wrong? It looks like you're looking at noticing the
> process taking a signal from SIG_KERNEL_COREDUMP_MASK ?
>
> (Better yet: what are fatal conditions that do NOT match
> SIG_KERNEL_COREDUMP_MASK, and should those be covered?)
>
> Regardless, *this* looks like the only place without an LSM hook. And it
> doesn't seem unreasonable to add one here. I assume it would probably
> just take the siginfo pointer, which is also what you're checking.

Good point, making this an LSM might be a good idea.

> e.g. for include/linux/lsm_hook_defs.h:
>
> LSM_HOOK(int, 0, task_coredump, const kernel_siginfo_t *siginfo);

I guess it should probably be an LSM_RET_VOID hook? And since, as you
said, it's not really semantically about core dumping, maybe it should
be named task_fatal_signal or something like that.