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

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Message-ID: <CAG48ez3aQXb3EuGRVvLLo7BxycqJ4Y2mL83QhY9-QMK_qkfCuQ@mail.gmail.com>
Date:   Mon, 14 Sep 2020 21:39:10 +0200
From:   Jann Horn <jannh@...gle.com>
To:     John Wood <john.wood@....com>
Cc:     Kees Cook <keescook@...omium.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.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 Sun, Sep 13, 2020 at 6:56 PM John Wood <john.wood@....com> wrote:
> On Fri, Sep 11, 2020 at 02:01:56AM +0200, Jann Horn wrote:
> > 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:
> > > > 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.
>
> If I understand correctly you propose to add a new LSM hook without return
> value and place it here:
>
> diff --git a/kernel/signal.c b/kernel/signal.c
> index a38b3edc6851..074492d23e98 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -2751,6 +2751,8 @@ bool get_signal(struct ksignal *ksig)
>                         do_coredump(&ksig->info);
>                 }
>
> +               // Add the new LSM hook here
> +
>                 /*
>                  * Death signals, no core dump.
>                  */

It should probably be in the "if (sig_kernel_coredump(signr)) {"
branch. And I'm not sure whether it should be before or after
do_coredump() - if you do it after do_coredump(), the hook will have
to wait until the core dump file has been written, which may take a
little bit of time.