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Message-ID: <20200913152724.GB2873@ubuntu>
Date: Sun, 13 Sep 2020 18:56:12 +0200
From: John Wood <john.wood@....com>
To: Jann Horn <jannh@...gle.com>, 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
Hi,
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.
*/
Thanks,
John Wood
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