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

[John Wood]

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 at chromium.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