RE: [PATCH v3 1/3] ptrace: Provide ___ptrace_may_access() that can be applied on arbitrary tasks

Schaufler, Casey
2018-09-05T15:37:56Z

> -----Original Message-----
> From: Peter Zijlstra [mailto:peterz@infradead.org]
> Sent: Wednesday, September 05, 2018 1:00 AM
> To: Jiri Kosina <jikos@kernel.org>
> Cc: Tim Chen <tim.c.chen@linux.intel.com>; Thomas Gleixner
> <tglx@linutronix.de>; Ingo Molnar <mingo@redhat.com>; Josh Poimboeuf
> <jpoimboe@redhat.com>; Andrea Arcangeli <aarcange@redhat.com>;
> Woodhouse, David <dwmw@amazon.co.uk>; Oleg Nesterov
> <oleg@redhat.com>; Schaufler, Casey <casey.schaufler@intel.com>; linux-
> kernel@vger.kernel.org; x86@kernel.org
> Subject: Re: [PATCH v3 1/3] ptrace: Provide ___ptrace_may_access() that can
> be applied on arbitrary tasks
>
> On Tue, Sep 04, 2018 at 07:35:29PM +0200, Jiri Kosina wrote:
> > On Tue, 4 Sep 2018, Tim Chen wrote:
> >
> > > > Current ptrace_may_access() implementation assumes that the 'source'
> task is
> > > > always the caller (current).
> > > >
> > > > Expose ___ptrace_may_access() that can be used to apply the check on
> arbitrary
> > > > tasks.
> > >
> > > Casey recently has proposed putting the decision making of whether to
> > > do IBPB in the security module.
> > >
> > > https://lwn.net/ml/kernel-hardening/20180815235355.14908-4-
> casey.schaufler@intel.com/
> > >
> > > That will have the advantage of giving the administrator a more flexibility
> > > of when to turn on IBPB. The policy is very similar to what you have
> proposed here
> > > but I think the security module is a more appropriate place for the security
> policy.
> >
> > Yeah, well, honestly, I have a bit hard time buying the "generic
> > sidechannel prevention security module" idea, given how completely
> > different in nature all the mitigations have been so far. I don't see that
> > trying to abstract this somehow provides more clarity.
> >
> > So if this should be done in LSM, it'd probably have to be written by
> > someone else than me :) who actually understands how the "sidechannel
> LSM"
> > idea works.
>
> Yeah, I'm not convinced on LSM either. Lets just do these here patches
> first and then Casey can try and convince us later.

Works for me. There are advantages to doing it either way.
The LSM approach allows you to consider implications of LSM
data, which you can't do otherwise. Once the hook is available
it becomes the obvious place to do other checks.