linux-kernel - RE: [PATCH v5 1/2] x86/speculation: apply IBPB more strictly to avoid cross-process data leak

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Date:   Mon, 10 Sep 2018 20:27:24 +0000
From:   "Schaufler, Casey" <casey.schaufler@...el.com>
To:     Jiri Kosina <jikos@...nel.org>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Schaufler, Casey" <casey.schaufler@...el.com>
Subject: RE: [PATCH v5 1/2] x86/speculation: apply IBPB more strictly to
 avoid cross-process data leak

> -----Original Message-----
> From: Jiri Kosina [mailto:jikos@...nel.org]
> Sent: Monday, September 10, 2018 12:36 PM
> To: Schaufler, Casey <casey.schaufler@...el.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>; Ingo Molnar <mingo@...hat.com>;
> Peter Zijlstra <peterz@...radead.org>; Josh Poimboeuf
> <jpoimboe@...hat.com>; Andrea Arcangeli <aarcange@...hat.com>;
> Woodhouse, David <dwmw@...zon.co.uk>; Andi Kleen <ak@...ux.intel.com>;
> Tim Chen <tim.c.chen@...ux.intel.com>; linux-kernel@...r.kernel.org;
> x86@...nel.org
> Subject: RE: [PATCH v5 1/2] x86/speculation: apply IBPB more strictly to avoid
> cross-process data leak
> 
> On Mon, 10 Sep 2018, Schaufler, Casey wrote:
> 
> > Yes, It would require that this patch be tested against all the existing
> > security modules that provide a ptrace_access_check hook. It's not like
> > the security module writers don't have a bunch of locking issues to deal
> > with.
> 
> Yeah, that was indeed my concern.
> 
> So can we agree on doing this in the 2nd envisioned step, when this is
> going to be replaced by LSM as discussed [1] previously?

It you're going to call __ptrace_access_check(), which already includes
an LSM hook, it makes a whole lot of sense to make that the path for doing
any module specific checks. It seems wrong to disable the LSM hook there,
then turn around and introduce a new one that does the check you just
disabled. The patches I had proposed created a new LSM hook because there
was not path to an existing hook. With your addition of __ptrace_access_check()
that is no longer an issue once any locking problems are resolved. Rather than
use a new hook, the existing ptrace hooks ought to work just fine, and any new
checks can be added in a new module that has its own ptrace_access_check hook.

> [1]
> http://lkml.kernel.org/r/99FC4B6EFCEFD44486C35F4C281DC67321447094@OR
> SMSX107.amr.corp.intel.com
> 
> Thanks,
> 
> --
> Jiri Kosina
> SUSE Labs