linux-kernel - Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when pti

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Message-ID: <20180112215422.GB16470@1wt.eu>
Date:   Fri, 12 Jan 2018 22:54:22 +0100
From:   Willy Tarreau <w@....eu>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when
 pti_disable is set

On Fri, Jan 12, 2018 at 01:18:06PM -0800, Andy Lutomirski wrote:
> FWIW, if we take this approach, then either dropping the capability should
> turn PTI back on or we need to deal with the corner case of PTI off and
> capability not present.  The latter is a bit awkward but not necessarily a
> show stopper.  I think that all we need to do is to update the ptrace rules
> and maybe make PTI turn back on when we execve.  At least there's no need to
> muck around with LSM hooks.

That's my point as well, just the same principle as the "NEXT" prctl : only
perform changes on execve(). At least we're sure to deal with something
consistent and it's the right moment for deciding on _PAGE_NX.

Willy