linux-kernel - Re: [RFC PATCH v2 2/6] x86/arch_prctl: add ARCH_GET_NOPTI and ARCH_SET

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Date:   Tue, 9 Jan 2018 15:54:22 +0100
From:   Willy Tarreau <w@....eu>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Kees Cook <keescook@...omium.org>
Subject: Re: [RFC PATCH v2 2/6] x86/arch_prctl: add ARCH_GET_NOPTI and
 ARCH_SET_NOPTI to enable/disable PTI

On Tue, Jan 09, 2018 at 03:51:57PM +0100, Borislav Petkov wrote:
> On Tue, Jan 09, 2018 at 03:36:53PM +0100, Willy Tarreau wrote:
> > I see and am not particularly against this, but what use case do you
> > have in mind precisely ? I doubt it's just saving a few tens of bytes,
> > so probably you're more concerned about the potential risks this opens ?
> > But given we only allow this for CAP_SYS_RAWIO and these ones already
> > have access to /dev/mem and many other things, don't you think there
> > are much easier ways to dump kernel memory in this case than trying to
> > inject some meltdown code into the victim process ? Or maybe you have
> > other cases in mind that I'm not seeing.
> 
> I'd like this to be config-controllable so that distros can make the
> decision whether/if they want to support the whole per-mm thing.

OK.

> Also, if CAP_SYS_RAWIO is going to protect, please make the
> ARCH_GET_NOPTI variant check it too.

Interestingly I removed the check consecutive to the discussions. But
I think I'll simply remove the whole ARCH_GET_NOPTI as it has no real
value beyond initial development.

Willy