linux-kernel - Re: [PATCH v2] x86/asm: Pin sensitive CR4 bits

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Date:   Thu, 21 Feb 2019 14:06:45 +0100
From:   Solar Designer <solar@...nwall.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>, Jann Horn <jannh@...gle.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        X86 ML <x86@...nel.org>
Subject: Re: [PATCH v2] x86/asm: Pin sensitive CR4 bits

On Wed, Feb 20, 2019 at 01:20:58PM -0800, Kees Cook wrote:
> On Wed, Feb 20, 2019 at 10:49 AM Solar Designer <solar@...nwall.com> wrote:
> >
> > On Wed, Feb 20, 2019 at 10:09:34AM -0800, Kees Cook wrote:
> > > +     if (WARN_ONCE((val & cr4_pin) != cr4_pin, "cr4 bypass attempt?!\n"))
> > > +             goto again;
> >
> > I think "goto again" is too mild a response given that it occurs after a
> > successful write of a non-pinned value to CR4.  I think it'd allow some
> > exploits to eventually win the race: make their desired use of whatever
> > functionality SMEP, etc. would have prevented - which may be just a few
> > instructions they need to run - before the CR4 value is reverted after
> > "goto again".  I think it's one of those cases where a kernel panic
> > would be more appropriate.
> 
> It will not land upstream with a BUG() or panic(). Linus has
> explicitly stated that none of this work can do that until it has
> "baked" in the kernel for a couple years.

OK.

> In his defense, anyone sufficiently paranoid can already raise the
> priority of a WARN() into a panic via sysctl kernel.panic_on_warn (and
> kernel.panic_on_oops).

I think there are too many uses of WARN() for anyone sane to enable
that in production, whereas it'd have made sense to enable it for the
few security-related uses.

> > Also, WARN_ONCE possibly introduces a delay sufficient to realistically
> > win this race on the first try.  If we choose to warn, we should do it
> > after having reverted the CR4 value, not before.
> 
> Isn't cr4 CPU-local though?

Good point.  I don't know.  If CR4 is per hardware thread, then the race
would require an interrupt and would be much harder to win.

> Couldn't we turn off interrupts to stop the race?

This won't help.  An attack would skip the code that disables interrupts
and land right on the MOV instruction.

Alexander