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

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Message-ID: <CAGXu5jLB=so4r169pq1KF3Kyfh2Kw_t6ZXGavpzVO2sg3=D1tw@mail.gmail.com>
Date:   Wed, 20 Feb 2019 13:20:58 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Solar Designer <solar@...nwall.com>
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 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.

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).

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

-- 
Kees Cook