linux-kernel - Re: [PATCH v2 1/3] x86/entry: Clear extra registers beyond syscall arguments for 64bit kernels

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Date:   Mon, 5 Feb 2018 17:30:46 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Andy Lutomirski <luto@...capital.net>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Dan Williams <dan.j.williams@...el.com>, tglx@...utronix.de,
        Andi Kleen <ak@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        luto@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        torvalds@...ux-foundation.org
Subject: Re: [PATCH v2 1/3] x86/entry: Clear extra registers beyond syscall
 arguments for 64bit kernels


* Andy Lutomirski <luto@...capital.net> wrote:

> 
> 
> > On Feb 5, 2018, at 3:42 AM, Ingo Molnar <mingo@...nel.org> wrote:
> > 
> > 
> > * Dan Williams <dan.j.williams@...el.com> wrote:
> > 
> >> +    /*
> >> +     * Sanitize extra registers of values that a speculation attack
> >> +     * might want to exploit. In the CONFIG_FRAME_POINTER=y case,
> >> +     * the expectation is that %ebp will be clobbered before it
> >> +     * could be used.
> >> +     */
> >> +    .macro CLEAR_EXTRA_REGS_NOSPEC
> >> +    xorq %r15, %r15
> >> +    xorq %r14, %r14
> >> +    xorq %r13, %r13
> >> +    xorq %r12, %r12
> >> +    xorl %ebx, %ebx
> >> +#ifndef CONFIG_FRAME_POINTER
> >> +    xorl %ebp, %ebp
> >> +#endif
> >> +    .endm
> > 
> > Yeah, so this series look pretty good to me, but there's one small detail: I think 
> > RBP should be cleared unconditionally here, even in the CONFIG_FRAME_POINTERS=y 
> > case, because:
> 
> ENCODE_FRAME_POINTER should take care of rbp, though.

AFAICS there's various entry paths where it's not used I think: for example the 
compat system calls in entry_64_compat.S don't seem to encode RBP in such a 
fashion (unless I missed some macro side effect).

Basically I'd iterate these things the following way:

 - where there's justified, demonstrable doubt we should be clearing input values 
   in a simple, robust fashion (as it seems to be in this particular case)

 - then we can micro-optimize again when we are convinced that it's safe

Otherwise I'm worried about the speed with which we converge to Spectre 
correctness.

Thanks,

	Ingo