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

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Message-ID: <CALCETrU4S3owtVF5_zVMxH_dXox6HXBbb-Q3+GzTQ-RU_xqgcw@mail.gmail.com>
Date:   Mon, 5 Feb 2018 16:38:33 +0000
From:   Andy Lutomirski <luto@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Brian Gerst <brgerst@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <ak@...ux.intel.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 1/3] x86/entry: Clear extra registers beyond syscall
 arguments for 64bit kernels

On Mon, Feb 5, 2018 at 4:26 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>
>> [...]
>>
>> But as the commit message says, the system call argument registers are
>> also likely to be aggressively clobbered unless used, since the low
>> registers are preferred for code generation (smaller code, and many of
>> them are special anyway in various ways and have forced uses for
>> shifts, function arguments, or just are special in general like %rax).
>>
>> So the actual argument registers tend to not be an issue anyway.
>

>
> So I submit that we should probably extend the register clearing/sanitization to
> R10 and R11 as well, because while they are technically caller-saved and freely
> clobberable, in practice they don't get clobbered all that often and there might
> be various code paths into complex system calls where these R10/R11 values survive
> just fine and can be used in Spectre gadgets.


Maybe R11, but we have to be careful, since R11 is used as scratch
space in a bunch of the asm.  Clearing R10 is mostly useless in the
syscall path because we'll just unconditionally reload it in
do_syscall_64().  If we manage to change the way syscall wrappers
work, then we can think about clearing R10 and maybe even more regs.