linux-kernel - Re: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast path with retpolines on

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Date:   Thu, 25 Jan 2018 22:05:05 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <alan@...ux.intel.com>, Jann Horn <jannh@...gle.com>,
        Samuel Neves <samuel.c.p.neves@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast
 path with retpolines on

On Thu, 25 Jan 2018, Andy Lutomirski wrote:
> On Thu, Jan 25, 2018 at 10:48 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> > On Mon, Jan 22, 2018 at 10:55 AM, Linus Torvalds
> > <torvalds@...ux-foundation.org> wrote:
> >>
> >> Honestly, I'd rather get rid of the fast-path entirely. Compared to
> >> all the PTI mess, it's not even noticeable.
> >
> > So I looked at how that would be.
> >
> > Patch attached. Not really "tested", but I'm running the kernel with
> > this patch now, and 'strace' etc works, and honestly, it seems very
> > obvious.
> >
> > Also, code generation for 'do_syscall_64()' does not look horrible. In
> > fact, it doesn't look all that much worse than the fast-path ever did.
> >
> > So the biggest impact of this is the extra register saves
> > (SAVE_EXTRA_REGS) from setting up the full ptregs. And honestly, I
> > hate how that stupid macro still uses "movq reg,off(%rsp)" instead of
> > "pushq %reg".
> >
> > Considering the diffstat:
> >
> >  2 files changed, 2 insertions(+), 121 deletions(-)
> >
> > and how those 100+ lines are nasty assembly code, I do think we should
> > just do it.
> 
> Feel free to Acked-by: Andy Lutomirski <luto@...nel.org> that patch.
> 
> Or I can grab it and send it to -tip.

That would be nice, so we can route it through x86/pti which provides it
for backporting cleanly.

Thanks,

	tglx