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

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Message-ID: <CAMzpN2h5_4yMQaKhvLw+eNuifVafpqRPcwzq81O4KvYGrcXFKg@mail.gmail.com>
Date:   Thu, 25 Jan 2018 15:04:27 -0500
From:   Brian Gerst <brgerst@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...nel.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

In Thu, Jan 25, 2018 at 2:16 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Jan 25, 2018 at 10:48 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> So the biggest impact of this is the extra register saves
>
> Actually, the other noticeable part is the reloading of the argument
> registers from ptregs. Together with just the extra level of
> 'call/ret' and the stack setup, I'm guessing we're talking maybe 20
> cycles or so.
>
> So there's the extra register saves, and simply the fact that the
> fastpath had a flatter calling structure.
>
> It still feels worth it. And if we do decide that we want to do the
> register clearing on kernel entry for some paranoid mode, we'd pretty
> much have to do this anyway.
>
>                  Linus

Another extra step the slow path does is checking to see if ptregs is
safe for SYSRET.  I think that can be mitigated by moving the check to
the places that do modify ptregs (ptrace, sigreturn, and exec) which
would set a flag to force return with IRET if the modified regs do not
satisfy the criteria for SYSRET.

--
Brian Gerst