linux-kernel - Re: [PATCH] x86/entry: Clarify the RF saving/restoring situation with SYSCALL/SYSRET

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Message-ID: <CALCETrX=wO69Jkuhb1SFOzpM9LeNfyaYUpMo-uxPEuw-GDTPMA@mail.gmail.com>
Date:	Wed, 3 Aug 2016 10:24:18 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/entry: Clarify the RF saving/restoring situation with SYSCALL/SYSRET

On Wed, Aug 3, 2016 at 10:14 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Wed, Aug 03, 2016 at 09:42:29AM -0700, Andy Lutomirski wrote:
>> I would change "so" and "and" -- the CPU designers could have make
>> SYSRET restore RF, but they chose not to.
>
> I'm assuming the reasoning behind it was that you should be able to
> break after SYSRET but then again, the kernel could've been left in
> control of that bit and set it or clear it however it likes before
> SYSRETing.
>
> Oh well.

AFAICT the AMD people didn't think of any use cases involving doing
anything interesting between SYSCALL and SYSRET.  Witness the
sysret_ss_attrs goof, for example: apparently SYSCALL; context switch;
IRET; interrupt; context switch; SYSRET didn't occur to AMD as a valid
use case.

--Andy