linux-kernel - Re: [PATCH v2 07/36] x86/entry/64/compat: After SYSENTER, move STI after the NT fixup

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Date:	Mon, 12 Oct 2015 14:11:10 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v2 07/36] x86/entry/64/compat: After SYSENTER, move STI
 after the NT fixup

On Mon, Oct 12, 2015 at 1:48 PM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
> On 10/09/2015 09:48 PM, Andy Lutomirski wrote:
>> Anyway, it's still a win, but I'll keep playing.  There may be even
>> better ways to do this.
>
> Such as this?
> Drop OR, check for cleared IF here:
>
>         testl   $X86_EFLAGS_NT, EFLAGS(%rsp)
>         jnz     sysenter_fix_flags
>         testl   $X86_EFLAGS_IF, EFLAGS(%rsp)
>         jz      sysenter_fix_flags
> sysenter_flags_fixed:
>
> and make sysenter_fix_flags code set IF in addition to TF clearing.
> This way, we don't even have an unconditional RMW op.

IF should normally be set in userspace, but since SYSENTER doesn't
save the old flags and always clears IF when entering the kernel, the
flags pushed on the stack will always have IF clear.

--
Brian Gerst
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