linux-kernel - Re: [PATCH] x86 : Ensure X86_FLAGS

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Date:	Mon, 29 Sep 2014 13:29:22 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Anish Bhatt <anish@...lsio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Sebastian Lackner <sebastian@...-team.de>
Subject: Re: [PATCH] x86 : Ensure X86_FLAGS_NT is cleared on syscall entry

On Mon, Sep 29, 2014 at 1:01 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 09/29/2014 12:57 PM, Thomas Gleixner wrote:
>>
>>> I think we should mask the bit anyway.
>>
>> I tend to disagree. If we clear it there we need to consequentely
>> audit ALL other possibilites and if there are any we need to clear the
>> bit there as well. Just to make buggy user space happy?
>>
>
> The entry options into the kernel are: interrupt/exception (already
> known to be OK),

> SYSENTER32,

I don't immediately see how to fix that without adding overhead.
Maybe do it in CLEAR_RREGS?  This won't do any good if EFI ever starts
using IRET, though.  We could suck it up and fix it on entry, adding
maybe ten cycles (wild guess).

> SYSCALL32

This should happen for free if we fix SYSCALL64, I think.

Also, I thought entry_64.S was a mess.  Eww, ia32entry.S.

I can try to write the patch later today.  But I don't want to touch
the actual 32-bit kernel code -- I'll stick to the 64-bit native and
compat stuff, thanks.

--Andy

 and SYSCALL64.  It is not too
> much to work through those issues, I don't think.
>
>         -hpa
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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