linux-kernel - Re: [PATCH 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace

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Date:	Tue, 30 Sep 2014 15:33:20 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Sebastian Lackner <sebastian@...-team.de>, X86 ML <x86@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Anish Bhatt <anish@...lsio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Chuck Ebbert <cebbert.lkml@...il.com>, stable@...nel.org
Subject: Re: [PATCH 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace

On Tue, Sep 30, 2014 at 3:27 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Tue, 30 Sep 2014, Andy Lutomirski wrote:
>> It would certainly be possible to clear NT and retry IRET if IRET
>> fails with NT set.  This would have no overhead for anything relevant.
>> That would be this alternative from my 0/2 email:
>>
>>  - Don't filter NT on sysenter.  Instead, filter it on EFI entry
>>    and modify the IRET code to retry without NT set if NT was set.
>>
>> Thomas hpa, etc: any thoughts?
>
> Filter it right away. That's solid and obvious. Anything else is just
> complex and prone for future brown paperbag failures.

Yeah, agreed.  That's exactly what these patches do, although, if you
put them in -tip and want to keep the stable CC, it's probably worth
fixing the address (oops).

>
> We get the context switch benefit from it, so there is some
> compensation for the extra cycles.

If we ever want those cycles back, I bet that the compat sysenter path
could be trimmed down a lot.  For example, I think that all of the
zero-extension stuff is unnecessary now that we have the magic syscall
wrappers for all (?) syscalls.

--Andy
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