linux-kernel - Re: [PATCH v4 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace

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Date:	Thu, 02 Oct 2014 08:36:20 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
	Ingo Molnar <mingo@...hat.com>
CC:	Sebastian Lackner <sebastian@...-team.de>,
	Anish Bhatt <anish@...lsio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Chuck Ebbert <cebbert.lkml@...il.com>,
	stable <stable@...r.kernel.org>
Subject: Re: [PATCH v4 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace

On 10/01/2014 12:49 PM, Andy Lutomirski wrote:
> On Wed, Oct 1, 2014 at 11:49 AM, Andy Lutomirski <luto@...capital.net> wrote:
>> The NT flag doesn't do anything in long mode other than causing IRET
>> to #GP.  Oddly, CPL3 code can still set NT using popf.
>>
> 
> [...]
> 
>> +
>> +       /*
>> +        * Sysenter doesn't filter flags, so we need to clear NT
>> +        * ourselves.  To save a few cycles, we can check whether
>> +        * NT was set instead of doing an unconditional popfq.
>> +        */
>> +       testl $X86_EFLAGS_NT,EFLAGS(%rsp)       /* saved EFLAGS match cpu */
>> +       jnz sysenter_fix_flags
>> +sysenter_flags_fixed:
>> +
> 
> Because this thread hasn't gone on long enough:
> 
> Do we need to clear IOPL here, too?  With patch 2 applied, an IOPL !=
> 0 program can leak IOPL into another task.  It should be cleared on
> iret, sysexit (via popf) and sysret (directly), so this shouldn't
> matter.  Am I missing something?
> 
> Adding IOPL to the test will add no overhead for non-iopl-using tasks,
> but it will slighly slow down 32-bit tasks that use iopl.
> 

As you correctly point out, IOPL is completely irrelevant in ring 0.  We
have to restore the user space flags before returning to user space, so
it shouldn't matter.

	-hpa


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