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

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Date:	Mon, 29 Sep 2014 12:43:32 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Andy Lutomirski <luto@...capital.net>
CC:	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 09/29/2014 12:41 PM, Thomas Gleixner wrote:
>>
>> If it weren't the case, then we'd be totally screwed.  Fortunately, it
>> is.  I found it: SDM Volume 3 6.12.1.2 says:
>>
>> (On calls to exception and interrupt
>> handlers, the processor also clears the VM, RF, and NT flags in the
>> EFLAGS register,
>> after they are saved on the stack.)
> 
> Sorry, I misunderstood your question.
> 
> And yes on exception and interrupt entry it is cleared. Otherwise the
> whole feature would not work at all ...
> 
> But that's why I'm really not worried about it. While we can mask out
> the stupid bit easily, it does not provide any value except protecting
> silly userspace from rightfully raised exceptions.
> 
> When I first saw that patch, I was worried about the security impact,
> but after staring long enough at the SDM and the code, the only way it
> can explode is when returning to user space. It cannot explode in the
> kernel.
> 
> So in IA-32e it creates a #GP otherwise it falls over the return to
> NULL (TSS.back_link). So what?
> 

How about "it's a bug, but it's not (necessarily) a security issue?"

I think we should mask the bit anyway.

	-hpa


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