linux-kernel - Re: [PATCH 4/7] x86: Add support for rd/wr fs/gs base

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Date:	Thu, 1 May 2014 14:39:20 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 4/7] x86: Add support for rd/wr fs/gs base

On Thu, May 1, 2014 at 2:15 PM, Andi Kleen <andi@...stfloor.org> wrote:
>> If usergs == kernelgs, then ebx will always be 1 and we'll never end
>> up in paranoid_userspace.
>
> You may miss a reschedule in this obscure case. It shouldn't really
> happen because loading a kernel pointer is not useful for user space.
>
> Doesn't seem like a real issue to me.
>
> We only happen need to handle it to avoid crashing.

Allowing userspace to prevent itself from being rescheduled by loading
something strange into gsbase seems unfortunate.

--Andy

>
>> Alternatively, what if the paranoid entry checked whether we're coming
>> from userspace at the very beginning and, if so, just jumped to the
>> non-paranoid entry?
>
> That would work, but I doubt it would be worth it.
>


> -Andi



-- 
Andy Lutomirski
AMA Capital Management, LLC
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