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

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Message-ID: <536082CA.40905@zytor.com>
Date:	Tue, 29 Apr 2014 21:57:46 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <andi@...stfloor.org>,
	Andy Lutomirski <luto@...capital.net>
CC:	x86@...nel.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 04/29/2014 09:52 PM, H. Peter Anvin wrote:
> 
> It matters -- greatly -- if (and only if) we can enter the kernel with
> usergs == kernelgs and then want to change usergs inside a paranoid
> routine.  At that point we risk being upside down, which basically means
> we're rooted.
> 
> However, I believe this patchset also means only IST entries can be
> paranoid, which in turn means we can't sleep inside them.  To the very
> best of my knowledge the only times we change usergs is on context
> switch or inside a system call.  We need to make sure that is actually
> the case, though.
> 
> I'm at ELC for a few days, so I'll have limited decent-sized-monitor
> time, but it shouldn't be too hard to convince ourselves of... mostly a
> matter of making sure something like ptrace can't to stupid crap.
> 

Just in case anyone is getting the wrong impression: this is a
discussion about details.  I'm glad to see this work getting done.

	-hpa


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