linux-kernel - Re: Updated version of RD/WR FS/GS BASE patchkit

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Message-ID: <20160321224152.GH5083@two.firstfloor.org>
Date:	Mon, 21 Mar 2016 23:41:52 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andi Kleen <andi@...stfloor.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Updated version of RD/WR FS/GS BASE patchkit

> Imagine that some brilliant lightweight threading library does:
> 
>  - set GS to nonzero (by whatever means -- arch_prctl(ARCH_SET_GS,
> whatever) on a pre-IVB host followed by migration, some modify_ldt
> garbage, simple bloody-mindedness, whatever);

Migration is only possible when the CPUID flags match.

>  - WRGSBASE
>  - Use GS for a bit
> 
> This will work most of the time until it gets unlucky with preemption.

As soon as a kernel thread or something else schedules the value
will be lost.

> And yes, runtime library authors really do mess up in amazing ways.
> 
> It's an issue.  It needs conscious design.

Ok. So your only objection is the order of the context switch
updates?

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.