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

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Message-ID: <20140501211554.GH2382@two.firstfloor.org>
Date:	Thu, 1 May 2014 23:15:54 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	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

> 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.

> 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
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