linux-kernel - Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*

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Date:	Fri, 25 Apr 2014 14:20:29 -0700
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Pavel Machek <pavel@....cz>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	Andy Lutomirski <amluto@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Borislav Petkov <bp@...en8.de>,
	Arjan van de Ven <arjan.van.de.ven@...el.com>,
	Brian Gerst <brgerst@...il.com>,
	Alexandre Julliard <julliard@...ehq.com>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*

On 04/25/2014 05:02 AM, Pavel Machek wrote:
> 
> Just to understand the consequences -- we leak 16 bit of kernel data
> to the userspace, right? Because it is %esp, we know that we leak
> stack address, which is not too sensitive, but will make kernel
> address randomization less useful...?
> 

It is rather sensitive, in fact.

>> The 64-bit implementation works like this:
>>
>> Set up a ministack for each CPU, which is then mapped 65536 times
>> using the page tables.  This implementation uses the second-to-last
>> PGD slot for this; with a 64-byte espfix stack this is sufficient for
>> 2^18 CPUs (currently we support a max of 2^13 CPUs.)
> 
> 16-bit stack segments on 64-bit machine. Who still uses it? Dosemu?
> Wine? Would the solution be to disallow that?

Welcome to the show.  We do, in fact disallow it now in the 3.15-rc
series.  The Wine guys are complaining.

	-hpa


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