Message-ID: <535AD19D.8000502@linux.intel.com>
Date: Fri, 25 Apr 2014 14:20:29 -0700
From: "H. Peter Anvin" <hpa@linux.intel.com>
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To: Pavel Machek <pavel@ucw.cz>
CC: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Alexander van Heukelum <heukelum@fastmail.fm>,
        Andy Lutomirski <amluto@gmail.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Borislav Petkov <bp@alien8.de>,
        Arjan van de Ven <arjan.van.de.ven@intel.com>,
        Brian Gerst <brgerst@gmail.com>,
        Alexandre Julliard <julliard@winehq.com>,
        Andi Kleen <andi@firstfloor.org>, Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*
References: <tip-kicdm89kzw9lldryb1br9od0@git.kernel.org> <1398120472-6190-1-git-send-email-hpa@linux.intel.com> <20140425120229.GA4691@amd.pavel.ucw.cz>
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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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