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

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Date:	Tue, 22 Apr 2014 23:24:04 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Lutomirski <amluto@...il.com>
CC:	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	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 04/22/2014 10:04 AM, Linus Torvalds wrote:
>
> The segment table is shared for a process. So you can have one thread
> doing a load_ldt() that invalidates a segment, while another thread is
> busy taking a page fault. The segment was valid at page fault time and
> is saved on the kernel stack, but by the time the page fault returns,
> it is no longer valid and the iretq will fault.
>
> Anyway, if done correctly, this whole espfix should be totally free
> for normal processes, since it should only trigger if SS is a LDT
> entry (bit #2 set in the segment descriptor). So the normal fast-path
> should just have a simple test for that.
>
> And if you have a SS that is a descriptor in the LDT, nobody cares
> about performance any more.
>

I just realized that with the LDT being a process-level object (unlike 
the GDT), we need to remove the filtering on the espfix hack, both for 
32-bit and 64-bit kernels.  Otherwise there is a race condition between 
executing the LAR instruction in the filter and the IRET, which could 
allow the leak to become manifest.

The "good" part is that I think the espfix hack is harmless even with a 
32/64-bit stack segment, although it has a substantial performance penalty.

Does anyone have any idea if there is a real use case for non-16-bit LDT 
segments used as the stack segment?  Does Wine use anything like that?

Very old NPTL Linux binaries use LDT segments, but only for data segments.

	-hpa


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