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

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Date:	Tue, 22 Apr 2014 10:29:45 -0700
From:	Andrew Lutomirski <amluto@...il.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"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 Tue, Apr 22, 2014 at 10:26 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Tue, Apr 22, 2014 at 10:11:27AM -0700, H. Peter Anvin wrote:
>> The fastpath interference is:
>>
>> 1. Testing for an LDT SS selector before IRET.  This is actually easier
>> than on 32 bits, because on 64 bits the SS:RSP on the stack is always valid.
>>
>> 2. Testing for an RSP inside the espfix region on exception entry, so we
>> can switch back the stack.  This has to be done very early in the
>> exception entry since the espfix stack is so small.  If NMI and #MC
>> didn't use IST it wouldn't work at all (but neither would SYSCALL).
>>
>> #2 currently saves/restores %rdi, which is also saved further down.
>> This is obviously wasteful.
>
> Btw, can we runtime-patch the fastpath interference chunk the moment we
> see a 16-bit segment? I.e., connect it to the write_idt() path, i.e. in
> the hunk you've removed in there and enable the espfix checks there the
> moment we load a 16-bit segment.
>
> I know, I know, this is not so important right now but let me put it out
> there just the same.

Or we could add a TIF_NEEDS_ESPFIX that gets set once you have a
16-bit LDT entry.

But I think it makes sense to nail down everything else first.  I
suspect that a single test-and-branch in the iret path will be lost in
the noise from iret itself.

--Andy
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