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

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Message-ID: <CAObL_7HdWs2hoNYd0gKzh6iVJr293Z9p+Dg1C6u+5GYQiDfgnA@mail.gmail.com>
Date:	Tue, 22 Apr 2014 10:00:26 -0700
From:	Andrew Lutomirski <amluto@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, 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 Tue, Apr 22, 2014 at 9:43 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Tue, Apr 22, 2014 at 9:33 AM, Andrew Lutomirski <amluto@...il.com> wrote:
>>
>> For the espfix_adjust_stack thing, when can it actually need to do
>> anything?  irqs should be off, I think, and MCE, NMI, and debug
>> exceptions use ist, so that leaves just #SS and #GP, I think.  How can
>> those actually occur?  Is there a way to trigger them deliberately
>> from userspace?  Why do you have three espfix_adjust_stack
>
> Yes, you can very much trigger GP deliberately.
>
> The way to do it is to just make an invalid segment descriptor on the
> iret stack. Or make it a valid 16-bit one, but make it a code segment
> for the stack pointer, or read-only, or whatever. All of which is
> trivial to do with a sigretun system call. But you can do it other
> ways too - enter with a SS that is valid, but do a load_ldt() system
> call that makes it invalid, so that by the time you exit it is no
> longer valid etc.
>
> There's a reason we mark that "iretq" as taking faults with that
>
>         _ASM_EXTABLE(native_iret, bad_iret)
>
> and that "bad_iret" creates a GP fault.
>
> And that's a lot of kernel stack. The whole initial GP fault path,
> which goes to the C code that finds the exception table etc. See
> do_general_protection_fault() and fixup_exception().

My point is that it may be safe to remove the special espfix fixup
from #PF, which is probably the most performance-critical piece here,
aside from iret itself.

--Andy
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