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

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Message-ID: <5356ADF9.5030401@zytor.com>
Date:	Tue, 22 Apr 2014 10:59:21 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andrew Lutomirski <amluto@...il.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 10:46 AM, Andrew Lutomirski wrote:
>>
>> That is the whole impact of the IRET path.
>>
>> If using IST for #GP won't cause trouble (ISTs don't nest, so we need to
>> make sure there is absolutely no way we could end up nested) then the
>> rest of the fixup code can go away and we kill the common path
>> exception-handling overhead; the only new overhead is the IST
>> indirection for #GP, which isn't a performance-critical fault (good
>> thing, because untangling #GP faults is a major effort.)
> 
> I'd be a bit nervous about read_msr_safe and friends.  Also, what
> happens if userspace triggers a #GP and the signal stack setup causes
> a page fault?
> 

Yes, #GPs inside the kernel could be a real problem.  MSRs generally
don't trigger #SS.  The second scenario shouldn't be a problem, the #PF
will be delivered on the currently active stack.

On the other hand, doing the espfix fixup only for #GP might be an
alternative, as long as we can convince ourselves that it really is the
only fault that could possibly be delivered on the espfix path.

	-hpa


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