linux-kernel - Re: [PATCH v2] x86: Return to kernel without IRET

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Message-ID: <536488C2.8040106@zytor.com>
Date:	Fri, 02 May 2014 23:12:18 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [PATCH v2] x86: Return to kernel without IRET

On 05/02/2014 09:32 PM, Linus Torvalds wrote:
> On Fri, May 2, 2014 at 4:53 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> On my box, this saves about 100ns on each interrupt and trap that
>> happens while running in kernel space.  This speeds up my kernel_pf
>> microbenchmark by about 17%.
> 
> Btw, would you mind _trying_ to do a similar trick for the "return to
> user space" case?
> 
> At least as a proof-of-concept, having a code sequence in user mode
> trampoline that does
> 
>    popq %rsi
>    popq %r11
>    retq $128
> 
> and building up a stack in user space at '%rsp-128' that has the
> values or rsi/r11/rip should allow us to use 'sysret'. Hmm?
> 

That would be a security hole if another userspace thread could muck
with the stack.

	-hpa


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