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

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Message-ID: <CAObL_7HN0S_zuqi=NSJARaV8HAQz=zQXAvBiE8QsiFQ0VxpO8Q@mail.gmail.com>
Date:	Wed, 23 Apr 2014 10:25:34 -0700
From:	Andrew Lutomirski <amluto@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	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 Wed, Apr 23, 2014 at 10:16 AM, H. Peter Anvin <hpa@...or.com> wrote:
> On 04/23/2014 10:08 AM, Andrew Lutomirski wrote:
>>
>> The only way I can see to trigger the race is with sigreturn, but it's
>> still there.  Sigh.
>>
>
> I don't see why sigreturn needs to be involved... all you need is
> modify_ldt() on one CPU while the other is in the middle of an IRET
> return.  Small window, so hard to hit, but still.
>

If you set the flag as soon as anyone calls modify_ldt, before any
descriptor is installed, then I don't think this can happen.  But
there's still sigreturn, and I don't think this is worth all the
complexity to save a single branch on #GP.

>> 2. I've often pondered changing the way we return *to* CPL 0 to bypass
>> iret entirely.  It could be something like:
>>
>> SS
>> RSP
>> EFLAGS
>> CS
>> RIP
>>
>> push 16($rsp)
>> popfq [does this need to force rex.w somehow?]
>> ret $64
>
> When you say return to CPL 0 you mean intra-kernel return?  That isn't
> really the problem here, though.  I think this will also break the
> kernel debugger since it will have the wrong behavior for TF and RF.

I do mean intra-kernel.  And yes, this has nothing to do with espfix,
but it would make write_msr_safe fail more quickly :)

--Andy
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