linux-kernel - Re: [PATCH v2] Linux VM workaround for Knights Landing A/D leak

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Message-ID: <fc551ed5-af19-9464-cee6-bb79bf35fa76@zytor.com>
Date:	Tue, 14 Jun 2016 14:08:32 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Dave Hansen <dave.hansen@...ux.intel.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Lukasz Anaczkowski <lukasz.anaczkowski@...el.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	tglx@...utronix.de, mingo@...hat.com, ak@...ux.intel.com,
	kirill.shutemov@...ux.intel.com, mhocko@...e.com,
	akpm@...ux-foundation.org, harish.srinivasappa@...el.com,
	lukasz.odzioba@...el.com, grzegorz.andrejczuk@...el.com,
	lukasz.daniluk@...el.com
Subject: Re: [PATCH v2] Linux VM workaround for Knights Landing A/D leak

On 06/14/16 14:02, Borislav Petkov wrote:
> On Tue, Jun 14, 2016 at 01:54:25PM -0700, H. Peter Anvin wrote:
>> There was that.  It is still possible that we end up with NOP a JMP
>> right before another JMP; we could perhaps make the patching code
>> smarter and see if we have a JMP immediately after.
> 
> Yeah, I still can't get reproduce that reliably - I remember seeing it
> at some point but then dismissing it for another, higher-prio thing. And
> now the whole memory is hazy at best.
> 
> But, you're giving me a great idea right now - I have this kernel
> disassembler tool which dumps alternative sections already and I could
> teach it to look for pathological cases around the patching sites and
> scream.
> 
> Something for my TODO list when I get a quiet moment.
> 

It's not really pathological; the issue is that asm goto() with an
unreachable clause after it doesn't tell gcc that a certain code path
ought to be linear, so we tell it to fall through.  However, if gcc then
wants to have a jump there for whatever reason (perhaps it is part of a
loop) we end up with a redundant jump, so a patch site followed by a JMP
is entirely reasonable.

	-hpa