linux-kernel - Re: [PATCH] x86: fix user address masking non-canonical speculation issue

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Message-ID: <m3xpbkhc3vuk3so5fxjs7stx4fmhkhvew27uhhxyht3s4yhm7d@etnhvnhzln6k>
Date: Fri, 25 Oct 2024 12:01:39 +0300
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, x86@...nel.org, Andrew Cooper <andrew.cooper3@...rix.com>, 
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] x86: fix user address masking non-canonical speculation
 issue

On Thu, Oct 24, 2024 at 10:35:33AM -0700, Linus Torvalds wrote:
> On Wed, 23 Oct 2024 at 23:13, Josh Poimboeuf <jpoimboe@...nel.org> wrote:
> >
> > I'm probably missing something but once LAM is enabled, how wouldn't
> > this allow non-canonical address speculation?
> 
> Once LAM is enabled, together with LASS, non-canonical addresses
> basically don't exit.

That's not true.

With LAM, canonically check is relaxed to bit 63 == bit 47/56.

I try to confirm internally that we don't speculate past this relaxed
canonically check. I believe we don't, but I want to double-check.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov