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

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Message-ID: <xonbgoepadskqagkprwpcse33sgl3yebo52liuskde2eiozcza@njkktzlcar6g>
Date: Thu, 24 Oct 2024 17:02:42 +0300
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.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 Wed, Oct 23, 2024 at 11:13:00PM -0700, Josh Poimboeuf wrote:
> On Wed, Oct 23, 2024 at 06:31:59PM -0700, Linus Torvalds wrote:
> > @@ -2389,6 +2390,15 @@ void __init arch_cpu_finalize_init(void)
> > +		/*
> > +		 * Enable this when LAM is gated on LASS support
> > +		if (cpu_feature_enabled(X86_FEATURE_LAM))
> > +			USER_PTR_MAX = (1ul << 63) - PAGE_SIZE;
> > +		 */
> 
> I'm probably missing something but once LAM is enabled, how wouldn't
> this allow non-canonical address speculation?  i.e. when bit 47/56 is
> set and 63 is cleared, would it not go untouched by mask_user_address()
> and thus be speculatively interpreted by AMD as a kernel address?

CPU with LAM enabled enforces bit 63 to be equal bit 47/56 and raises #GP
otherwise.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov