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

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Message-ID: <20241024154254.c2ylwy3b7kilvoir@treble.attlocal.net>
Date: Thu, 24 Oct 2024 08:42:54 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: "Kirill A. Shutemov" <kirill@...temov.name>
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 Thu, Oct 24, 2024 at 05:02:42PM +0300, Kirill A. Shutemov wrote:
> 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.

Right, but I'm asking about the speculation bug which happens before the
exception (and which prompted this patch in the first place).

-- 
Josh