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

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Message-ID: <yuxpa4k3hdpplol35cdbvcaiwbi2i4mwwkww6tuab6loopdezm@qkhzxo3zlf7g>
Date: Thu, 24 Oct 2024 15:32:32 +0300
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: Borislav Petkov <bp@...en8.de>
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>, 
	Josh Poimboeuf <jpoimboe@...nel.org>
Subject: Re: [PATCH] x86: fix user address masking non-canonical speculation
 issue

On Thu, Oct 24, 2024 at 01:08:22PM +0200, Borislav Petkov wrote:
> > @@ -2389,6 +2390,15 @@ void __init arch_cpu_finalize_init(void)
> >  	alternative_instructions();
> >  
> >  	if (IS_ENABLED(CONFIG_X86_64)) {
> > +		unsigned long USER_PTR_MAX = TASK_SIZE_MAX;
> > +
> > +		/*
> > +		 * Enable this when LAM is gated on LASS support
> > +		if (cpu_feature_enabled(X86_FEATURE_LAM))
> > +			USER_PTR_MAX = (1ul << 63) - PAGE_SIZE;
> > +		 */
> > +		runtime_const_init(ptr, USER_PTR_MAX);
> 
> Looking at Documentation/arch/x86/x86_64/mm.rst, 5 level page tables define
> USR_PTR_MAX as 0x00ffffffffffffff, i.e., bits [55:0].
> 
> So I guess that USER_PTR_MAX needs to look at X86_FEATURE_LA57, no?

X86_FEATURE_LA57 is already handled inside TASK_SIZE_MAX definition.

Although, it might be worth updating end of userspace VA in mm.rst with
-PAGE_SIZE.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov