linux-kernel - Re: [PATCHv8 12/17] x86/traps: Communicate a LASS violation in #GP message

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Message-ID: <wuo2m6qyhdub2bw4lovdzyxygu4rsbsvsoronmmm4vmn4vvyba@bletrxfbct4c>
Date: Wed, 2 Jul 2025 13:10:18 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Sohil Mehta <sohil.mehta@...el.com>
Cc: Andy Lutomirski <luto@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, 
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	"Paul E. McKenney" <paulmck@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	Xiongwei Song <xiongwei.song@...driver.com>, Xin Li <xin3.li@...el.com>, 
	"Mike Rapoport (IBM)" <rppt@...nel.org>, Brijesh Singh <brijesh.singh@....com>, 
	Michael Roth <michael.roth@....com>, Tony Luck <tony.luck@...el.com>, 
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	linux-mm@...ck.org
Subject: Re: [PATCHv8 12/17] x86/traps: Communicate a LASS violation in #GP
 message

On Tue, Jul 01, 2025 at 05:36:06PM -0700, Sohil Mehta wrote:
> On 7/1/2025 2:58 AM, Kirill A. Shutemov wrote:
> >  /*
> > @@ -672,6 +681,12 @@ static enum kernel_gp_hint get_kernel_gp_address(struct pt_regs *regs,
> >  	if (*addr < ~__VIRTUAL_MASK &&
> >  	    *addr + insn.opnd_bytes - 1 > __VIRTUAL_MASK)
> >  		return GP_NON_CANONICAL;
> > +	else if (*addr < ~__VIRTUAL_MASK &&
> > +		 cpu_feature_enabled(X86_FEATURE_LASS)) {
> > +		if (*addr < PAGE_SIZE)
> > +			return GP_NULL_POINTER;
> > +		return GP_LASS_VIOLATION;
> > +	}
> 
> The comments above this section of code say:
> 
> /*
>  * Check that:
>  *  - the operand is not in the kernel half
>  *  - the last byte of the operand is not in the user canonical half
>  */
> 
> They should be updated since we are updating the logic.

Okay.

> Also, below is easier to read than above:
> 
> 	if (*addr < ~__VIRTUAL_MASK) {
> 
> 		if (*addr + insn.opnd_bytes - 1 > __VIRTUAL_MASK)
> 			return EXC_NON_CANONICAL;
> 
> 		if (cpu_feature_enabled(X86_FEATURE_LASS)) {
> 			if (*addr < PAGE_SIZE)
> 				return EXC_NULL_POINTER;
> 			return EXC_LASS_VIOLATION;
> 		}
> 	}
> 
> I am wondering if the NULL pointer exception should be made
> unconditional, even if it is unlikely to reach here without LASS. So
> maybe something like this:
> 
> 	if (*addr < ~__VIRTUAL_MASK) {
> 
> 		if (*addr + insn.opnd_bytes - 1 > __VIRTUAL_MASK)
> 			return EXC_NON_CANONICAL;
> 
> 		if (*addr < PAGE_SIZE)
> 			return EXC_NULL_POINTER;
> 
> 		if (cpu_feature_enabled(X86_FEATURE_LASS))
> 			return EXC_LASS_VIOLATION;
> 	}

That's cleaner.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov