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

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Message-ID: <63a44def-fd5d-4f21-8d09-bb608cfab524@intel.com>
Date: Tue, 1 Jul 2025 17:36:06 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Andy Lutomirski
	<luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar
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 Gupta" <pawan.kumar.gupta@...ux.intel.com>, Daniel Sneddon
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Subject: Re: [PATCHv8 12/17] x86/traps: Communicate a LASS violation in #GP
 message

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.

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;
	}