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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
	<mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
	<dave.hansen@...ux.intel.com>, <x86@...nel.org>, "H. Peter Anvin"
	<hpa@...or.com>, Peter Zijlstra <peterz@...radead.org>, Ard Biesheuvel
	<ardb@...nel.org>, "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>, Alexey Kardashevskiy <aik@....com>, Alexander Shishkin
	<alexander.shishkin@...ux.intel.com>
CC: Jonathan Corbet <corbet@....net>, Ingo Molnar <mingo@...nel.org>, "Pawan
 Gupta" <pawan.kumar.gupta@...ux.intel.com>, Daniel Sneddon
	<daniel.sneddon@...ux.intel.com>, Kai Huang <kai.huang@...el.com>, "Sandipan
 Das" <sandipan.das@....com>, Breno Leitao <leitao@...ian.org>, Rick Edgecombe
	<rick.p.edgecombe@...el.com>, Alexei Starovoitov <ast@...nel.org>, Hou Tao
	<houtao1@...wei.com>, Juergen Gross <jgross@...e.com>, Vegard Nossum
	<vegard.nossum@...cle.com>, Kees Cook <kees@...nel.org>, Eric Biggers
	<ebiggers@...gle.com>, Jason Gunthorpe <jgg@...pe.ca>, "Masami Hiramatsu
 (Google)" <mhiramat@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>,
	Luis Chamberlain <mcgrof@...nel.org>, Yuntao Wang <ytcoode@...il.com>,
	"Rasmus Villemoes" <linux@...musvillemoes.dk>, Christophe Leroy
	<christophe.leroy@...roup.eu>, Tejun Heo <tj@...nel.org>, Changbin Du
	<changbin.du@...wei.com>, Huang Shijie <shijie@...amperecomputing.com>,
	"Geert Uytterhoeven" <geert+renesas@...der.be>, Namhyung Kim
	<namhyung@...nel.org>, Arnaldo Carvalho de Melo <acme@...hat.com>,
	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-efi@...r.kernel.org>, <linux-mm@...ck.org>
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;
	}



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