linux-kernel - Re: [PATCH v12 6/8] x86/traps: Communicate a LASS violation in #GP message

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Message-ID: <20251117182958.GBaRtppoY2uANW2JI8@fat_crate.local>
Date: Mon, 17 Nov 2025 19:29:58 +0100
From: Borislav Petkov <bp@...en8.de>
To: Sohil Mehta <sohil.mehta@...el.com>
Cc: x86@...nel.org, Dave Hansen <dave.hansen@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Jonathan Corbet <corbet@....net>,
	"H . Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...nel.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ard Biesheuvel <ardb@...nel.org>,
	"Kirill A . Shutemov" <kas@...nel.org>, Xin Li <xin@...or.com>,
	David Woodhouse <dwmw@...zon.co.uk>,
	Sean Christopherson <seanjc@...gle.com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Vegard Nossum <vegard.nossum@...cle.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Kees Cook <kees@...nel.org>, Tony Luck <tony.luck@...el.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-efi@...r.kernel.org
Subject: Re: [PATCH v12 6/8] x86/traps: Communicate a LASS violation in #GP
 message

On Mon, Nov 17, 2025 at 09:24:20AM -0800, Sohil Mehta wrote:
> The page fault error messages have similar logic:
> 
> if (address < PAGE_SIZE && !user_mode(regs))
> 	pr_alert("BUG: kernel NULL pointer dereference, address: %px\n",
> 		(void *)address);
> 
> I believe the check is to account for arithmetic or other operations
> that may have happened on the "NULL" pointer before it is dereferenced.

Ah ok, that makes sense. That comment still reads weird:

+       /*
+        * If LASS is active, a NULL pointer dereference generates a #GP
+        * instead of a #PF.
+        */
+       if (*addr < PAGE_SIZE)

It says "if LASS is active" but the check if
(cpu_feature_enabled(X86_FEATURE_LASS)) is below it.

I guess you want to have

	if (cpu_feature_enabled(X86_FEATURE_LASS)) {
		if (*addr < PAGE_SIZE)
			return GP_NULL_POINTER;
		else
			return GP_LASS_VIOLATION;
	}

so that it is perfectly clear.

Then the catch-all GP_CANONICAL will take care of the absurd cases, if we ever
hit them.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette