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

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Message-ID: <bfce23cc-bf7f-494e-a443-baea41f33381@intel.com>
Date: Mon, 17 Nov 2025 09:24:20 -0800
From: Sohil Mehta <sohil.mehta@...el.com>
To: Borislav Petkov <bp@...en8.de>
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
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	<rick.p.edgecombe@...el.com>, Vegard Nossum <vegard.nossum@...cle.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>, Randy Dunlap
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	<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 11/17/2025 6:48 AM, Borislav Petkov wrote:

> Why is the first page frame special to justify a special check?
> 

Any dereference to a very low address (the first page frame) is
typically considered as a "NULL" pointer dereference.

$ git log | grep "BUG: kernel NULL pointer dereference"

    BUG: kernel NULL pointer dereference, address: 0000000000000000
    BUG: kernel NULL pointer dereference, address: 0000000000000010
    BUG: kernel NULL pointer dereference, address: 0000000000000008
    BUG: kernel NULL pointer dereference, address: 0000000000000008
    BUG: kernel NULL pointer dereference, address: 0000000000000040
    BUG: kernel NULL pointer dereference, address: 0000000000000264
    BUG: kernel NULL pointer dereference, address: 0000000000000000
    BUG: kernel NULL pointer dereference, address: 0000000000000098
    BUG: kernel NULL pointer dereference, address: 0000000000000000
    BUG: kernel NULL pointer dereference, address: 00000000000003c0

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.