linux-kernel - Re: [PATCH v11 7/9] x86/traps: Communicate a LASS violation in #GP message

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Message-Id: <fe662d7c-d537-495e-92e5-baf64ebbcc2d@app.fastmail.com>
Date: Fri, 31 Oct 2025 13:03:24 -0700
From: "Andy Lutomirski" <luto@...nel.org>
To: "Sohil Mehta" <sohil.mehta@...el.com>,
 "Dave Hansen" <dave.hansen@...el.com>,
 "the arch/x86 maintainers" <x86@...nel.org>,
 "Dave Hansen" <dave.hansen@...ux.intel.com>,
 "Thomas Gleixner" <tglx@...utronix.de>, "Ingo Molnar" <mingo@...hat.com>,
 "Borislav Petkov" <bp@...en8.de>
Cc: "Jonathan Corbet" <corbet@....net>, "H. Peter Anvin" <hpa@...or.com>,
 "Josh Poimboeuf" <jpoimboe@...nel.org>,
 "Peter Zijlstra (Intel)" <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 P 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 Mailing List" <linux-kernel@...r.kernel.org>,
 linux-efi@...r.kernel.org
Subject: Re: [PATCH v11 7/9] x86/traps: Communicate a LASS violation in #GP message

On Fri, Oct 31, 2025, at 12:59 PM, Sohil Mehta wrote:
> On 10/31/2025 10:16 AM, Dave Hansen wrote:
>> On 10/29/25 14:03, Sohil Mehta wrote:
>>> To make the transition easier, enhance the #GP Oops message to include a
>>> hint about LASS violations. Also, add a special hint for kernel NULL
>>> pointer dereferences to match with the existing #PF message.
>> 
>> Reviewed-by: Dave Hansen <dave.hansen@...ux.intel.com>
>> 
>> This also reminds me... Are there tests for this somewhere? How did you
>> test all these new messages?
>
> I have some very simple kernel modules that access invalid user memory
> and generate these faults. I configure the kernel not to panic/reboot.
> But, I have been running them manually.
>
> Invalid accesses from the kernel generate:
> #PF (without LASS):
>   BUG: kernel NULL pointer dereference, address: 0000000000000000
>   BUG: unable to handle page fault for address: 0000000000100000
>
> #GP (with LASS):
>   Oops: general protection fault, kernel NULL pointer dereference 0x0: 0000
>   Oops: general protection fault, probably LASS violation for address
> 0x100000: 0000
>
> For testing user SIGSEGVs, the Vsyscall tests have been sufficient to
> cover all scenarios.
>
> Were you looking for anything specific? I can clean them up and post
> them if required.

LKDTM is basically meant for this use case. If you can’t provoke a LASS failure from there, maybe just add another failure type?  I would expect that LKDTM can already do a SMAP violation.