linux-kernel - Re: [PATCH v11 4/9] x86/alternatives: Disable LASS when patching kernel code

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Message-ID: <404E225C-B6FC-4FCF-B4D9-0D079C72E8B2@zytor.com>
Date: Mon, 10 Nov 2025 11:09:56 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Sohil Mehta <sohil.mehta@...el.com>, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>
CC: Jonathan Corbet <corbet@....net>, 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, Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v11 4/9] x86/alternatives: Disable LASS when patching kernel code

On November 10, 2025 10:15:23 AM PST, Sohil Mehta <sohil.mehta@...el.com> wrote:
>Hi Boris,
>
>On 10/29/2025 2:03 PM, Sohil Mehta wrote:
>> +/*
>> + * LASS enforcement is based on bit 63 of the virtual address. The
>> + * kernel is not allowed to touch memory in the lower half of the
>> + * virtual address space.
>> + *
>> + * Use lass_disable()/lass_enable() to toggle the AC bit for kernel data
>> + * accesses (!_PAGE_USER) that are blocked by LASS, but not by SMAP.
>> + *
>> + * Even with the AC bit set, LASS will continue to block instruction
>> + * fetches from the user half of the address space. To allow those,
>> + * clear CR4.LASS to disable the LASS mechanism entirely.
>> + *
>
>Based on the EFI discussion, it looks like we would now need to toggle
>CR4.LASS every time we switch to efi_mm. The lass_enable()/_disable()
>naming would be more suitable for those wrappers.
>
>I am thinking of reverting this back to lass_clac()/lass_stac().
>
>lass_clac()/_stac():
>	Disable enforcement for kernel data accesses similar to SMAP.
>
>lass_enable()/_disable():
>	Disable the entire LASS mechanism (Data and instruction fetch)
>	by toggling CR4.LASS
>
>Would that work? Any other suggestions?
>
>
>> +
>> +static __always_inline void lass_enable(void)
>> +{
>> +	alternative("", "clac", X86_FEATURE_LASS);
>> +}
>> +
>> +static __always_inline void lass_disable(void)
>> +{
>> +	alternative("", "stac", X86_FEATURE_LASS);
>> +}
>> +

That would be my suggestion for making, too.