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Mon, 10 Nov 2025 18:15:26 +0000 Message-ID: <29f2d16f-361f-475c-957e-0ebcefcd1a8c@intel.com> Date: Mon, 10 Nov 2025 10:15:23 -0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v11 4/9] x86/alternatives: Disable LASS when patching kernel code To: , Borislav Petkov CC: Jonathan Corbet , "H . Peter Anvin" , "Andy Lutomirski" , Josh Poimboeuf , "Peter Zijlstra" , Ard Biesheuvel , "Kirill A . 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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); > +} > +