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

Sohil Mehta
2025-11-10T18:15:31Z

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);
> +}
> +