Re: [PATCH v10 03/15] x86/alternatives: Disable LASS when patching kernel alternatives

Sohil Mehta
2025-10-21T20:56:00Z

On 10/21/2025 1:03 PM, Borislav Petkov wrote:
> On Mon, Oct 06, 2025 at 11:51:07PM -0700, Sohil Mehta wrote:
>> +static __always_inline void lass_clac(void)
>> +{
>> + alternative("", "clac", X86_FEATURE_LASS);
>> +}
>> +
>> +static __always_inline void lass_stac(void)
>> +{
>> + alternative("", "stac", X86_FEATURE_LASS);
>> +}
>
> So I probably missed the whole discussion on how we arrived at
> lass_{stac,clac}() but just in case, those names sound silly.
>

I am okay with lass_enable()/lass_disable() if we can all agree on it.

PeterZ didn't like lass_enable_enforcement()/lass_enable_enforcement()
when it was proposed. But your suggestion is much shorter, so maybe it
would work for him.
https://lore.kernel.org/lkml/20250626134921.GK1613200@noisy.programming.kicks-ass.net/

Though, there is a slight semantic difference we need to be careful
about. LASS manages 2 types of kernel accesses: Data and Instruction fetch.

1) The STAC/CLAC only control the kernel *data* accesses into the lower
half.

2) CR4.LASS is what truly controls the entire mechanism. If an
instruction fetch needs to happen from a lower address, CR4.LASS must be
cleared to disable LASS completely. (See patch 6 and 7).

In the series, we directly write to the CR4 bits, so they don't have any
wrappers. But in the future, lass_enable()/lass_disable() could be
confusing if wrappers were added for the CR4 toggling.

> IOW, I'd do this ontop:
>

> +#define lass_disable() stac()
> +#define lass_enable() clac()
>

There is an issue here which you had originally objected to.
https://lore.kernel.org/lkml/20240710171836.GGZo7CbFJeZwLCZUAt@fat_crate.local/
https://lore.kernel.org/lkml/20240711012333.GAZo80FU30_x77otP4@fat_crate.local/

These new versions of lass_disable()/lass_enable() will toggle the AC
flag on older platforms without X86_FEATURE_LASS. It definitely makes
the code easier to read and maintain if we are okay with the minor
performance penalty.
> static void text_poke_memcpy(void *dst, const void *src, size_t len)
> {
> - lass_stac();
> + lass_disable();
> __inline_memcpy(dst, src, len);
> - lass_clac();
> + lass_enable();
> }
>
> static void text_poke_memset(void *dst, const void *src, size_t len)
> {
> int c = *(const int *)src;
>
> - lass_stac();
> + lass_disable();
> __inline_memset(dst, c, len);
> - lass_clac();
> + lass_enable();
> }