x86/alternatives: Disable LASS when patching kernel code

For patching, the kernel initializes a temporary mm area in the lower
half of the address range. LASS blocks these accesses because its
enforcement relies on bit 63 of the virtual address as opposed to SMAP
which depends on the _PAGE_BIT_USER bit in the page table. Disable LASS
enforcement by toggling the RFLAGS.AC bit during patching to avoid
triggering a #GP fault.

Introduce LASS-specific STAC/CLAC helpers to set the AC bit only on
platforms that need it. Name the wrappers as lass_stac()/_clac() instead
of lass_disable()/_enable() because they only control the kernel data
access enforcement. The entire LASS mechanism (including instruction
fetch enforcement) is controlled by the CR4.LASS bit.

Describe the usage of the new helpers in comparison to the ones used for
SMAP. Also, add comments to explain when the existing stac()/clac()
should be used. While at it, move the duplicated "barrier" comment to
the same block.

The Text poking functions use standard memcpy()/memset() while patching
kernel code. However, objtool complains about calling such dynamic
functions within an AC=1 region. See warning #9, regarding function
calls with UACCESS enabled, in tools/objtool/Documentation/objtool.txt.

To pacify objtool, one option is to add memcpy() and memset() to the
list of allowed-functions. However, that would provide a blanket
exemption for all usages of memcpy() and memset(). Instead, replace the
standard calls in the text poking functions with their unoptimized,
always-inlined versions. Considering that patching is usually small,
there is no performance impact expected.

Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://patch.msgid.link/20251118182911.2983253-5-sohil.mehta%40intel.com

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