x86/alternatives: Disable KASAN in apply_alternatives()

In the Linux kernel, the following vulnerability has been resolved:

x86/alternatives: Disable KASAN in apply_alternatives()

Fei has reported that KASAN triggers during apply_alternatives() on

a 5-level paging machine:

BUG: KASAN: out-of-bounds in rcu_is_watching()

Read of size 4 at addr ff110003ee6419a0 by task swapper/0/0

...

__asan_load4()

rcu_is_watching()

trace_hardirqs_on()

text_poke_early()

apply_alternatives()

...

On machines with 5-level paging, cpu_feature_enabled(X86_FEATURE_LA57)

gets patched. It includes KASAN code, where KASAN_SHADOW_START depends on

__VIRTUAL_MASK_SHIFT, which is defined with cpu_feature_enabled().

KASAN gets confused when apply_alternatives() patches the

KASAN_SHADOW_START users. A test patch that makes KASAN_SHADOW_START

static, by replacing __VIRTUAL_MASK_SHIFT with 56, works around the issue.

Fix it for real by disabling KASAN while the kernel is patching alternatives.

[ mingo: updated the changelog ]