CVE-2023-52504

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, cpufeatureenabled(X86FEATURELA57) gets patched. It includes KASAN code, where KASANSHADOWSTART depends on __VIRTUALMASKSHIFT, which is defined with cpufeatureenabled().

KASAN gets confused when applyalternatives() patches the KASANSHADOWSTART users. A test patch that makes KASANSHADOW_START static, by replacing __VIRTUALMASKSHIFT with 56, works around the issue.

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

[ mingo: updated the changelog ]