Re: [PATCH] x86/alternatives: Disable KASAN on text_poke_early() in apply_alternatives()

Kirill A. Shutemov
2023-10-10T08:41:06Z

On Tue, Oct 10, 2023 at 10:19:38AM +0200, Borislav Petkov wrote:
> On Tue, Oct 10, 2023 at 08:37:16AM +0300, Kirill A. Shutemov wrote:
> > On machines with 5-level paging, cpu_feature_enabled(X86_FEATURE_LA57)
> > got patched. It includes KASAN code, where KASAN_SHADOW_START depends on
> > __VIRTUAL_MASK_SHIFT, which is defined with the cpu_feature_enabled().
>
> So use boot_cpu_has(X86_FEATURE_LA57).

__VIRTUAL_MASK_SHIFT used in many places. I don't think it is good idea to
give up on patching completely.

> > It seems that KASAN gets confused when apply_alternatives() patches the
>
> It seems?

Admittedly, I don't understand KASAN well enough. I confirmed my idea
indirectly, by patching KASASN_SHADOW_START, as I mentioned.

> > KASAN_SHADOW_START users. A test patch that makes KASAN_SHADOW_START
> > static, by replacing __VIRTUAL_MASK_SHIFT with 56, fixes the issue.
> >
> > During text_poke_early() in apply_alternatives(), KASAN should be
> > disabled. KASAN is already disabled in non-_early() text_poke().
> >
> > It is unclear why the issue was not reported earlier. Bisecting does not
> > help. Older kernels trigger the issue less frequently, but it still
> > occurs. In the absence of any other clear offenders, the initial dynamic
> > 5-level paging support is to blame.
>
> This whole thing sounds like it is still not really clear what is
> actually happening...

Maybe KASAN folks can help to understand the situation.

--
Kiryl Shutsemau / Kirill A. Shutemov