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

[Peter Zijlstra]

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

somewhere along the line __asan_loadN() gets tripped, this then ends up
in kasan_check_range() -> check_region_inline() -> addr_has_metadata().

This latter has: kasan_shadow_to_mem() which is compared against
KASAN_SHADOW_START, which includes, as Kirill says __VIRTUAL_MASK_SHIFT.

Now, obviously you really don't want boot_cpu_has() in
__VIRTUAL_MASK_SHIFT, that would be really bad (Linus recently
complained about how horrible the code-gen is around this already, must
not make it far worse).


Anyway, being half-way through patching X86_FEATURE_LA57 thing *are*
inconsistent and I really can't blame things for going sideways.

That said, I don't particularly like the patch, I think it should, at
the veyr least, cover all of apply_alternatives, not just
text_poke_early().