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x86/cpu: Don't transiently clear the boot CPU's capabilities

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On the boot CPU, identify_cpu() runs from arch_cpu_finalize_init(), with
interrupts enabled and before alternatives are patched. So
cpu_feature_enabled() still evaluates against boot_cpu_data.

identify_cpu() rebuilds c->x86_capability from scratch: the reset zeroes the
array and the CPUID rescan fills it in again. An interrupt delivered in that
window finds X86_FEATURE_LA57 clear in boot_cpu_data, so pgtable_l5_enabled()
is false and KASAN checks a 5-level address against the 4-level addressability
limit. The result is a bogus "wild-memory-access" report, and under
kasan_multi_shot a report storm that wedges the boot.

The boot CPU has already been scanned by early_identify_cpu(), with interrupts
disabled, and its capabilities cannot have changed since. Reset only the CPUs
which have not been scanned yet.

The window is as old as identify_cpu() rebuilding the capabilities.  Commit

  39b9552281 ("x86/mm: Optimize boot-time paging mode switching cost")

merely let KASAN notice it by making pgtable_l5_enabled() read the feature
bit. So no Fixes: tag.

Closes: https://lore.kernel.org/bpf/20260610175651.647515-1-ihor.solodrai@linux.dev/
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://patch.msgid.link/20260916195203.1099646-6-ihor.solodrai@linux.dev
This commit is contained in:
Ihor Solodrai 2026-09-16 12:52:03 -07:00 committed by Borislav Petkov (AMD)
parent db349783ca
commit 2711d67bc3
1 changed files with 1 additions and 2 deletions
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3
arch/x86/kernel/cpu/common.c
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@ -2000,8 +2000,6 @@ static void identify_cpu(struct cpuinfo_x86 *c)
c->loops_per_jiffy = loops_per_jiffy;
init_cpu_info(c);
if (!cpuid_feature())
identify_cpu_without_cpuid(c);
@ -2186,6 +2184,7 @@ void identify_secondary_cpu(unsigned int cpu)
*c = boot_cpu_data;
c->cpu_index = cpu;
init_cpu_info(c);
identify_cpu(c);
x86_spec_ctrl_setup_ap();
update_srbds_msr();