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 ]
Fixes: 6657fca06e3f ("x86/mm: Allow to boot without LA57 if CONFIG_X86_5LEVEL=y") Reported-by: Fei Yang fei.yang@intel.com Signed-off-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com Signed-off-by: Ingo Molnar mingo@kernel.org Acked-by: Peter Zijlstra (Intel) peterz@infradead.org Cc: Linus Torvalds torvalds@linux-foundation.org Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/20231012100424.1456-1-kirill.shutemov@linux.intel.... (cherry picked from commit d35652a5fc9944784f6f50a5c979518ff8dacf61) Signed-off-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com --- arch/x86/kernel/alternative.c | 13 +++++++++++++ 1 file changed, 13 insertions(+)
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c index e6e63a9d27cb..d58621f0fe51 100644 --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -424,6 +424,17 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start, u8 insn_buff[MAX_PATCH_LEN];
DPRINTK("alt table %px, -> %px", start, end); + + /* + * In the case CONFIG_X86_5LEVEL=y, KASAN_SHADOW_START is defined using + * cpu_feature_enabled(X86_FEATURE_LA57) and is therefore patched here. + * During the process, KASAN becomes confused seeing partial LA57 + * conversion and triggers a false-positive out-of-bound report. + * + * Disable KASAN until the patching is complete. + */ + kasan_disable_current(); + /* * The scan order should be from start to end. A later scanned * alternative code can overwrite previously scanned alternative code. @@ -491,6 +502,8 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start, next: optimize_nops(instr, a->instrlen); } + + kasan_enable_current(); }
#if defined(CONFIG_RETPOLINE) && defined(CONFIG_STACK_VALIDATION)