[PATCHv3] x86/alternatives: Disable KASAN in apply_alternatives()

[Kirill A. Shutemov]

Fei has reported that KASAN triggers during apply_alternatives() on
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)
got patched. It includes KASAN code, where KASAN_SHADOW_START depends on
__VIRTUAL_MASK_SHIFT, which is defined with the 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, fixes the issue.

Disable KASAN while kernel patches alternatives.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reported-by: Fei Yang <fei.yang@xxxxxxxxx>
Fixes: 6657fca06e3f ("x86/mm: Allow to boot without LA57 if CONFIG_X86_5LEVEL=y")
Cc: stable@xxxxxxxxxxxxxxx
---
 v3:
  - Summarize KASAN splat;
  - Update comment in apply_alternatives();

 v2:
  - Move kasan_disable/_enable_current() to cover whole loop, not only
    text_poke_early();
  - Adjust commit message.
---
 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 517ee01503be..73be3931e4f0 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -403,6 +403,17 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 	u8 insn_buff[MAX_PATCH_LEN];
 
 	DPRINTK(ALT, "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.
@@ -452,6 +463,8 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 
 		text_poke_early(instr, insn_buff, insn_buff_sz);
 	}
+
+	kasan_enable_current();
 }
 
 static inline bool is_jcc32(struct insn *insn)
-- 
2.41.0