kmsan: allow using __msan_instrument_asm_store() inside runtime

In certain cases (e.g.  when handling a softirq)
__msan_instrument_asm_store(&var, sizeof(var)) may be called with from
within KMSAN runtime, but later the value of @var is used with
!kmsan_in_runtime(), leading to false positives.

Because kmsan_internal_unpoison_memory() doesn't take locks, it should be
fine to call it without kmsan_in_runtime() checks, which fixes the
mentioned false positives.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Alexander Potapenko <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Eric Biggers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Marco Elver <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/kmsan/instrumentation.c

@@ -81,12 +81,16 @@ DECLARE_METADATA_PTR_GETTER(8);
  * Handle a memory store performed by inline assembly. KMSAN conservatively
  * attempts to unpoison the outputs of asm() directives to prevent false
  * positives caused by missed stores.
+ *
+ * __msan_instrument_asm_store() may be called for inline assembly code when
+ * entering or leaving IRQ. We omit the check for kmsan_in_runtime() to ensure
+ * the memory written to in these cases is also marked as initialized.
  */
 void __msan_instrument_asm_store(void *addr, uintptr_t size)
 {
  unsigned long ua_flags;
 
- if (!kmsan_enabled || kmsan_in_runtime())
+ if (!kmsan_enabled)
  return;
 
  ua_flags = user_access_save();
@@ -103,10 +107,8 @@ void __msan_instrument_asm_store(void *addr, uintptr_t size)
  user_access_restore(ua_flags);
  return;
  }
- kmsan_enter_runtime();
  /* Unpoisoning the memory on best effort. */
  kmsan_internal_unpoison_memory(addr, size, /*checked*/ false);
- kmsan_leave_runtime();
  user_access_restore(ua_flags);
 }
 EXPORT_SYMBOL(__msan_instrument_asm_store);