linux-kernel - [PATCH 2/2] kmsan: allow using __msan_instrument_asm

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Message-ID: <20221128094541.2645890-2-glider@google.com>
Date:   Mon, 28 Nov 2022 10:45:41 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     glider@...gle.com
Cc:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        peterz@...radead.org, mingo@...hat.com, will@...nel.org,
        elver@...gle.com, dvyukov@...gle.com, linux-mm@...ck.org,
        kasan-dev@...glegroups.com
Subject: [PATCH 2/2] 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.

Signed-off-by: Alexander Potapenko <glider@...gle.com>
---
 mm/kmsan/instrumentation.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/kmsan/instrumentation.c b/mm/kmsan/instrumentation.c
index 271f135f97a16..770fe02904f36 100644
--- a/mm/kmsan/instrumentation.c
+++ b/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);
-- 
2.38.1.584.g0f3c55d4c2-goog