linux-kernel - [PATCH kcsan 2/3] kcsan: Never set up watchpoints on NULL pointers

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Date:   Thu,  5 Nov 2020 14:03:23 -0800
From:   paulmck@...nel.org
To:     linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        kernel-team@...com, mingo@...nel.org
Cc:     elver@...gle.com, andreyknvl@...gle.com, glider@...gle.com,
        dvyukov@...gle.com, cai@....pw, boqun.feng@...il.com,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH kcsan 2/3] kcsan: Never set up watchpoints on NULL pointers

From: Marco Elver <elver@...gle.com>

Avoid setting up watchpoints on NULL pointers, as otherwise we would
crash inside the KCSAN runtime (when checking for value changes) instead
of the instrumented code.

Because that may be confusing, skip any address less than PAGE_SIZE.

Reviewed-by: Dmitry Vyukov <dvyukov@...gle.com>
Signed-off-by: Marco Elver <elver@...gle.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/kcsan/encoding.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/kcsan/encoding.h b/kernel/kcsan/encoding.h
index 1a6db2f..4f73db6 100644
--- a/kernel/kcsan/encoding.h
+++ b/kernel/kcsan/encoding.h
@@ -48,7 +48,11 @@
 
 static inline bool check_encodable(unsigned long addr, size_t size)
 {
-	return size <= MAX_ENCODABLE_SIZE;
+	/*
+	 * While we can encode addrs<PAGE_SIZE, avoid crashing with a NULL
+	 * pointer deref inside KCSAN.
+	 */
+	return addr >= PAGE_SIZE && size <= MAX_ENCODABLE_SIZE;
 }
 
 static inline long
-- 
2.9.5