[tip: core/rcu] kcsan: Never set up watchpoints on NULL pointers

[tip-bot2 for Marco Elver]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     55a2346c7ac4bbf6ee6972394237bf31e29a1c05
Gitweb:        https://git.kernel.org/tip/55a2346c7ac4bbf6ee6972394237bf31e29a1c05
Author:        Marco Elver <elver@xxxxxxxxxx>
AuthorDate:    Thu, 22 Oct 2020 13:45:53 +02:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Mon, 02 Nov 2020 17:08:51 -08:00

kcsan: Never set up watchpoints on NULL pointers

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@xxxxxxxxxx>
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 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