[PATCH] kcsan: Never set up watchpoints on NULL pointers

[PATCH] kcsan: Never set up watchpoints on NULL pointers

[Marco Elver]

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.

Signed-off-by: Marco Elver <elver@google.com>
---
 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 f03562aaf2eb..64b3c0f2a685 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
-- 
2.29.0.rc1.297.gfa9743e501-goog

Re: [PATCH] kcsan: Never set up watchpoints on NULL pointers

[Marco Elver]

On Thu, 22 Oct 2020 at 13:30, Marco Elver <elver@google.com> wrote:
>
> 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.
>
> Signed-off-by: Marco Elver <elver@google.com>
> ---

Please use the series at:
https://lkml.kernel.org/r/20201022114553.2440135-1-elver@google.com
which includes a prerequisite patch that was missing.

Thanks,
-- Marco