[PATCH v2 1/2] kcsan: selftest: Ensure that address is at least PAGE_SIZE

[PATCH v2 1/2] kcsan: selftest: Ensure that address is at least PAGE_SIZE

[Marco Elver]

In preparation of supporting only addresses not within the NULL page,
change the selftest to never use addresses that are less than PAGE_SIZE.

Signed-off-by: Marco Elver <elver@google.com>
---
v2:
* Introduce patch to series.
---
 kernel/kcsan/selftest.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/kcsan/selftest.c b/kernel/kcsan/selftest.c
index d98bc208d06d..9014a3a82cf9 100644
--- a/kernel/kcsan/selftest.c
+++ b/kernel/kcsan/selftest.c
@@ -33,6 +33,9 @@ static bool test_encode_decode(void)
 		unsigned long addr;
 
 		prandom_bytes(&addr, sizeof(addr));
+		if (addr < PAGE_SIZE)
+			addr = PAGE_SIZE;
+
 		if (WARN_ON(!check_encodable(addr, size)))
 			return false;
 
-- 
2.29.0.rc1.297.gfa9743e501-goog

[PATCH v2 2/2] 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 v2 1/2] kcsan: selftest: Ensure that address is at least PAGE_SIZE

[Dmitry Vyukov]

On Thu, Oct 22, 2020 at 1:45 PM Marco Elver <elver@google.com> wrote:
>
> In preparation of supporting only addresses not within the NULL page,
> change the selftest to never use addresses that are less than PAGE_SIZE.
>
> Signed-off-by: Marco Elver <elver@google.com>

Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

> ---
> v2:
> * Introduce patch to series.
> ---
>  kernel/kcsan/selftest.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/kernel/kcsan/selftest.c b/kernel/kcsan/selftest.c
> index d98bc208d06d..9014a3a82cf9 100644
> --- a/kernel/kcsan/selftest.c
> +++ b/kernel/kcsan/selftest.c
> @@ -33,6 +33,9 @@ static bool test_encode_decode(void)
>                 unsigned long addr;
>
>                 prandom_bytes(&addr, sizeof(addr));
> +               if (addr < PAGE_SIZE)
> +                       addr = PAGE_SIZE;
> +
>                 if (WARN_ON(!check_encodable(addr, size)))
>                         return false;
>
> --
> 2.29.0.rc1.297.gfa9743e501-goog
>

Re: [PATCH v2 2/2] kcsan: Never set up watchpoints on NULL pointers

[Dmitry Vyukov]

On Thu, Oct 22, 2020 at 1:45 PM 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>

Reviewed-by: Dmitry Vyukov <dvyukov@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
>