Defining a prctl flag as an int is a footgun because on a 64 bit machine
and with a variadic implementation of prctl (like in musl and glibc),
when used directly as a prctl argument, it can get casted to long with
garbage upper bits which would result in unexpected behaviors.

This patch changes the constant to an unsigned long to eliminate that
possibilities. This does not break UAPI.

Fixes: b507808ebce2 ("mm: implement memory-deny-write-execute as a prctl")
Cc: linux-stable@vger.kernel.org
Signed-off-by: Florent Revest <revest@chromium.org>
Suggested-by: Alexey Izbyshev <izbyshev@ispras.ru>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
---
 include/uapi/linux/prctl.h       | 2 +-
 tools/include/uapi/linux/prctl.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index f23d9a16507f..6e9af6cbc950 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -283,7 +283,7 @@ struct prctl_mm_map {
 
 /* Memory deny write / execute */
 #define PR_SET_MDWE			65
-# define PR_MDWE_REFUSE_EXEC_GAIN	1
+# define PR_MDWE_REFUSE_EXEC_GAIN	(1UL << 0)
 
 #define PR_GET_MDWE			66
 
diff --git a/tools/include/uapi/linux/prctl.h b/tools/include/uapi/linux/prctl.h
index f23d9a16507f..6e9af6cbc950 100644
--- a/tools/include/uapi/linux/prctl.h
+++ b/tools/include/uapi/linux/prctl.h
@@ -283,7 +283,7 @@ struct prctl_mm_map {
 
 /* Memory deny write / execute */
 #define PR_SET_MDWE			65
-# define PR_MDWE_REFUSE_EXEC_GAIN	1
+# define PR_MDWE_REFUSE_EXEC_GAIN	(1UL << 0)
 
 #define PR_GET_MDWE			66
 
-- 
2.41.0.255.g8b1d071c50-goog

On Tue, Jul 04, 2023 at 05:36:27PM +0200, Florent Revest wrote:
> Defining a prctl flag as an int is a footgun because on a 64 bit machine
> and with a variadic implementation of prctl (like in musl and glibc),
> when used directly as a prctl argument, it can get casted to long with
> garbage upper bits which would result in unexpected behaviors.
> 
> This patch changes the constant to an unsigned long to eliminate that
> possibilities. This does not break UAPI.
> 
> Fixes: b507808ebce2 ("mm: implement memory-deny-write-execute as a prctl")
> Cc: linux-stable@vger.kernel.org
> Signed-off-by: Florent Revest <revest@chromium.org>

Ah yes. I remember this pain with seccomp. :)

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook

On Tue, Jul 4, 2023 at 5:37 PM Florent Revest <revest@chromium.org> wrote:
>
> Defining a prctl flag as an int is a footgun because on a 64 bit machine
> and with a variadic implementation of prctl (like in musl and glibc),
> when used directly as a prctl argument, it can get casted to long with
> garbage upper bits which would result in unexpected behaviors.
>
> This patch changes the constant to an unsigned long to eliminate that
> possibilities. This does not break UAPI.
>
> Fixes: b507808ebce2 ("mm: implement memory-deny-write-execute as a prctl")
> Cc: linux-stable@vger.kernel.org

Oops, this was supposed to be stable@vger.kernel.org... I'll fix that
tag as part of v4.