queueing: use CFI-safe ptr_ring cleanup function

We make too nuanced use of ptr_ring to entirely move to the skb_array
wrappers, but we at least should avoid the naughty function pointer cast
when cleaning up skbs. Otherwise RAP/CFI will honk at us. This patch
uses the __skb_array_destroy_skb wrapper for the cleanup, rather than
directly providing kfree_skb, which is what other drivers in the same
situation do too.

Reported-by: PaX Team <pageexec@freemail.hu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Author: zx2c4

src/compat/Kbuild.include

@@ -12,6 +12,10 @@ ifeq ($(wildcard $(srctree)/include/linux/ptr_ring.h),)
 ccflags-y += -I$(kbuild-dir)/compat/ptr_ring/include
 endif
 
+ifeq ($(wildcard $(srctree)/include/linux/skb_array.h),)
+ccflags-y += -I$(kbuild-dir)/compat/skb_array/include
+endif
+
 ifeq ($(wildcard $(srctree)/include/linux/siphash.h),)
 ccflags-y += -I$(kbuild-dir)/compat/siphash/include
 wireguard-y += compat/siphash/siphash.o

src/compat/skb_array/include/linux/skb_array.h

@@ -0,0 +1,11 @@
+#ifndef _WG_SKB_ARRAY_H
+#define _WG_SKB_ARRAY_H
+
+#include <linux/skbuff.h>
+
+static void __skb_array_destroy_skb(void *ptr)
+{
+	kfree_skb(ptr);
+}
+
+#endif

src/queueing.c

@@ -4,6 +4,7 @@
  */
 
 #include "queueing.h"
+#include <linux/skb_array.h>
 
 struct multicore_worker __percpu *
 wg_packet_percpu_multicore_worker_alloc(work_func_t function, void *ptr)
@@ -42,7 +43,7 @@ void wg_packet_queue_free(struct crypt_queue *queue, bool purge)
 {
 	free_percpu(queue->worker);
 	WARN_ON(!purge && !__ptr_ring_empty(&queue->ring));
-	ptr_ring_cleanup(&queue->ring, purge ? (void(*)(void*))kfree_skb : NULL);
+	ptr_ring_cleanup(&queue->ring, purge ? __skb_array_destroy_skb : NULL);
 }
 
 #define NEXT(skb) ((skb)->prev)