ptr_ring: prevent integer overflow when calculating size

[ Upstream commit 54e02162 ]

Switch to use dividing to prevent integer overflow when size is too
big to calculate allocation size properly.
Reported-by: Eric Biggers <ebiggers3@gmail.com>
Fixes: 6e6e41c3 ("ptr_ring: fail early if queue occupies more than KMALLOC_MAX_SIZE")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/ptr_ring.h

@@ -351,7 +351,7 @@ static inline void *ptr_ring_consume_bh(struct ptr_ring *r)
 
 static inline void **__ptr_ring_init_queue_alloc(unsigned int size, gfp_t gfp)
 {
-	if (size * sizeof(void *) > KMALLOC_MAX_SIZE)
+	if (size > KMALLOC_MAX_SIZE / sizeof(void *))
 		return NULL;
 	return kcalloc(size, sizeof(void *), gfp);
 }