mm/gup_benchmark.c: prevent integer overflow in ioctl

The concern here is that "gup->size" is a u64 and "nr_pages" is unsigned
long. On 32 bit systems we could trick the kernel into allocating fewer
pages than expected.

Link: http://lkml.kernel.org/r/[e-mail]
Fixes: 64c349f4ae78 ("mm: add infrastructure for get_user_pages_fast() benchmarking")
Signed-off-by: Dan Carpenter <[e-mail]>
Acked-by: Kirill A. Shutemov <[e-mail]>
Reviewed-by: Andrew Morton <[e-mail]>
Cc: Stephen Rothwell <[e-mail]>
Cc: Keith Busch <[e-mail]>
Cc: "Michael S. Tsirkin" <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: YueHaibing <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/gup_benchmark.c

···
 	int nr;
 	struct page **pages;
 
+	if (gup->size > ULONG_MAX)
+		return -EINVAL;
+
 	nr_pages = gup->size / PAGE_SIZE;
 	pages = kvcalloc(nr_pages, sizeof(void *), GFP_KERNEL);
 	if (!pages)