fs/proc/task_mmu: prevent integer overflow in pagemap_scan_get_args()

The "arg->vec_len" variable is a u64 that comes from the user at the start
of the function.  The "arg->vec_len * sizeof(struct page_region))"
multiplication can lead to integer wrapping.  Use size_mul() to avoid
that.

Also the size_add/mul() functions work on unsigned long so for 32bit
systems we need to ensure that "arg->vec_len" fits in an unsigned long.

Link: https://lkml.kernel.org/r/39d41335-dd4d-48ed-8a7f-402c57d8ea84@stanley.mountain
Fixes: 52526ca ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Andrei Vagin <avagin@google.com>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Dan Carpenter

fs/proc/task_mmu.c

@@ -2665,8 +2665,10 @@ static int pagemap_scan_get_args(struct pm_scan_arg *arg,
 		return -EFAULT;
 	if (!arg->vec && arg->vec_len)
 		return -EINVAL;
+	if (UINT_MAX == SIZE_MAX && arg->vec_len > SIZE_MAX)
+		return -EINVAL;
 	if (arg->vec && !access_ok((void __user *)(long)arg->vec,
-			      arg->vec_len * sizeof(struct page_region)))
+				   size_mul(arg->vec_len, sizeof(struct page_region))))
 		return -EFAULT;
 
 	/* Fixup default values */