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/[e-mail]
Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs")
Signed-off-by: Dan Carpenter <[e-mail]>
Cc: Andrei Vagin <[e-mail]>
Cc: Andrii Nakryiko <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Michał Mirosław <[e-mail]>
Cc: Muhammad Usama Anjum <[e-mail]>
Cc: Oscar Salvador <[e-mail]>
Cc: Peter Xu <[e-mail]>
Cc: Ryan Roberts <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

fs/proc/task_mmu.c

···
 		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 */