swap: divide-by-zero when zero length swap file on ssd

Calling swapon() on a zero length swap file on SSD can lead to a
divide-by-zero.

Although creating such files isn't possible with mkswap and they woud be
considered invalid, it would be better for the swapon code to be more
robust and handle this condition gracefully (return -EINVAL).
Especially since the fix is small and straightforward.

To help with wear leveling on SSD, the swapon syscall calculates a
random position in the swap file using modulo p->highest_bit, which is
set to maxpages - 1 in read_swap_header.

If the swap file is zero length, read_swap_header sets maxpages=1 and
last_page=0, resulting in p->highest_bit=0 and we divide-by-zero when we
modulo p->highest_bit in swapon syscall.

This can be prevented by having read_swap_header return zero if
last_page is zero.

Link: http://lkml.kernel.org/r/5AC747C1020000A7001FA82C@prv-mh.provo.novell.com
Signed-off-by: Thomas Abraham <tabraham@suse.com>
Reported-by: <Mark.Landis@Teradata.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swapfile.c

@@ -2961,6 +2961,10 @@ static unsigned long read_swap_header(struct swap_info_struct *p,
 	maxpages = swp_offset(pte_to_swp_entry(
 			swp_entry_to_pte(swp_entry(0, ~0UL)))) + 1;
 	last_page = swap_header->info.last_page;
+	if (!last_page) {
+		pr_warn("Empty swap-file\n");
+		return 0;
+	}
 	if (last_page > maxpages) {
 		pr_warn("Truncating oversized swap area, only using %luk out of %luk\n",
 			maxpages << (PAGE_SHIFT - 10),