libceph: fix overflow in osdmap_apply_incremental()

On 32-bit systems, a large `pglen' would overflow `pglen*sizeof(u32)'
and bypass the check ceph_decode_need(p, end, pglen*sizeof(u32), bad).
It would also overflow the subsequent kmalloc() size, leading to
out-of-bounds write.
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Reviewed-by: Alex Elder <elder@inktank.com>

net/ceph/osdmap.c

@@ -893,6 +893,10 @@ struct ceph_osdmap *osdmap_apply_incremental(void **p, void *end,
 			(void) __remove_pg_mapping(&map->pg_temp, pgid);
 
 			/* insert */
+			if (pglen > (UINT_MAX - sizeof(*pg)) / sizeof(u32)) {
+				err = -EINVAL;
+				goto bad;
+			}
 			pg = kmalloc(sizeof(*pg) + sizeof(u32)*pglen, GFP_NOFS);
 			if (!pg) {
 				err = -ENOMEM;