[PATCH] reject corrupt swapfiles earlier

The fsfuzzer found this; with a corrupt small swapfile that claims to have
many pages:

  [root]# file swap.741.img
  swap.741.img: Linux/i386 swap file (new style) 1 (4K pages) size 1040191487 pages
  [root]# ls -l swap.741.img
  -rw-r--r-- 1 root root 16777216 Nov 22 05:18 swap.741.img

sys_swapon() will try to vmalloc all those pages, and -then- check to see if
the file is actually that large:

                if (!(p->swap_map = vmalloc(maxpages * sizeof(short)))) {
  <snip>
        if (swapfilesize && maxpages > swapfilesize) {
                printk(KERN_WARNING
                       "Swap area shorter than signature indicates\n");

It seems to me that it would make more sense to move this test up before
the vmalloc, with the other checks, to avoid the OOM-killer in this
situation...
Signed-off-by: NEric Sandeen <sandeen@redhat.com>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

mm/swapfile.c

@@ -1552,6 +1552,11 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags)
 		error = -EINVAL;
 		if (!maxpages)
 			goto bad_swap;
+		if (swapfilesize && maxpages > swapfilesize) {
+			printk(KERN_WARNING
+			       "Swap area shorter than signature indicates\n");
+			goto bad_swap;
+		}
 		if (swap_header->info.nr_badpages && S_ISREG(inode->i_mode))
 			goto bad_swap;
 		if (swap_header->info.nr_badpages > MAX_SWAP_BADPAGES)
@@ -1579,12 +1584,6 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags)
 			goto bad_swap;
 	}
 
-	if (swapfilesize && maxpages > swapfilesize) {
-		printk(KERN_WARNING
-		       "Swap area shorter than signature indicates\n");
-		error = -EINVAL;
-		goto bad_swap;
-	}
 	if (nr_good_pages) {
 		p->swap_map[0] = SWAP_MAP_BAD;
 		p->max = maxpages;