fs/partitions/ldm.c: fix oops caused by corrupted partition table

The kernel automatically evaluates partition tables of storage devices.
The code for evaluating LDM partitions (in fs/partitions/ldm.c) contains
a bug that causes a kernel oops on certain corrupted LDM partitions.
A kernel subsystem seems to crash, because, after the oops, the kernel no
longer recognizes newly connected storage devices.

The patch validates the value of vblk_size.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Timo Warns <warns@pre-sense.de>
Cc: Eugene Teo <eugeneteo@kernel.sg>
Cc: Harvey Harrison <harvey.harrison@gmail.com>
Cc: Richard Russon <rich@flatcap.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/partitions/ldm.c

@@ -1299,13 +1299,22 @@ static bool ldm_frag_add (const u8 *data, int size, struct list_head *frags)
 
 	BUG_ON (!data || !frags);
 
+	if (size < 2 * VBLK_SIZE_HEAD) {
+		ldm_error("Value of size is to small.");
+		return false;
+	}
+
 	group = get_unaligned_be32(data + 0x08);
 	rec   = get_unaligned_be16(data + 0x0C);
 	num   = get_unaligned_be16(data + 0x0E);
 	if ((num < 1) || (num > 4)) {
 		ldm_error ("A VBLK claims to have %d parts.", num);
 		return false;
 	}
+	if (rec >= num) {
+		ldm_error("REC value (%d) exceeds NUM value (%d)", rec, num);
+		return false;
+	}
 
 	list_for_each (item, frags) {
 		f = list_entry (item, struct frag, list);
@@ -1334,10 +1343,9 @@ static bool ldm_frag_add (const u8 *data, int size, struct list_head *frags)
 
 	f->map |= (1 << rec);
 
-	if (num > 0) {
-		data += VBLK_SIZE_HEAD;
-		size -= VBLK_SIZE_HEAD;
-	}
+	data += VBLK_SIZE_HEAD;
+	size -= VBLK_SIZE_HEAD;
+
 	memcpy (f->data+rec*(size-VBLK_SIZE_HEAD)+VBLK_SIZE_HEAD, data, size);
 
 	return true;