hfs_mdb_get() loops "while (1)" around hfs_part_find(), rereading the
volume header wherever the partition table points.  A partition entry
with a zero start (pdStart or pmPyPartStart) leaves part_start where it
was, and the loop reads the same blocks forever.

An Apple partition map sits at the start of the raw device: the boot ROM
reads it from block 1 (TN1189), and nothing Apple documents nests one
inside a partition.  Follow at most one partition-table hop and fail
with -EIO after that, as for a table that cannot be parsed.  A zero
start is caught on the second pass, when the loop comes round again.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Matthias Goergens <matthias.goergens@gmail.com>
---
A partition entry pointing at its own table hangs the mount forever
without this patch and fails at once with it:

  img=hfs-partmap-loop.img
  put() { printf "$2" | dd of=$img bs=1 seek=$1 conv=notrunc status=none; }
  truncate --size=64K $img
  put $((512+0x00)) '\x50\x4d'          # pmSig 'PM'
  put $((512+0x04)) '\x00\x00\x00\x01'  # pmMapBlkCnt 1
  put $((512+0x08)) '\x00\x00\x00\x00'  # pmPyPartStart 0 (self)
  put $((512+0x0c)) '\x00\x00\x00\x64'  # pmPartBlkCnt 100
  put $((512+0x30)) 'Apple_HFS'         # pmPartType
  mount -o ro,loop -t hfs $img /mnt
---
 fs/hfs/mdb.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/hfs/mdb.c b/fs/hfs/mdb.c
index 277de712f9d4..1704f332e28d 100644
--- a/fs/hfs/mdb.c
+++ b/fs/hfs/mdb.c
@@ -134,6 +134,7 @@ int hfs_mdb_get(struct super_block *sb)
 	sector_t part_start, part_size;
 	loff_t off;
 	__be16 attrib;
+	bool part_hop_done = false;
 
 	/* set the device driver to 512-byte blocks */
 	size = sb_min_blocksize(sb, HFS_SECTOR_SIZE);
@@ -152,11 +153,18 @@ int hfs_mdb_get(struct super_block *sb)
 			break;
 		brelse(bh);
 
-		/* check for a partition block
+		/*
+		 * check for a partition block
 		 * (should do this only for cdrom/loop though)
+		 *
+		 * The partition map is at the start of the device: follow
+		 * it once, or an entry pointing at itself loops forever.
 		 */
+		if (part_hop_done)
+			return -EIO;
 		if (hfs_part_find(sb, &part_start, &part_size))
 			return -EIO;
+		part_hop_done = true;
 	}
 
 	HFS_SB(sb)->alloc_blksz = size = be32_to_cpu(mdb->drAlBlkSiz);

base-commit: 6812ce4e4379ffc99c52401ec28f0d7ffbc36206
-- 
2.55.0