Re: [PATCH 1/2] hfs: bound the partition-table hop in hfs_mdb_get()

[Viacheslav Dubeyko <slava@dubeyko.com>]

On Sat, 2026-09-26 at 16:40 +0800, Matthias Goergens wrote:
> 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))

If I understood the problem correctly, then we  have issue in
hfs_part_find(). This method should have the proper sanity check.

Thanks,
Slava.

>  			return -EIO;
> +		part_hop_done = true;
>  	}
>  
>  	HFS_SB(sb)->alloc_blksz = size = be32_to_cpu(mdb-
> >drAlBlkSiz);
> 
> base-commit: 6812ce4e4379ffc99c52401ec28f0d7ffbc36206