udf: validate VAT header length against the VAT inode size

udf: validate VAT header length against the VAT inode size

udf_load_vat() takes the virtual partition's start offset straight from
the on-disk VAT 2.0 header without checking it against the VAT inode
size:

	map->s_type_specific.s_virtual.s_start_offset =
		le16_to_cpu(vat20->lengthHeader);
	map->s_type_specific.s_virtual.s_num_entries =
		(sbi->s_vat_inode->i_size -
			map->s_type_specific.s_virtual.s_start_offset) >> 2;

lengthHeader is a fully attacker-controlled 16-bit value.  If it exceeds
the VAT inode size, the s_num_entries subtraction underflows to a huge
count, which defeats the "block > s_num_entries" bound in
udf_get_pblock_virt15(); and on the ICB-inline path that function reads

	((__le32 *)(iinfo->i_data + s_start_offset))[block]

so a large s_start_offset indexes past the inode's in-ICB data.  Mounting
a crafted UDF image with a virtual (VAT) partition then triggers an
out-of-bounds read.

Reject a VAT whose header length does not leave room for at least one
entry within the VAT inode.

Fixes: fa5e08156335 ("udf: Handle VAT packed inside inode properly")
Cc: stable@vger.kernel.org
Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
Link: https://patch.msgid.link/20260612-b4-disp-9a2317ee-v1-1-fefef5736154@proton.me
Signed-off-by: Jan Kara <jack@suse.cz>