udf: prevent allocation beyond UDF partition

The UDF bitmap allocation code assumes that a recorded
Unallocated Space Bitmap is compliant with ECMA-167 4/13,
which requires that pad bytes between the end of the bitmap
and the end of a logical block are all zero.

When a recorded bitmap does not comply with this requirement,
for example one padded with FF to the block boundary instead
of 00, the allocator may "allocate" blocks that are outside
the UDF partition extent. This can result in UDF volume descriptors
being overwritten by file data or by partition-level descriptors,
and in extreme cases, even in scribbling on a subsequent disk partition.

Add a check that the block selected by the allocator actually
resides within the UDF partition extent.

Signed-off-by: Steven J. Magnani <steve@digidescorp.com>

Link: https://lore.kernel.org/r/1564341552-129750-1-git-send-email-steve@digidescorp.com
Signed-off-by: Jan Kara <jack@suse.cz>

fs/udf/balloc.c

@@ -325,6 +325,17 @@ got_block:
 	newblock = bit + (block_group << (sb->s_blocksize_bits + 3)) -
 		(sizeof(struct spaceBitmapDesc) << 3);
 
+	if (newblock >= sbi->s_partmaps[partition].s_partition_len) {
+		/*
+		 * Ran off the end of the bitmap, and bits following are
+		 * non-compliant (not all zero)
+		 */
+		udf_err(sb, "bitmap for partition %d corrupted (block %u marked"
+			" as free, partition length is %u)\n", partition,
+			newblock, sbi->s_partmaps[partition].s_partition_len);
+		goto error_return;
+	}
+
 	if (!udf_clear_bit(bit, bh->b_data)) {
 		udf_debug("bit already cleared for block %d\n", bit);
 		goto repeat;