udf: Check consistency of Space Bitmap Descriptor

Bits, which are related to Bitmap Descriptor logical blocks,
are not reset when buffer headers are allocated for them. As the
result, these logical blocks can be treated as free and
be used for other blocks.This can cause usage of one buffer header
for several types of data. UDF issues WARNING in this situation:

WARNING: CPU: 0 PID: 2703 at fs/udf/inode.c:2014
  __udf_add_aext+0x685/0x7d0 fs/udf/inode.c:2014

RIP: 0010:__udf_add_aext+0x685/0x7d0 fs/udf/inode.c:2014
Call Trace:
 udf_setup_indirect_aext+0x573/0x880 fs/udf/inode.c:1980
 udf_add_aext+0x208/0x2e0 fs/udf/inode.c:2067
 udf_insert_aext fs/udf/inode.c:2233 [inline]
 udf_update_extents fs/udf/inode.c:1181 [inline]
 inode_getblk+0x1981/0x3b70 fs/udf/inode.c:885

Found by Linux Verification Center (linuxtesting.org) with syzkaller.

[JK: Somewhat cleaned up the boundary checks]

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Vladislav Efanov <VEfanov@ispras.ru>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/udf/balloc.c

@@ -36,18 +36,41 @@ static int read_block_bitmap(struct super_block *sb,
 			     unsigned long bitmap_nr)
 {
 	struct buffer_head *bh = NULL;
-	int retval = 0;
+	int i;
+	int max_bits, off, count;
 	struct kernel_lb_addr loc;
 
 	loc.logicalBlockNum = bitmap->s_extPosition;
 	loc.partitionReferenceNum = UDF_SB(sb)->s_partition;
 
 	bh = sb_bread(sb, udf_get_lb_pblock(sb, &loc, block));
+	bitmap->s_block_bitmap[bitmap_nr] = bh;
 	if (!bh)
-		retval = -EIO;
+		return -EIO;
 
-	bitmap->s_block_bitmap[bitmap_nr] = bh;
-	return retval;
+	/* Check consistency of Space Bitmap buffer. */
+	max_bits = sb->s_blocksize * 8;
+	if (!bitmap_nr) {
+		off = sizeof(struct spaceBitmapDesc) << 3;
+		count = min(max_bits - off, bitmap->s_nr_groups);
+	} else {
+		/*
+		 * Rough check if bitmap number is too big to have any bitmap
+ 		 * blocks reserved.
+		 */
+		if (bitmap_nr >
+		    (bitmap->s_nr_groups >> (sb->s_blocksize_bits + 3)) + 2)
+			return 0;
+		off = 0;
+		count = bitmap->s_nr_groups - bitmap_nr * max_bits +
+				(sizeof(struct spaceBitmapDesc) << 3);
+		count = min(count, max_bits);
+	}
+
+	for (i = 0; i < count; i++)
+		if (udf_test_bit(i + off, bh->b_data))
+			return -EFSCORRUPTED;
+	return 0;
 }
 
 static int __load_block_bitmap(struct super_block *sb,