ext4: fix ext4_empty_dir() for directories with holes

Function ext4_empty_dir() doesn't correctly handle directories with
holes and crashes on bh->b_data dereference when bh is NULL. Reorganize
the loop to use 'offset' variable all the times instead of comparing
pointers to current direntry with bh->b_data pointer. Also add more
strict checking of '.' and '..' directory entries to avoid entering loop
in possibly invalid state on corrupted filesystems.

References: CVE-2019-19037
CC: stable@vger.kernel.org
Fixes: 4e19d6b6 ("ext4: allow directory holes")
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20191202170213.4761-2-jack@suse.czSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/namei.c

@@ -2822,7 +2822,7 @@ bool ext4_empty_dir(struct inode *inode)
 {
 	unsigned int offset;
 	struct buffer_head *bh;
-	struct ext4_dir_entry_2 *de, *de1;
+	struct ext4_dir_entry_2 *de;
 	struct super_block *sb;
 
 	if (ext4_has_inline_data(inode)) {
@@ -2847,19 +2847,25 @@ bool ext4_empty_dir(struct inode *inode)
 		return true;
 
 	de = (struct ext4_dir_entry_2 *) bh->b_data;
-	de1 = ext4_next_entry(de, sb->s_blocksize);
-	if (le32_to_cpu(de->inode) != inode->i_ino ||
-			le32_to_cpu(de1->inode) == 0 ||
-			strcmp(".", de->name) || strcmp("..", de1->name)) {
-		ext4_warning_inode(inode, "directory missing '.' and/or '..'");
+	if (ext4_check_dir_entry(inode, NULL, de, bh, bh->b_data, bh->b_size,
+				 0) ||
+	    le32_to_cpu(de->inode) != inode->i_ino || strcmp(".", de->name)) {
+		ext4_warning_inode(inode, "directory missing '.'");
+		brelse(bh);
+		return true;
+	}
+	offset = ext4_rec_len_from_disk(de->rec_len, sb->s_blocksize);
+	de = ext4_next_entry(de, sb->s_blocksize);
+	if (ext4_check_dir_entry(inode, NULL, de, bh, bh->b_data, bh->b_size,
+				 offset) ||
+	    le32_to_cpu(de->inode) == 0 || strcmp("..", de->name)) {
+		ext4_warning_inode(inode, "directory missing '..'");
 		brelse(bh);
 		return true;
 	}
-	offset = ext4_rec_len_from_disk(de->rec_len, sb->s_blocksize) +
-		 ext4_rec_len_from_disk(de1->rec_len, sb->s_blocksize);
-	de = ext4_next_entry(de1, sb->s_blocksize);
+	offset += ext4_rec_len_from_disk(de->rec_len, sb->s_blocksize);
 	while (offset < inode->i_size) {
-		if ((void *) de >= (void *) (bh->b_data+sb->s_blocksize)) {
+		if (!(offset & (sb->s_blocksize - 1))) {
 			unsigned int lblock;
 			brelse(bh);
 			lblock = offset >> EXT4_BLOCK_SIZE_BITS(sb);
@@ -2870,12 +2876,11 @@ bool ext4_empty_dir(struct inode *inode)
 			}
 			if (IS_ERR(bh))
 				return true;
-			de = (struct ext4_dir_entry_2 *) bh->b_data;
 		}
+		de = (struct ext4_dir_entry_2 *) (bh->b_data +
+					(offset & (sb->s_blocksize - 1)));
 		if (ext4_check_dir_entry(inode, NULL, de, bh,
 					 bh->b_data, bh->b_size, offset)) {
-			de = (struct ext4_dir_entry_2 *)(bh->b_data +
-							 sb->s_blocksize);
 			offset = (offset | (sb->s_blocksize - 1)) + 1;
 			continue;
 		}
@@ -2884,7 +2889,6 @@ bool ext4_empty_dir(struct inode *inode)
 			return false;
 		}
 		offset += ext4_rec_len_from_disk(de->rec_len, sb->s_blocksize);
-		de = ext4_next_entry(de, sb->s_blocksize);
 	}
 	brelse(bh);
 	return true;