ext4: make sure the first directory block is not a hole

JIRA: https://issues.redhat.com/browse/RHEL-54975
CVE: CVE-2024-42304

commit f9ca515
Author: Baokun Li <libaokun1@huawei.com>
Date:   Tue Jul 2 21:23:49 2024 +0800

    ext4: make sure the first directory block is not a hole

    The syzbot constructs a directory that has no dirblock but is non-inline,
    i.e. the first directory block is a hole. And no errors are reported when
    creating files in this directory in the following flow.

        ext4_mknod
         ...
          ext4_add_entry
            // Read block 0
            ext4_read_dirblock(dir, block, DIRENT)
              bh = ext4_bread(NULL, inode, block, 0)
              if (!bh && (type == INDEX || type == DIRENT_HTREE))
              // The first directory block is a hole
              // But type == DIRENT, so no error is reported.

    After that, we get a directory block without '.' and '..' but with a valid
    dentry. This may cause some code that relies on dot or dotdot (such as
    make_indexed_dir()) to crash.

    Therefore when ext4_read_dirblock() finds that the first directory block
    is a hole report that the filesystem is corrupted and return an error to
    avoid loading corrupted data from disk causing something bad.

    Reported-by: syzbot+ae688d469e36fb5138d0@syzkaller.appspotmail.com
    Closes: https://syzkaller.appspot.com/bug?extid=ae688d469e36fb5138d0
    Fixes: 4e19d6b ("ext4: allow directory holes")
    Cc: stable@kernel.org
    Signed-off-by: Baokun Li <libaokun1@huawei.com>
    Reviewed-by: Jan Kara <jack@suse.cz>
    Link: https://patch.msgid.link/20240702132349.2600605-3-libaokun@huaweicloud.com
    Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Signed-off-by: CKI Backport Bot <cki-ci-bot+cki-gitlab-backport-bot@redhat.com>

Author: CKI Backport Bot

fs/ext4/namei.c

@@ -152,10 +152,11 @@ static struct buffer_head *__ext4_read_dirblock(struct inode *inode,
 
 		return bh;
 	}
-	if (!bh && (type == INDEX || type == DIRENT_HTREE)) {
+	/* The first directory block must not be a hole. */
+	if (!bh && (type == INDEX || type == DIRENT_HTREE || block == 0)) {
 		ext4_error_inode(inode, func, line, block,
-				 "Directory hole found for htree %s block",
-				 (type == INDEX) ? "index" : "leaf");
+				 "Directory hole found for htree %s block %u",
+				 (type == INDEX) ? "index" : "leaf", block);
 		return ERR_PTR(-EFSCORRUPTED);
 	}
 	if (!bh)
@@ -3085,10 +3086,7 @@ bool ext4_empty_dir(struct inode *inode)
 		EXT4_ERROR_INODE(inode, "invalid size");
 		return false;
 	}
-	/* The first directory block must not be a hole,
-	 * so treat it as DIRENT_HTREE
-	 */
-	bh = ext4_read_dirblock(inode, 0, DIRENT_HTREE);
+	bh = ext4_read_dirblock(inode, 0, EITHER);
 	if (IS_ERR(bh))
 		return false;
 
@@ -3532,10 +3530,7 @@ static struct buffer_head *ext4_get_first_dir_block(handle_t *handle,
 		struct ext4_dir_entry_2 *de;
 		unsigned int offset;
 
-		/* The first directory block must not be a hole, so
-		 * treat it as DIRENT_HTREE
-		 */
-		bh = ext4_read_dirblock(inode, 0, DIRENT_HTREE);
+		bh = ext4_read_dirblock(inode, 0, EITHER);
 		if (IS_ERR(bh)) {
 			*retval = PTR_ERR(bh);
 			return NULL;