exfat: fix the infinite loop in exfat_readdir()

JIRA: https://issues.redhat.com/browse/RHEL-75663
CVE: CVE-2024-57940

commit fee8737
Author: Yuezhang Mo <Yuezhang.Mo@sony.com>
Date:   Fri Dec 13 13:08:37 2024 +0800

    exfat: fix the infinite loop in exfat_readdir()

    If the file system is corrupted so that a cluster is linked to
    itself in the cluster chain, and there is an unused directory
    entry in the cluster, 'dentry' will not be incremented, causing
    condition 'dentry < max_dentries' unable to prevent an infinite
    loop.

    This infinite loop causes s_lock not to be released, and other
    tasks will hang, such as exfat_sync_fs().

    This commit stops traversing the cluster chain when there is unused
    directory entry in the cluster to avoid this infinite loop.

    Reported-by: syzbot+205c2644abdff9d3f9fc@syzkaller.appspotmail.com
    Closes: https://syzkaller.appspot.com/bug?extid=205c2644abdff9d3f9fc
    Tested-by: syzbot+205c2644abdff9d3f9fc@syzkaller.appspotmail.com
    Fixes: ca06197 ("exfat: add directory operations")
    Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
    Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
    Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

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

Author: CKI Backport Bot

fs/exfat/dir.c

@@ -125,7 +125,7 @@ static int exfat_readdir(struct inode *inode, loff_t *cpos, struct exfat_dir_ent
 			type = exfat_get_entry_type(ep);
 			if (type == TYPE_UNUSED) {
 				brelse(bh);
-				break;
+				goto out;
 			}
 
 			if (type != TYPE_FILE && type != TYPE_DIR) {
@@ -189,6 +189,7 @@ static int exfat_readdir(struct inode *inode, loff_t *cpos, struct exfat_dir_ent
 		}
 	}
 
+out:
 	dir_entry->namebuf.lfn[0] = '\0';
 	*cpos = EXFAT_DEN_TO_B(dentry);
 	return 0;