f2fs: Add sanity checks before unlinking and loading inodes

Add check for inode->i_nlink == 1 for directories during unlink,
as their value is decremented twice, which can trigger a warning in
drop_nlink. In such case mark the filesystem as corrupted and return
from the function call with the relevant failure return value.

Additionally add the check for i_nlink == 1 in
sanity_check_inode in order to detect on-disk corruption early.

Reported-by: syzbot+c07d47c7bc68f47b9083@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c07d47c7bc68f47b9083
Tested-by: syzbot+c07d47c7bc68f47b9083@syzkaller.appspotmail.com
Signed-off-by: Nikola Z. Ivanov <zlatistiv@gmail.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Author: zlatistiv

fs/f2fs/inode.c

@@ -294,6 +294,12 @@ static bool sanity_check_inode(struct inode *inode, struct folio *node_folio)
 		return false;
 	}
 
+	if (S_ISDIR(inode->i_mode) && unlikely(inode->i_nlink == 1)) {
+		f2fs_warn(sbi, "%s: directory inode (ino=%lx) has a single i_nlink",
+			  __func__, inode->i_ino);
+		return false;
+	}
+
 	if (f2fs_has_extra_attr(inode)) {
 		if (!f2fs_sb_has_extra_attr(sbi)) {
 			f2fs_warn(sbi, "%s: inode (ino=%lx) is with extra_attr, but extra_attr feature is off",

fs/f2fs/namei.c

@@ -570,12 +570,13 @@ static int f2fs_unlink(struct inode *dir, struct dentry *dentry)
 	}
 
 	if (unlikely(inode->i_nlink == 0)) {
-		f2fs_warn(F2FS_I_SB(inode), "%s: inode (ino=%lx) has zero i_nlink",
+		f2fs_warn(sbi, "%s: inode (ino=%lx) has zero i_nlink",
 			  __func__, inode->i_ino);
-		err = -EFSCORRUPTED;
-		set_sbi_flag(F2FS_I_SB(inode), SBI_NEED_FSCK);
-		f2fs_folio_put(folio, false);
-		goto out;
+		goto corrupted;
+	} else if (S_ISDIR(inode->i_mode) && unlikely(inode->i_nlink == 1)) {
+		f2fs_warn(sbi, "%s: directory inode (ino=%lx) has a single i_nlink",
+			  __func__, inode->i_ino);
+		goto corrupted;
 	}
 
 	f2fs_balance_fs(sbi, true);
@@ -601,6 +602,12 @@ static int f2fs_unlink(struct inode *dir, struct dentry *dentry)
 
 	if (IS_DIRSYNC(dir))
 		f2fs_sync_fs(sbi->sb, 1);
+
+	goto out;
+corrupted:
+	err = -EFSCORRUPTED;
+	set_sbi_flag(sbi, SBI_NEED_FSCK);
+	f2fs_folio_put(folio, false);
 out:
 	trace_f2fs_unlink_exit(inode, err);
 	return err;