[PATCH] fs: Prevent non-symlinks from entering pick link

[Edward Adam Davis <eadavis@qq.com>]

The reproducer uses a file0 on a ntfs3 file system with a corrupted i_link.
When renaming, the file0's inode is marked as a bad inode because the file
name cannot be deleted. However, before renaming, file0 is a directory.
After the renaming fails, it is marked as a bad inode, which makes it a
regular file. In any case, when opening it after creating a hard link,
pick_link() should not be entered because it is not a symbolic link from
beginning to end.

Add a check on the symbolic link before entering pick_link() to avoid
triggering unknown exceptions when performing the i_link acquisition
operation on other types of files.

Reported-by: syzbot+1aa90f0eb1fc3e77d969@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=1aa90f0eb1fc3e77d969
Tested-by: syzbot+1aa90f0eb1fc3e77d969@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
 fs/namei.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/namei.c b/fs/namei.c
index 4bb889fc980b..1524a5359d46 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2005,6 +2005,10 @@ static const char *step_into(struct nameidata *nd, int flags,
 		if (path.mnt == nd->path.mnt)
 			mntget(path.mnt);
 	}
+
+	if (inode && !S_ISLNK(inode->i_mode))
+		return NULL;
+
 	return pick_link(nd, &path, inode, flags);
 }
 
-- 
2.43.0