rename(): avoid a deadlock in the case of parents having no common ancestor

... and fix the directory locking documentation and proof of correctness.
Holding ->s_vfs_rename_mutex *almost* prevents ->d_parent changes; the
case where we really don't want it is splicing the root of disconnected
tree to somewhere.

In other words, ->s_vfs_rename_mutex is sufficient to stabilize "X is an
ancestor of Y" only if X and Y are already in the same tree.  Otherwise
it can go from false to true, and one can construct a deadlock on that.

Make lock_two_directories() report an error in such case and update the
callers of lock_rename()/lock_rename_child() to handle such errors.

And yes, such conditions are not impossible to create ;-/
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Documentation/filesystems/porting.rst

@@ -1079,3 +1079,12 @@ On same-directory ->rename() the (tautological) update of .. is not protected
 by any locks; just don't do it if the old parent is the same as the new one.
 We really can't lock two subdirectories in same-directory rename - not without
 deadlocks.
+
+---
+
+**mandatory**
+
+lock_rename() and lock_rename_child() may fail in cross-directory case, if
+their arguments do not have a common ancestor.  In that case ERR_PTR(-EXDEV)
+is returned, with no locks taken.  In-tree users updated; out-of-tree ones
+would need to do so.

fs/cachefiles/namei.c

@@ -305,6 +305,8 @@ int cachefiles_bury_object(struct cachefiles_cache *cache,
 
 	/* do the multiway lock magic */
 	trap = lock_rename(cache->graveyard, dir);
+	if (IS_ERR(trap))
+		return PTR_ERR(trap);
 
 	/* do some checks before getting the grave dentry */
 	if (rep->d_parent != dir || IS_DEADDIR(d_inode(rep))) {

fs/ecryptfs/inode.c

@@ -599,6 +599,8 @@ ecryptfs_rename(struct mnt_idmap *idmap, struct inode *old_dir,
 	target_inode = d_inode(new_dentry);
 
 	trap = lock_rename(lower_old_dir_dentry, lower_new_dir_dentry);
+	if (IS_ERR(trap))
+		return PTR_ERR(trap);
 	dget(lower_new_dentry);
 	rc = -EINVAL;
 	if (lower_old_dentry->d_parent != lower_old_dir_dentry)

fs/namei.c

@@ -3014,21 +3014,37 @@ static inline int may_create(struct mnt_idmap *idmap,
 	return inode_permission(idmap, dir, MAY_WRITE | MAY_EXEC);
 }
 
+// p1 != p2, both are on the same filesystem, ->s_vfs_rename_mutex is held
 static struct dentry *lock_two_directories(struct dentry *p1, struct dentry *p2)
 {
-	struct dentry *p;
+	struct dentry *p = p1, *q = p2, *r;
 
-	p = d_ancestor(p2, p1);
-	if (p) {
+	while ((r = p->d_parent) != p2 && r != p)
+		p = r;
+	if (r == p2) {
+		// p is a child of p2 and an ancestor of p1 or p1 itself
 		inode_lock_nested(p2->d_inode, I_MUTEX_PARENT);
 		inode_lock_nested(p1->d_inode, I_MUTEX_PARENT2);
 		return p;
 	}
-
-	p = d_ancestor(p1, p2);
+	// p is the root of connected component that contains p1
+	// p2 does not occur on the path from p to p1
+	while ((r = q->d_parent) != p1 && r != p && r != q)
+		q = r;
+	if (r == p1) {
+		// q is a child of p1 and an ancestor of p2 or p2 itself
 		inode_lock_nested(p1->d_inode, I_MUTEX_PARENT);
 		inode_lock_nested(p2->d_inode, I_MUTEX_PARENT2);
-	return p;
+		return q;
+	} else if (likely(r == p)) {
+		// both p2 and p1 are descendents of p
+		inode_lock_nested(p1->d_inode, I_MUTEX_PARENT);
+		inode_lock_nested(p2->d_inode, I_MUTEX_PARENT2);
+		return NULL;
+	} else { // no common ancestor at the time we'd been called
+		mutex_unlock(&p1->d_sb->s_vfs_rename_mutex);
+		return ERR_PTR(-EXDEV);
+	}
 }
 
 /*
@@ -4947,6 +4963,10 @@ int do_renameat2(int olddfd, struct filename *from, int newdfd,
 
 retry_deleg:
 	trap = lock_rename(new_path.dentry, old_path.dentry);
+	if (IS_ERR(trap)) {
+		error = PTR_ERR(trap);
+		goto exit_lock_rename;
+	}
 
 	old_dentry = lookup_one_qstr_excl(&old_last, old_path.dentry,
 					  lookup_flags);
@@ -5014,6 +5034,7 @@ int do_renameat2(int olddfd, struct filename *from, int newdfd,
 	dput(old_dentry);
 exit3:
 	unlock_rename(new_path.dentry, old_path.dentry);
+exit_lock_rename:
 	if (delegated_inode) {
 		error = break_deleg_wait(&delegated_inode);
 		if (!error)

fs/nfsd/vfs.c

@@ -1813,6 +1813,10 @@ nfsd_rename(struct svc_rqst *rqstp, struct svc_fh *ffhp, char *fname, int flen,
 	}
 
 	trap = lock_rename(tdentry, fdentry);
+	if (IS_ERR(trap)) {
+		err = (rqstp->rq_vers == 2) ? nfserr_acces : nfserr_xdev;
+		goto out;
+	}
 	err = fh_fill_pre_attrs(ffhp);
 	if (err != nfs_ok)
 		goto out_unlock;

fs/overlayfs/copy_up.c

@@ -722,7 +722,7 @@ static int ovl_copy_up_workdir(struct ovl_copy_up_ctx *c)
 	struct inode *inode;
 	struct inode *udir = d_inode(c->destdir), *wdir = d_inode(c->workdir);
 	struct path path = { .mnt = ovl_upper_mnt(ofs) };
-	struct dentry *temp, *upper;
+	struct dentry *temp, *upper, *trap;
 	struct ovl_cu_creds cc;
 	int err;
 	struct ovl_cattr cattr = {
@@ -760,9 +760,11 @@ static int ovl_copy_up_workdir(struct ovl_copy_up_ctx *c)
 	 * If temp was moved, abort without the cleanup.
 	 */
 	ovl_start_write(c->dentry);
-	if (lock_rename(c->workdir, c->destdir) != NULL ||
-	    temp->d_parent != c->workdir) {
+	trap = lock_rename(c->workdir, c->destdir);
+	if (trap || temp->d_parent != c->workdir) {
 		err = -EIO;
+		if (IS_ERR(trap))
+			goto out;
 		goto unlock;
 	} else if (err) {
 		goto cleanup;
@@ -803,6 +805,7 @@ static int ovl_copy_up_workdir(struct ovl_copy_up_ctx *c)
 		ovl_set_flag(OVL_WHITEOUTS, inode);
 unlock:
 	unlock_rename(c->workdir, c->destdir);
+out:
 	ovl_end_write(c->dentry);
 
 	return err;

fs/overlayfs/dir.c

@@ -1180,6 +1180,10 @@ static int ovl_rename(struct mnt_idmap *idmap, struct inode *olddir,
 	}
 
 	trap = lock_rename(new_upperdir, old_upperdir);
+	if (IS_ERR(trap)) {
+		err = PTR_ERR(trap);
+		goto out_revert_creds;
+	}
 
 	olddentry = ovl_lookup_upper(ofs, old->d_name.name, old_upperdir,
 				     old->d_name.len);

fs/overlayfs/super.c

@@ -439,8 +439,10 @@ static bool ovl_workdir_ok(struct dentry *workdir, struct dentry *upperdir)
 	bool ok = false;
 
 	if (workdir != upperdir) {
-		ok = (lock_rename(workdir, upperdir) == NULL);
+		struct dentry *trap = lock_rename(workdir, upperdir);
+		if (!IS_ERR(trap))
 			unlock_rename(workdir, upperdir);
+		ok = (trap == NULL);
 	}
 	return ok;
 }

fs/overlayfs/util.c

@@ -1198,12 +1198,17 @@ void ovl_nlink_end(struct dentry *dentry)
 
 int ovl_lock_rename_workdir(struct dentry *workdir, struct dentry *upperdir)
 {
+	struct dentry *trap;
+
 	/* Workdir should not be the same as upperdir */
 	if (workdir == upperdir)
 		goto err;
 
 	/* Workdir should not be subdir of upperdir and vice versa */
-	if (lock_rename(workdir, upperdir) != NULL)
+	trap = lock_rename(workdir, upperdir);
+	if (IS_ERR(trap))
+		goto err;
+	if (trap)
 		goto err_unlock;
 
 	return 0;

fs/smb/server/vfs.c

@@ -708,6 +708,10 @@ int ksmbd_vfs_rename(struct ksmbd_work *work, const struct path *old_path,
 		goto out2;
 
 	trap = lock_rename_child(old_child, new_path.dentry);
+	if (IS_ERR(trap)) {
+		err = PTR_ERR(trap);
+		goto out_drop_write;
+	}
 
 	old_parent = dget(old_child->d_parent);
 	if (d_unhashed(old_child)) {
@@ -770,6 +774,7 @@ int ksmbd_vfs_rename(struct ksmbd_work *work, const struct path *old_path,
 out3:
 	dput(old_parent);
 	unlock_rename(old_parent, new_path.dentry);
+out_drop_write:
 	mnt_drop_write(old_path->mnt);
 out2:
 	path_put(&new_path);