nfs: fix ->d_revalidate() UAF on ->d_name accesses

Pass the stable name all the way down to ->rpc_ops->lookup() instances.

Note that passing &dentry->d_name is safe in e.g. nfs_lookup() - it *is*
stable there, as it is in ->create() et.al.

dget_parent() in nfs_instantiate() should be redundant - it'd better be
stable there; if it's not, we have more trouble, since ->d_name would
also be unsafe in such case.

nfs_submount() and nfs4_submount() may or may not require fixes - if
they ever get moved on server with fhandle preserved, we are in trouble
there...

UAF window is fairly narrow here and exfiltration requires the ability
to watch the traffic.

Reviewed-by: Jeff Layton <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/nfs/dir.c

···
 	return nfs_lookup_revalidate_done(dir, dentry, inode, 1);
 }
 
-static int nfs_lookup_revalidate_dentry(struct inode *dir,
+static int nfs_lookup_revalidate_dentry(struct inode *dir, const struct qstr *name,
 					struct dentry *dentry,
 					struct inode *inode, unsigned int flags)
 {
···
 		goto out;
 
 	dir_verifier = nfs_save_change_attribute(dir);
-	ret = NFS_PROTO(dir)->lookup(dir, dentry, fhandle, fattr);
+	ret = NFS_PROTO(dir)->lookup(dir, dentry, name, fhandle, fattr);
 	if (ret < 0)
 		goto out;
 
···
 	if (NFS_STALE(inode))
 		goto out_bad;
 
-	return nfs_lookup_revalidate_dentry(dir, dentry, inode, flags);
+	return nfs_lookup_revalidate_dentry(dir, name, dentry, inode, flags);
 out_valid:
 	return nfs_lookup_revalidate_done(dir, dentry, inode, 1);
 out_bad:
···
 
 	dir_verifier = nfs_save_change_attribute(dir);
 	trace_nfs_lookup_enter(dir, dentry, flags);
-	error = NFS_PROTO(dir)->lookup(dir, dentry, fhandle, fattr);
+	error = NFS_PROTO(dir)->lookup(dir, dentry, &dentry->d_name,
+				       fhandle, fattr);
 	if (error == -ENOENT) {
 		if (nfs_server_capable(dir, NFS_CAP_CASE_INSENSITIVE))
 			dir_verifier = inode_peek_iversion_raw(dir);
···
 reval_dentry:
 	if (flags & LOOKUP_RCU)
 		return -ECHILD;
-	return nfs_lookup_revalidate_dentry(dir, dentry, inode, flags);
+	return nfs_lookup_revalidate_dentry(dir, name, dentry, inode, flags);
 
 full_reval:
 	return nfs_do_lookup_revalidate(dir, name, dentry, flags);
···
 	d_drop(dentry);
 
 	if (fhandle->size == 0) {
-		error = NFS_PROTO(dir)->lookup(dir, dentry, fhandle, fattr);
+		error = NFS_PROTO(dir)->lookup(dir, dentry, &dentry->d_name,
+					       fhandle, fattr);
 		if (error)
 			goto out_error;
 	}

fs/nfs/namespace.c

···
 	int err;
 
 	/* Look it up again to get its attributes */
-	err = server->nfs_client->rpc_ops->lookup(d_inode(parent), dentry,
+	err = server->nfs_client->rpc_ops->lookup(d_inode(parent), dentry, &dentry->d_name,
 						  ctx->mntfh, ctx->clone_data.fattr);
 	dput(parent);
 	if (err != 0)

fs/nfs/nfs3proc.c

···
 }
 
 static int
-nfs3_proc_lookup(struct inode *dir, struct dentry *dentry,
+nfs3_proc_lookup(struct inode *dir, struct dentry *dentry, const struct qstr *name,
 		 struct nfs_fh *fhandle, struct nfs_fattr *fattr)
 {
 	unsigned short task_flags = 0;
···
 		task_flags |= RPC_TASK_TIMEOUT;
 
 	dprintk("NFS call  lookup %pd2\n", dentry);
-	return __nfs3_proc_lookup(dir, dentry->d_name.name,
-				  dentry->d_name.len, fhandle, fattr,
+	return __nfs3_proc_lookup(dir, name->name, name->len, fhandle, fattr,
 				  task_flags);
 }
 

fs/nfs/nfs4proc.c

···
 }
 
 static int _nfs4_proc_lookup(struct rpc_clnt *clnt, struct inode *dir,
-		struct dentry *dentry, struct nfs_fh *fhandle,
-		struct nfs_fattr *fattr)
+		struct dentry *dentry, const struct qstr *name,
+		struct nfs_fh *fhandle, struct nfs_fattr *fattr)
 {
 	struct nfs_server *server = NFS_SERVER(dir);
 	int		       status;
 	struct nfs4_lookup_arg args = {
 		.bitmask = server->attr_bitmask,
 		.dir_fh = NFS_FH(dir),
-		.name = &dentry->d_name,
+		.name = name,
 	};
 	struct nfs4_lookup_res res = {
 		.server = server,
···
 }
 
 static int nfs4_proc_lookup_common(struct rpc_clnt **clnt, struct inode *dir,
-				   struct dentry *dentry, struct nfs_fh *fhandle,
-				   struct nfs_fattr *fattr)
+				   struct dentry *dentry, const struct qstr *name,
+				   struct nfs_fh *fhandle, struct nfs_fattr *fattr)
 {
 	struct nfs4_exception exception = {
 		.interruptible = true,
 	};
 	struct rpc_clnt *client = *clnt;
-	const struct qstr *name = &dentry->d_name;
 	int err;
 	do {
-		err = _nfs4_proc_lookup(client, dir, dentry, fhandle, fattr);
+		err = _nfs4_proc_lookup(client, dir, dentry, name, fhandle, fattr);
 		trace_nfs4_lookup(dir, name, err);
 		switch (err) {
 		case -NFS4ERR_BADNAME:
···
 	return err;
 }
 
-static int nfs4_proc_lookup(struct inode *dir, struct dentry *dentry,
+static int nfs4_proc_lookup(struct inode *dir, struct dentry *dentry, const struct qstr *name,
 			    struct nfs_fh *fhandle, struct nfs_fattr *fattr)
 {
 	int status;
 	struct rpc_clnt *client = NFS_CLIENT(dir);
 
-	status = nfs4_proc_lookup_common(&client, dir, dentry, fhandle, fattr);
+	status = nfs4_proc_lookup_common(&client, dir, dentry, name, fhandle, fattr);
 	if (client != NFS_CLIENT(dir)) {
 		rpc_shutdown_client(client);
 		nfs_fixup_secinfo_attributes(fattr);
···
 	struct rpc_clnt *client = NFS_CLIENT(dir);
 	int status;
 
-	status = nfs4_proc_lookup_common(&client, dir, dentry, fhandle, fattr);
+	status = nfs4_proc_lookup_common(&client, dir, dentry, &dentry->d_name,
+					 fhandle, fattr);
 	if (status < 0)
 		return ERR_PTR(status);
 	return (client == NFS_CLIENT(dir)) ? rpc_clone_client(client) : client;

fs/nfs/proc.c

···
 }
 
 static int
-nfs_proc_lookup(struct inode *dir, struct dentry *dentry,
+nfs_proc_lookup(struct inode *dir, struct dentry *dentry, const struct qstr *name,
 		struct nfs_fh *fhandle, struct nfs_fattr *fattr)
 {
 	struct nfs_diropargs	arg = {
 		.fh		= NFS_FH(dir),
-		.name		= dentry->d_name.name,
-		.len		= dentry->d_name.len
+		.name		= name->name,
+		.len		= name->len
 	};
 	struct nfs_diropok	res = {
 		.fh		= fhandle,

include/linux/nfs_xdr.h

···
 			    struct nfs_fattr *, struct inode *);
 	int	(*setattr) (struct dentry *, struct nfs_fattr *,
 			    struct iattr *);
-	int	(*lookup)  (struct inode *, struct dentry *,
+	int	(*lookup)  (struct inode *, struct dentry *, const struct qstr *,
 			    struct nfs_fh *, struct nfs_fattr *);
 	int	(*lookupp) (struct inode *, struct nfs_fh *,
 			    struct nfs_fattr *);