Subject: Re: [PATCH v4 03/11] fs: Add fd_install_received() wrapper for __fd_install_received()

From: Kees Cook <[email-masked]>
Date: Wed, 17 Jun 2020 12:58:00 -0700

On Wed, Jun 17, 2020 at 03:35:20PM +0000, David Laight wrote:
> From: Kees Cook
> > Sent: 16 June 2020 04:25
> > 
> > For both pidfd and seccomp, the __user pointer is not used. Update
> > __fd_install_received() to make writing to ufd optional. (ufd
> > itself cannot checked for NULL because this changes the SCM_RIGHTS
> > interface behavior.) In these cases, the new fd needs to be returned
> > on success.  Update the existing callers to handle it. Add new wrapper
> > fd_install_received() for pidfd and seccomp that does not use the ufd
> > argument.
> ...> 
> >  static inline int fd_install_received_user(struct file *file, int __user *ufd,
> >  					   unsigned int o_flags)
> >  {
> > -	return __fd_install_received(file, ufd, o_flags);
> > +	return __fd_install_received(file, true, ufd, o_flags);
> > +}
> 
> Can you get rid of the 'return user' parameter by adding
> 	if (!ufd) return -EFAULT;
> to the above wrapper, then checking for NULL in the function?
> 
> Or does that do the wrong horrid things in the fail path?

Oh, hm. No, that shouldn't break the failure path, since everything gets
unwound in __fd_install_received if the ufd write fails.

Effectively this (I'll chop it up into the correct patches):

diff --git a/fs/file.c b/fs/file.c
index b583e7c60571..3b80324a31cc 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -939,18 +939,16 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
  *
  * @fd: fd to install into (if negative, a new fd will be allocated)
  * @file: struct file that was received from another process
- * @ufd_required: true to use @ufd for writing fd number to userspace
  * @ufd: __user pointer to write new fd number to
  * @o_flags: the O_* flags to apply to the new fd entry
  *
  * Installs a received file into the file descriptor table, with appropriate
  * checks and count updates. Optionally writes the fd number to userspace, if
- * @ufd_required is true (@ufd cannot just be tested for NULL because NULL may
- * actually get passed into SCM_RIGHTS).
+ * @ufd is non-NULL.
  *
  * Returns newly install fd or -ve on error.
  */
-int __fd_install_received(int fd, struct file *file, bool ufd_required,
+int __fd_install_received(int fd, struct file *file,
 			  int __user *ufd, unsigned int o_flags)
 {
 	struct socket *sock;
@@ -967,7 +965,7 @@ int __fd_install_received(int fd, struct file *file, bool ufd_required,
 			return new_fd;
 	}
 
-	if (ufd_required) {
+	if (ufd) {
 		error = put_user(new_fd, ufd);
 		if (error) {
 			put_unused_fd(new_fd);
diff --git a/include/linux/file.h b/include/linux/file.h
index f1d16e24a12e..2ade0d90bc5e 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -91,20 +91,22 @@ extern void put_unused_fd(unsigned int fd);
 
 extern void fd_install(unsigned int fd, struct file *file);
 
-extern int __fd_install_received(int fd, struct file *file, bool ufd_required,
+extern int __fd_install_received(int fd, struct file *file,
 				 int __user *ufd, unsigned int o_flags);
 static inline int fd_install_received_user(struct file *file, int __user *ufd,
 					   unsigned int o_flags)
 {
-	return __fd_install_received(-1, file, true, ufd, o_flags);
+	if (ufd == NULL)
+		return -EFAULT;
+	return __fd_install_received(-1, file, ufd, o_flags);
 }
 static inline int fd_install_received(struct file *file, unsigned int o_flags)
 {
-	return __fd_install_received(-1, file, false, NULL, o_flags);
+	return __fd_install_received(-1, file, NULL, o_flags);
 }
 static inline int fd_replace_received(int fd, struct file *file, unsigned int o_flags)
 {
-	return __fd_install_received(fd, file, false, NULL, o_flags);
+	return __fd_install_received(fd, file, NULL, o_flags);
 }
 
 extern void flush_delayed_fput(void);

-- 
Kees Cook