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

[Kees Cook <keescook@xxxxxxxxxxxx>]

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.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 fs/file.c            | 26 +++++++++++++++++---------
 include/linux/file.h | 10 +++++++---
 net/compat.c         |  2 +-
 net/core/scm.c       |  2 +-
 4 files changed, 26 insertions(+), 14 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index fcfddae0d252..14a8ef74efb2 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -944,11 +944,14 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
  * @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.
+ * 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).
  *
- * Returns -ve on error.
+ * Returns newly install fd or -ve on error.
  */
-int __fd_install_received(struct file *file, int __user *ufd, unsigned int o_flags)
+int __fd_install_received(struct file *file, bool ufd_required, int __user *ufd,
+			  unsigned int o_flags)
 {
 	struct socket *sock;
 	int new_fd;
@@ -962,20 +965,25 @@ int __fd_install_received(struct file *file, int __user *ufd, unsigned int o_fla
 	if (new_fd < 0)
 		return new_fd;
 
-	error = put_user(new_fd, ufd);
-	if (error) {
-		put_unused_fd(new_fd);
-		return error;
+	if (ufd_required) {
+		error = put_user(new_fd, ufd);
+		if (error) {
+			put_unused_fd(new_fd);
+			return error;
+		}
 	}
 
-	/* Bump the usage count and install the file. */
+	/* Bump the usage count and install the file. The resulting value of
+	 * "error" is ignored here since we only need to take action when
+	 * the file is a socket and testing "sock" for NULL is sufficient.
+	 */
 	sock = sock_from_file(file, &error);
 	if (sock) {
 		sock_update_netprioidx(&sock->sk->sk_cgrp_data);
 		sock_update_classid(&sock->sk->sk_cgrp_data);
 	}
 	fd_install(new_fd, get_file(file));
-	return 0;
+	return new_fd;
 }
 
 static int ksys_dup3(unsigned int oldfd, unsigned int newfd, int flags)
diff --git a/include/linux/file.h b/include/linux/file.h
index fe18a1a0d555..999a2c56db07 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -91,12 +91,16 @@ extern void put_unused_fd(unsigned int fd);
 
 extern void fd_install(unsigned int fd, struct file *file);
 
-extern int __fd_install_received(struct file *file, int __user *ufd,
-				 unsigned int o_flags);
+extern int __fd_install_received(struct file *file, bool ufd_required,
+				 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(file, ufd, o_flags);
+	return __fd_install_received(file, true, ufd, o_flags);
+}
+static inline int fd_install_received(struct file *file, unsigned int o_flags)
+{
+	return __fd_install_received(file, false, NULL, o_flags);
 }
 
 extern void flush_delayed_fput(void);
diff --git a/net/compat.c b/net/compat.c
index 94f288e8dac5..71494337cca7 100644
--- a/net/compat.c
+++ b/net/compat.c
@@ -299,7 +299,7 @@ void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm)
 
 	for (i = 0; i < fdmax; i++) {
 		err = fd_install_received_user(scm->fp->fp[i], cmsg_data + i, o_flags);
-		if (err)
+		if (err < 0)
 			break;
 	}
 
diff --git a/net/core/scm.c b/net/core/scm.c
index df190f1fdd28..b9a0442ebd26 100644
--- a/net/core/scm.c
+++ b/net/core/scm.c
@@ -307,7 +307,7 @@ void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm)
 
 	for (i = 0; i < fdmax; i++) {
 		err = fd_install_received_user(scm->fp->fp[i], cmsg_data + i, o_flags);
-		if (err)
+		if (err < 0)
 			break;
 	}
 
-- 
2.25.1