[PATCH v5 3/7] 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 via a NULL check.
However, for the fd_install_received_user() wrapper, ufd is NULL checked
so an -EFAULT can be returned to avoid changing the SCM_RIGHTS interface
behavior. Add new wrapper fd_install_received() for pidfd and seccomp
that does not use the ufd argument. For the new helper, the new fd needs
to be returned on success. Update the existing callers to handle it.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 fs/file.c            | 22 ++++++++++++++--------
 include/linux/file.h |  7 +++++++
 net/compat.c         |  2 +-
 net/core/scm.c       |  2 +-
 4 files changed, 23 insertions(+), 10 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index f2167d6feec6..de85a42defe2 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -942,9 +942,10 @@ 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. Writes the fd number to userspace.
+ * checks and count updates. Optionally writes the fd number to userspace, if
+ * @ufd is non-NULL.
  *
- * 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)
 {
@@ -960,20 +961,25 @@ 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) {
+		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..e19974ed9322 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -9,6 +9,7 @@
 #include <linux/compiler.h>
 #include <linux/types.h>
 #include <linux/posix_types.h>
+#include <linux/errno.h>
 
 struct file;
 
@@ -96,8 +97,14 @@ extern int __fd_install_received(struct file *file, int __user *ufd,
 static inline int fd_install_received_user(struct file *file, int __user *ufd,
 					   unsigned int o_flags)
 {
+	if (ufd == NULL)
+		return -EFAULT;
 	return __fd_install_received(file, ufd, o_flags);
 }
+static inline int fd_install_received(struct file *file, unsigned int o_flags)
+{
+	return __fd_install_received(file, NULL, o_flags);
+}
 
 extern void flush_delayed_fput(void);
 extern void __fput_sync(struct file *);
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