On Tue, Jul 07, 2020 at 02:38:54PM +0200, Christian Brauner wrote:

On Mon, Jul 06, 2020 at 01:17:18PM -0700, Kees Cook wrote:

quoted

Expand __receive_fd() with support for replace_fd() for the coming seccomp
"addfd" ioctl(). Add new wrapper receive_fd_replace() for the new behavior
and update existing wrappers to retain old behavior.

Thanks to Colin Ian King [off-list ref] for pointing out an
uninitialized variable exposure in an earlier version of this patch.

Reviewed-by: Sargun Dhillon <redacted>
Signed-off-by: Kees Cook <redacted>
---

Thanks!
(One tiny-nit below.)
Acked-by: Christian Brauner <redacted>

quoted

 fs/file.c            | 24 ++++++++++++++++++------
 include/linux/file.h | 10 +++++++---
 2 files changed, 25 insertions(+), 9 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 0efdcf413210..11313ff36802 100644
--- a/fs/file.c
+++ b/fs/file.c

@@ -937,6 +937,7 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
 /**
  * __receive_fd() - Install received file into file descriptor table
  *
+ * @fd: fd to install into (if negative, a new fd will be allocated)
  * @file: struct file that was received from another process
  * @ufd: __user pointer to write new fd number to
  * @o_flags: the O_* flags to apply to the new fd entry

@@ -950,7 +951,7 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
  *
  * Returns newly install fd or -ve on error.
  */
-int __receive_fd(struct file *file, int __user *ufd, unsigned int o_flags)
+int __receive_fd(int fd, struct file *file, int __user *ufd, unsigned int o_flags)
 {
 	struct socket *sock;
 	int new_fd;

@@ -960,18 +961,30 @@ int __receive_fd(struct file *file, int __user *ufd, unsigned int o_flags)
 	if (error)
 		return error;
 
-	new_fd = get_unused_fd_flags(o_flags);
-	if (new_fd < 0)
-		return new_fd;
+	if (fd < 0) {
+		new_fd = get_unused_fd_flags(o_flags);
+		if (new_fd < 0)
+			return new_fd;
+	} else
+		new_fd = fd;

This is nitpicky but coding style technically wants us to use braces
around both branches if one of them requires them. ;)

Ah yeah, good point. Fixed. Thanks!

-- 
Kees Cook