re: fs: Expand __fd_install_received() to accept fd

[Colin Ian King <colin.king@canonical.com>]

Hi,

static analysis with Coverity has detected a potential issue with the
following commit:

commit 8336af9ab8c5d64a309cbf72648054af61548899
Author: Kees Cook <keescook@chromium.org>
Date:   Wed Jun 10 08:46:58 2020 -0700

    fs: Expand __fd_install_received() to accept fd

Calling __fd_install_received() with fd >= 0 and ufd being non-null will
cause a put_user of an uninitialized new_fd hence potentially leaking
data on the stack back to the user.

static analysis is as follows:

1050 int __fd_install_received(int fd, struct file *file, int __user *ufd,
1051                          unsigned int o_flags)
1052 {
1053        struct socket *sock;

    1. var_decl: Declaring variable new_fd without initializer.

1054        int new_fd;
1055        int error;
1056
1057        error = security_file_receive(file);

    2. Condition error, taking false branch.

1058        if (error)
1059                return error;
1060

    3. Condition fd < 0, taking false branch.

1061        if (fd < 0) {
1062                new_fd = get_unused_fd_flags(o_flags);
1063                if (new_fd < 0)
1064                        return new_fd;
1065        }
1066

    4. Condition ufd, taking true branch.
1067        if (ufd) {

CID: Uninitialized scalar variable (UNINIT)5. uninit_use: Using
uninitialized value new_fd.

1068                error = put_user(new_fd, ufd);

Colin