[2/2] pidfd: Replace open-coded partial __scm_install_fd()

The sock counting (sock_update_netprioidx() and sock_update_classid())
was missing from this implementation of fd installation, compared to
SCM_RIGHTS. Use the new scm helper to get the work done, after adjusting
it to return the installed fd and accept a NULL user pointer.

Fixes: 8649c322f75c ("pid: Implement pidfd_getfd syscall")
Signed-off-by: Kees Cook <keescook@chromium.org>
---
AFAICT, the following patches are needed for back-porting this to stable:

0462b6bdb644 ("net: add a CMSG_USER_DATA macro")
2618d530dd8b ("net/scm: cleanup scm_detach_fds")
1f466e1f15cf ("net: cleanly handle kernel vs user buffers for ->msg_control")
6e8a4f9dda38 ("net: ignore sock_from_file errors in __scm_install_fd")
---
 kernel/pid.c   | 12 ++----------
 net/compat.c   |  2 +-
 net/core/scm.c | 27 ++++++++++++++++++++-------
 3 files changed, 23 insertions(+), 18 deletions(-)

On Tue, Jun 09, 2020 at 09:52:14PM -0700, Kees Cook wrote:
> The sock counting (sock_update_netprioidx() and sock_update_classid())
> was missing from this implementation of fd installation, compared to
> SCM_RIGHTS. Use the new scm helper to get the work done, after adjusting
> it to return the installed fd and accept a NULL user pointer.
> 
> Fixes: 8649c322f75c ("pid: Implement pidfd_getfd syscall")
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> AFAICT, the following patches are needed for back-porting this to stable:
> 
> 0462b6bdb644 ("net: add a CMSG_USER_DATA macro")
> 2618d530dd8b ("net/scm: cleanup scm_detach_fds")
> 1f466e1f15cf ("net: cleanly handle kernel vs user buffers for ->msg_control")
> 6e8a4f9dda38 ("net: ignore sock_from_file errors in __scm_install_fd")
> ---
>  kernel/pid.c   | 12 ++----------
>  net/compat.c   |  2 +-
>  net/core/scm.c | 27 ++++++++++++++++++++-------
>  3 files changed, 23 insertions(+), 18 deletions(-)
> 
> diff --git a/kernel/pid.c b/kernel/pid.c
> index f1496b757162..a7ce4ba898d3 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -42,6 +42,7 @@
>  #include <linux/sched/signal.h>
>  #include <linux/sched/task.h>
>  #include <linux/idr.h>
> +#include <net/scm.h>
>  
>  struct pid init_struct_pid = {
>  	.count		= REFCOUNT_INIT(1),
> @@ -635,18 +636,9 @@ static int pidfd_getfd(struct pid *pid, int fd)
>  	if (IS_ERR(file))
>  		return PTR_ERR(file);
>  
> -	ret = security_file_receive(file);
> -	if (ret) {
> -		fput(file);
> -		return ret;
> -	}
> -
> -	ret = get_unused_fd_flags(O_CLOEXEC);
> +	ret = __scm_install_fd(file, NULL, O_CLOEXEC);
>  	if (ret < 0)
>  		fput(file);
> -	else
> -		fd_install(ret, file);
> -
>  	return ret;
>  }
>  
> diff --git a/net/compat.c b/net/compat.c
> index 117f1869bf3b..f8575555b6d7 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 = __scm_install_fd(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 86d96152646f..e80648fb4da7 100644
> --- a/net/core/scm.c
> +++ b/net/core/scm.c
> @@ -280,6 +280,14 @@ void put_cmsg_scm_timestamping(struct msghdr *msg, struct scm_timestamping_inter
>  }
>  EXPORT_SYMBOL(put_cmsg_scm_timestamping);
>  
> +/**
> + * __scm_install_fd() - Install received file into file descriptor table
Any reason not to rename this remote_install_* or similar, and move it to fs/?
> + *
> + * Installs a received file into the file descriptor table, with appropriate
> + * checks and count updates.
> + *
> + * Returns fd installed or -ve on error.
> + */
>  int __scm_install_fd(struct file *file, int __user *ufd, int o_flags)
>  {
>  	struct socket *sock;
> @@ -294,20 +302,25 @@ int __scm_install_fd(struct file *file, int __user *ufd, int o_flags)
>  	if (new_fd < 0)
>  		return new_fd;
>  
> -	error = put_user(new_fd, ufd);
> -	if (error) {
> -		put_unused_fd(new_fd);
> -		return error;
> +	if (ufd) {
See my comment elsewhere about not being able to use NULL here.
> +		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 scm_max_fds(struct msghdr *msg)
> @@ -337,7 +350,7 @@ void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm)
>  
>  	for (i = 0; i < fdmax; i++) {
>  		err = __scm_install_fd(scm->fp->fp[i], cmsg_data + i, o_flags);
> -		if (err)
> +		if (err < 0)
>  			break;
>  	}
>  
> -- 
> 2.25.1
>