Re: [PATCH v5 3/7] fs: Add fd_install_received() wrapper for __fd_install_received()

[Sargun Dhillon <sargun@sargun.me>]

On Wed, Jun 17, 2020 at 03:03:23PM -0700, Kees Cook wrote:
> 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@chromium.org>
> ---
>  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;
Isn't this *technically* a behvaiour change? Nonetheless, I think this is a much better
approach than forcing everyone to do null checking, and avoids at least one error case
where the kernel installs FDs for SCM_RIGHTS, and they're not actualy usable.

>  	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
> 

Reviewed-by: Sargun Dhillon <sargun@sargun.me>