Re: [PATCH 3/3] net: cleanly handle kernel vs user buffers for ->msg_control

[Eric Dumazet]

On 5/11/20 4:59 AM, Christoph Hellwig wrote:
> The msg_control field in struct msghdr can either contain a user
> pointer when used with the recvmsg system call, or a kernel pointer
> when used with sendmsg.  To complicate things further kernel_recvmsg
> can stuff a kernel pointer in and then use set_fs to make the uaccess
> helpers accept it.
> 
> Replace it with a union of a kernel pointer msg_control field, and
> a user pointer msg_control_user one, and allow kernel_recvmsg operate
> on a proper kernel pointer using a bitfield to override the normal
> choice of a user pointer for recvmsg.
> 
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
>  include/linux/socket.h | 12 ++++++++++-
>  net/compat.c           |  5 +++--
>  net/core/scm.c         | 49 ++++++++++++++++++++++++------------------
>  net/ipv4/ip_sockglue.c |  3 ++-
>  net/socket.c           | 22 ++++++-------------
>  5 files changed, 50 insertions(+), 41 deletions(-)
> 
> diff --git a/include/linux/socket.h b/include/linux/socket.h
> index 4cc64d611cf49..04d2bc97f497d 100644
> --- a/include/linux/socket.h
> +++ b/include/linux/socket.h
> @@ -50,7 +50,17 @@ struct msghdr {
>  	void		*msg_name;	/* ptr to socket address structure */
>  	int		msg_namelen;	/* size of socket address structure */
>  	struct iov_iter	msg_iter;	/* data */
> -	void		*msg_control;	/* ancillary data */
> +
> +	/*
> +	 * Ancillary data. msg_control_user is the user buffer used for the
> +	 * recv* side when msg_control_is_user is set, msg_control is the kernel
> +	 * buffer used for all other cases.
> +	 */
> +	union {
> +		void		*msg_control;
> +		void __user	*msg_control_user;
> +	};
> +	bool		msg_control_is_user : 1;

Adding a field in this structure seems dangerous.

Some users of 'struct msghdr '  define their own struct on the stack,
and are unaware of this new mandatory field.

This bit contains garbage, crashes are likely to happen ?

Look at IPV6_2292PKTOPTIONS for example.