Re: [PATCH 1/1] net: ioctl: Use kernel memory on protocol ioctl callbacks

Willem de Bruijn
2023-05-19T15:10:15Z

On Fri, May 19, 2023 at 9:59 AM Breno Leitao <leitao@debian.org> wrote:
>
> Most of the ioctls to net protocols operates directly on userspace
> argument (arg). Usually doing get_user()/put_user() directly in the
> ioctl callback. This is not flexible, because it is hard to reuse these
> functions without passing userspace buffers.
>
> Change the "struct proto" ioctls to avoid touching userspace memory and
> operate on kernel buffers, i.e., all protocol's ioctl callbacks is
> adapted to operate on a kernel memory other than on userspace (so, no
> more {put,get}_user() and friends being called in the ioctl callback).
>
> This changes the "struct proto" ioctl format in the following way:
>
> int (*ioctl)(struct sock *sk, int cmd,
> - unsigned long arg);
> + int *karg);
>
> So, the "karg" argument, which is passed to the ioctl callback, is a
> pointer allocated to kernel space memory (inside a function wrapper -
> sock_skprot_ioctl()). This buffer (karg) may contain input argument
> (copied from userspace in a prep function) and it might return a
> value/buffer, which is copied back to userspace if necessary. There is
> not one-size-fits-all format (that is I am using 'may' above), but
> basically, there are three type of ioctls:
>
> 1) Do not read from userspace, returns a result to userspace
> 2) Read an input parameter from userspace, and does not return anything
> to userspace
> 3) Read an input from userspace, and return a buffer to userspace.
>
> The default case (1) (where no input parameter is given, and an "int" is
> returned to userspace) encompasses more than 90% of the cases, but there
> are two other exceptions. Here is a list of exceptions:
>
> * Protocol RAW:
> * cmd = SIOCGETVIFCNT:
> * input and output = struct sioc_vif_req
> * cmd = SIOCGETSGCNT
> * input and output = struct sioc_sg_req
> * Explanation: for the SIOCGETVIFCNT case, userspace passes the input
> argument, which is struct sioc_vif_req. Then the callback populates
> the struct, which is copied back to userspace.
>
> * Protocol RAW6:
> * cmd = SIOCGETMIFCNT_IN6
> * input and output = struct sioc_mif_req6
> * cmd = SIOCGETSGCNT_IN6
> * input and output = struct sioc_sg_req6
>
> * Protocol PHONET:
> * cmd == SIOCPNADDRESOURCE | SIOCPNDELRESOURCE
> * input int (4 bytes)
> * Nothing is copied back to userspace.
>
> For the exception cases, functions sock_skproto_ioctl_in{out}() will
> copy the userspace input, and copy it back to kernel space.
>
> The wrapper that prepares the buffer and puts the buffer back to user is
> sock_skprot_ioctl(), so, instead of calling sk->sk_prot->ioctl(), the
> callee now calls sock_skprot_ioctl().
>
> Signed-off-by: Breno Leitao <leitao@debian.org>

Overall this looks great to me.

Thanks for the detailed commit message that lists all exceptions, Bruno.

Since that is a limited well understood list, I'm not in favor of the
suggestion to add an explicit length argument that then needs to be
checked in each callee.

> +/* Copy 'size' bytes from userspace and return `size` back to userspace */
> +int sock_skproto_ioctl_inout(struct sock *sk, unsigned int cmd,
> + void __user *arg, size_t size)
> +{
> + void *ptr;
> + int ret;
> +
> + ptr = kmalloc(size, GFP_KERNEL);
> + if (!ptr)
> + return -ENOMEM;

> +/* A wrapper around sock ioctls, which copies the data from userspace
> + * (depending on the protocol/ioctl), and copies back the result to userspace.
> + * The main motivation for this function is to pass kernel memory to the
> + * protocol ioctl callsback, instead of userspace memory.
> + */
> +int sock_skprot_ioctl(struct sock *sk, unsigned int cmd,
> + void __user *arg)
> +{
> +#ifdef CONFIG_IP_MROUTE
> + if (!strcmp(sk->sk_prot->name, "RAW")) {

This must check both sk_family and sk_protocol. That is preferable
over string match.

For these exception cases, instead of having sock_skproto_ioctl_inout
dynamically allocate the struct, how about stack allocating them here
and passing to the function?

Nit: the function name is quite a mouthful. Just sock_ioctl_inout?

> + switch (cmd) {
> + /* These userspace buffers will be consumed by ipmr_ioctl() */
> + case SIOCGETVIFCNT:
> + return sock_skproto_ioctl_inout(sk, cmd,
> + arg, sizeof(struct sioc_vif_req));
> + case SIOCGETSGCNT:
> + return sock_skproto_ioctl_inout(sk, cmd,
> + arg, sizeof(struct sioc_sg_req));
> + }
> + }
> +#endif
> +#ifdef CONFIG_IPV6_MROUTE
> + if (!strcmp(sk->sk_prot->name, "RAW6")) {
> + switch (cmd) {
> + /* These userspace buffers will be consumed by ip6mr_ioctl() */
> + case SIOCGETMIFCNT_IN6:
> + return sock_skproto_ioctl_inout(sk, cmd,
> + arg, sizeof(struct sioc_mif_req6));
> + case SIOCGETSGCNT_IN6:
> + return sock_skproto_ioctl_inout(sk, cmd,
> + arg, sizeof(struct sioc_sg_req6));
> + }
> + }
> +#endif
> +#ifdef CONFIG_PHONET
> + if (!strcmp(sk->sk_prot->name, "PHONET")) {
> + /* This userspace buffers will be consumed by pn_ioctl() */
> + switch (cmd) {
> + case SIOCPNADDRESOURCE:
> + case SIOCPNDELRESOURCE:
> + return sock_skproto_ioctl_in(sk, cmd, arg);
> + }
> + }
> +#endif
> +
> + return sock_skproto_ioctl_out(sk, cmd, arg);
> +}