Subject: Re: [PATCH v2] net: ioctl: Use kernel memory on protocol ioctl callbacks

From: Breno Leitao <[email-masked]>
Date: Tue, 23 May 2023 06:54:21 -0700

On Mon, May 22, 2023 at 03:26:55PM -0400, Willem de Bruijn wrote:
> On Mon, May 22, 2023 at 9:51 AM Breno Leitao <[email-masked]> 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 -
> > sk_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 sk_ioctl_in{out}() will
> > copy the userspace input, and copy it back to kernel space.
> >
> > The wrapper that prepare the buffer and put the buffer back to user is
> > sk_ioctl(), so, instead of calling sk->sk_prot->ioctl(), the
> > callee now calls sk_ioctl(), which will handle all cases.
> >
> > Signed-off-by: Breno Leitao <[email-masked]>
> 
> Going forward, please mark patches for net-next with [PATCH net-next v2]
> 
> > --- a/include/net/udp.h
> > +++ b/include/net/udp.h
> > @@ -283,7 +283,7 @@ void udp_flush_pending_frames(struct sock *sk);
> >  int udp_cmsg_send(struct sock *sk, struct msghdr *msg, u16 *gso_size);
> >  void udp4_hwcsum(struct sk_buff *skb, __be32 src, __be32 dst);
> >  int udp_rcv(struct sk_buff *skb);
> > -int udp_ioctl(struct sock *sk, int cmd, unsigned long arg);
> > +int udp_ioctl(struct sock *sk, int cmd, int *karg);
> >  int udp_init_sock(struct sock *sk);
> >  int udp_pre_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len);
> >  int __udp_disconnect(struct sock *sk, int flags);
> > diff --git a/net/core/sock.c b/net/core/sock.c
> > index 5440e67bcfe3..a2cea95aec99 100644
> > --- a/net/core/sock.c
> > +++ b/net/core/sock.c
> > @@ -114,6 +114,8 @@
> >  #include <linux/memcontrol.h>
> >  #include <linux/prefetch.h>
> >  #include <linux/compat.h>
> > +#include <linux/mroute.h>
> > +#include <linux/mroute6.h>
> 
> This is for the ioctl constants only, right.

Right.

> Then like those header files, include the uapi header, and only that,
> to minimize the dependencies added to net/core/sock.c

ack!

> 
> >  #include <linux/uaccess.h>
> >
> > @@ -138,6 +140,7 @@
> >
> >  #include <net/tcp.h>
> >  #include <net/busy_poll.h>
> > +#include <net/phonet/phonet.h>
> >
> >  #include <linux/ethtool.h>
> >
> > @@ -4106,3 +4109,112 @@ int sock_bind_add(struct sock *sk, struct sockaddr *addr, int addr_len)
> >         return sk->sk_prot->bind_add(sk, addr, addr_len);
> >  }
> >  EXPORT_SYMBOL(sock_bind_add);
> > +
> > +#ifdef CONFIG_PHONET
> > +/* Copy u32 value from userspace and do not return anything back */
> > +static int sk_ioctl_in(struct sock *sk, unsigned int cmd, void __user *arg)
> 
> The pointer can be const.
> 
> > +{
> > +       int karg;
> > +
> > +       if (get_user(karg, (u32 __user *)arg))
> > +               return -EFAULT;
> 
> The comment and cast are u32, but the datatype is int. Is there a
> reason for that.

I just copied what we have in pn_ioctl()[1]

	static int pn_ioctl(struct sock *sk, int cmd, unsigned long arg)
	{

		switch (cmd) {
		case SIOCPNADDRESOURCE:
		case SIOCPNDELRESOURCE: {
				u32 res;
				if (get_user(res, (u32 __user *)arg))
				....

I will cast it to "int" on V3.

[1] https://github.com/torvalds/linux/blob/ae8373a5add4ea39f032563cf12a02946d1e3546/net/phonet/datagram.c#L47