Re: [PATCH v3 1/4] fs, net: Standardize on file_receive helper to move fds across processes

[Kees Cook <keescook@chromium.org>]

On Wed, Jun 10, 2020 at 08:12:38AM +0000, Sargun Dhillon wrote:
> On Tue, Jun 09, 2020 at 10:27:54PM -0700, Kees Cook wrote:
> > On Tue, Jun 09, 2020 at 11:27:30PM +0200, Christian Brauner wrote:
> > > On June 9, 2020 10:55:42 PM GMT+02:00, Kees Cook <keescook@chromium.org> wrote:
> > > >LOL. And while we were debating this, hch just went and cleaned stuff up:
> > > >
> > > >2618d530dd8b ("net/scm: cleanup scm_detach_fds")
> > > >
> > > >So, um, yeah, now my proposal is actually even closer to what we already
> > > >have there. We just add the replace_fd() logic to __scm_install_fd() and
> > > >we're done with it.
> > > 
> > > Cool, you have a link? :)
> > 
> > How about this:
> > 
> Thank you.
> > https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=devel/seccomp/addfd/v3.1&id=bb94586b9e7cc88e915536c2e9fb991a97b62416
> > 
> > -- 
> > Kees Cook
> 
> +		if (ufd) {
> +			error = put_user(new_fd, ufd);
> +			if (error) {
> +				put_unused_fd(new_fd);
> +				return error;
> +			}
> + 		}
> I'm fairly sure this introduces a bug[1] if the user does:

Ah, sorry, I missed this before I posted my "v3.2" tree link.

> 
> struct msghdr msg = {};
> struct cmsghdr *cmsg;
> struct iovec io = {
> 	.iov_base = &c,
> 	.iov_len = 1,
> };
> 
> msg.msg_iov = &io;
> msg.msg_iovlen = 1;
> msg.msg_control = NULL;
> msg.msg_controllen = sizeof(buf);
> 
> recvmsg(sock, &msg, 0);
> 
> They will have the FD installed, no error message, but FD number wont be written 
> to memory AFAICT. If two FDs are passed, you will get an efault. They will both
> be installed, but memory wont be written to. Maybe instead of 0, make it a
> poison pointer, or -1 instead?

Hmmm. I see what you mean -- SCM_RIGHTS effectively _requires_ a valid
__user pointer, so we can't use NULL to indicate "we don't want this".
I'm not sure I can pass this through directly at all, though.

> -----
> As an aside, all of this junk should be dropped:
> +	ret = get_user(size, &uaddfd->size);
> +	if (ret)
> +		return ret;
> +
> +	ret = copy_struct_from_user(&addfd, sizeof(addfd), uaddfd, size);
> +	if (ret)
> +		return ret;
> 
> and the size member of the seccomp_notif_addfd struct. I brought this up 
> off-list with Tycho that ioctls have the size of the struct embedded in them. We 
> should just use that. The ioctl definition is based on this[2]:
> #define _IOC(dir,type,nr,size) \
> 	(((dir)  << _IOC_DIRSHIFT) | \
> 	 ((type) << _IOC_TYPESHIFT) | \
> 	 ((nr)   << _IOC_NRSHIFT) | \
> 	 ((size) << _IOC_SIZESHIFT))
> 
> 
> We should just use copy_from_user for now. In the future, we can either 
> introduce new ioctl names for new structs, or extract the size dynamically from 
> the ioctl (and mask it out on the switch statement in seccomp_notify_ioctl.

Okay, sounds good.

> ----
> +#define SECCOMP_IOCTL_NOTIF_ADDFD	SECCOMP_IOR(3,	\
> +						struct seccomp_notif_addfd)
> 
> Lastly, what I believe to be a small mistake, it should be SECCOMP_IOW, based on 
> the documentation in ioctl.h -- "_IOW means userland is writing and kernel is 
> reading."

Okay, let me tweak things and get a "v3.3". ;)

-- 
Kees Cook