linux-kernel - Re: [PATCH v2 2/3] seccomp: Introduce addfd ioctl to seccomp user notifier

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Date:   Sat, 30 May 2020 03:58:18 +0000
From:   Sargun Dhillon <sargun@...gun.me>
To:     Kees Cook <keescook@...omium.org>
Cc:     christian.brauner@...ntu.com,
        containers@...ts.linux-foundation.org, cyphar@...har.com,
        jannh@...gle.com, jeffv@...gle.com, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org, palmer@...gle.com, rsesek@...gle.com,
        tycho@...ho.ws, Matt Denton <mpdenton@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v2 2/3] seccomp: Introduce addfd ioctl to seccomp user
 notifier

> 
> I mean, yes, that's certainly better, but it just seems a shame that
> everyone has to do the get_unused/put_unused dance just because of how
> SCM_RIGHTS does this weird put_user() in the middle.
> 
> Can anyone clarify the expected failure mode from SCM_RIGHTS? Can we
> move the put_user() after instead? I think cleanup would just be:
> replace_fd(fd, NULL, 0)
> 
> So:
> 
> (updated to skip sock updates on failure; thank you Christian!)
> 
> int file_receive(int fd, unsigned long flags, struct file *file)
> {
> 	struct socket *sock;
> 	int ret;
> 
> 	ret = security_file_receive(file);
> 	if (ret)
> 		return ret;
> 
> 	/* Install the file. */
> 	if (fd == -1) {
> 		ret = get_unused_fd_flags(flags);
> 		if (ret >= 0)
> 			fd_install(ret, get_file(file));
> 	} else {
> 		ret = replace_fd(fd, file, flags);
> 	}
> 
> 	/* Bump the sock usage counts. */
> 	if (ret >= 0) {
> 		sock = sock_from_file(addfd->file, &err);
> 		if (sock) {
> 			sock_update_netprioidx(&sock->sk->sk_cgrp_data);
> 			sock_update_classid(&sock->sk->sk_cgrp_data);
> 		}
> 	}
> 
> 	return ret;
> }
> 
> scm_detach_fds()
> 	...
> 	for (i=0, cmfptr=(__force int __user *)CMSG_DATA(cm); i<fdmax;
>              i++, cmfptr++)
> 	{
> 		int new_fd;
> 
> 		err = file_receive(-1, MSG_CMSG_CLOEXEC & msg->msg_flags
>                                           ? O_CLOEXEC : 0, fp[i]);
> 		if (err < 0)
> 			break;
> 		new_fd = err;
> 
Isn't the "right" way to do this to allocate a bunch of file descriptors,
and fill up the user buffer with them, and then install the files? This
seems to like half-install the file descriptors and then error out.

I know that's the current behaviour, but that seems like a bad idea. Do
we really want to perpetuate this half-broken state? I guess that some
userspace programs could be depending on this -- and their recovery
semantics could rely on this. I mean this is 10+ year old code.

> 		err = put_user(err, cmfptr);
> 		if (err) {
> 			/*
> 			 * If we can't notify userspace that it got the
> 			 * fd, we need to unwind and remove it again.
> 			 */
> 			replace_fd(new_fd, NULL, 0);
> 			break;
> 		}
> 	}
> 	...
> 
> 
> 
> -- 
> Kees Cook