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From: Kees Cook <keescook@chromium.org>
To: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Sargun Dhillon <sargun@sargun.me>,
	Giuseppe Scrivano <gscrivan@redhat.com>,
	Robert Sesek <rsesek@google.com>,
	Chris Palmer <palmer@google.com>, Jann Horn <jannh@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	containers@lists.linux-foundation.org,
	Daniel Wagner <daniel.wagner@bmw-carit.de>,
	linux-kernel@vger.kernel.org, Matt Denton <mpdenton@google.com>,
	John Fastabend <john.r.fastabend@intel.com>,
	linux-fsdevel@vger.kernel.org, Al Viro <viro@zeniv.linux.org.uk>,
	cgroups@vger.kernel.org, Tejun Heo <tj@kernel.org>,
	stable@vger.kernel.org, "David S . Miller" <davem@davemloft.net>
Subject: Re: [PATCH v3 1/4] fs, net: Standardize on file_receive helper to move fds across processes
Date: Tue, 9 Jun 2020 13:55:42 -0700	[thread overview]
Message-ID: <202006091346.66B79E07@keescook> (raw)
In-Reply-To: <20200609200346.3fthqgfyw3bxat6l@wittgenstein>

On Tue, Jun 09, 2020 at 10:03:46PM +0200, Christian Brauner wrote:
> I'm looking at __scm_install_fd() and I wonder what specifically you
> mean by that? The put_user() seems to be placed such that the install
> occurrs only if it succeeded. Sure, it only handles a single fd but
> whatever. Userspace knows that already. Just look at systemd when a msg
> fails:
> 
> void cmsg_close_all(struct msghdr *mh) {
>         struct cmsghdr *cmsg;
> 
>         assert(mh);
> 
>         CMSG_FOREACH(cmsg, mh)
>                 if (cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SCM_RIGHTS)
>                         close_many((int*) CMSG_DATA(cmsg), (cmsg->cmsg_len - CMSG_LEN(0)) / sizeof(int));
> }
> 
> The only reasonable scenario for this whole mess I can think of is sm like (pseudo code):
> 
> fd_install_received(int fd, struct file *file)
> {
>  	sock = sock_from_file(fd, &err);
>  	if (sock) {
>  		sock_update_netprioidx(&sock->sk->sk_cgrp_data);
>  		sock_update_classid(&sock->sk->sk_cgrp_data);
>  	}
> 
> 	fd_install();
> }
> 
> error = 0;
> fdarray = malloc(fdmax);
> for (i = 0; i < fdmax; i++) {
> 	fdarray[i] = get_unused_fd_flags(o_flags);
> 	if (fdarray[i] < 0) {
> 		error = -EBADF;
> 		break;
> 	}
> 
> 	error = security_file_receive(file);
> 	if (error)
> 		break;
> 
> 	error = put_user(fd_array[i], ufd);
> 	if (error)
> 		break;
> }
> 
> for (i = 0; i < fdmax; i++) {
> 	if (error) {
> 		/* ignore errors */
> 		put_user(-EBADF, ufd); /* If this put_user() fails and the first one succeeded userspace might now close an fd it didn't intend to. */
> 		put_unused_fd(fdarray[i]);
> 	} else {
> 		fd_install_received(fdarray[i], file);
> 	}
> }

I see 4 cases of the same code pattern (get_unused_fd_flags(),
sock_update_*(), fd_install()), one of them has this difficult put_user()
in the middle, and one of them has a potential replace_fd() instead of
the get_used/fd_install. So, to me, it makes sense to have a helper that
encapsulates the common work that each of those call sites has to do,
which I keep cringing at all these suggestions that leave portions of it
outside the helper.

If it's too ugly to keep the put_user() in the helper, then we can try
what was suggested earlier, and just totally rework the failure path for
SCM_RIGHTS.

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.

-- 
Kees Cook

  reply	other threads:[~2020-06-09 20:55 UTC|newest]

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20200603011044.7972-1-sargun@sargun.me>
2020-06-03  1:10 ` [PATCH v3 1/4] fs, net: Standardize on file_receive helper to move fds across processes Sargun Dhillon
2020-06-04  1:24   ` Christian Brauner
2020-06-04  2:22     ` Kees Cook
2020-06-04  5:20       ` Sargun Dhillon
2020-06-04 12:52       ` Christian Brauner
2020-06-04 13:28         ` David Laight
2020-06-05  7:54         ` Sargun Dhillon
2020-06-09 19:43           ` Kees Cook
2020-06-09 20:03             ` Christian Brauner
2020-06-09 20:55               ` Kees Cook [this message]
2020-06-09 21:27                 ` Christian Brauner
2020-06-10  5:27                   ` Kees Cook
2020-06-10  8:12                     ` Sargun Dhillon
2020-06-10  8:48                       ` David Laight
2020-06-11  3:02                         ` Kees Cook
2020-06-11  7:51                           ` David Laight
2020-06-10 17:10                       ` Kees Cook
2020-06-11  2:59                       ` Kees Cook
2020-06-11  4:41                         ` Sargun Dhillon
2020-06-11  9:19                         ` Christian Brauner
2020-06-11 10:39                           ` Sargun Dhillon
2020-06-11 23:23                             ` Kees Cook
2020-06-11 10:01                         ` Christian Brauner
2020-06-11 11:06                           ` Sargun Dhillon
2020-06-11 14:42                             ` Christian Brauner
2020-06-11 14:56                             ` David Laight
2020-06-11 23:49                               ` Kees Cook
2020-06-12  6:58                                 ` Kees Cook
2020-06-12  8:36                                 ` David Laight
2020-06-12 10:46                                   ` Sargun Dhillon
2020-06-12 15:13                                     ` Kees Cook
2020-06-12 15:55                                       ` David Laight
2020-06-12 18:28                                       ` Christian Brauner
2020-06-12 18:38                                         ` Kees Cook
2020-06-12 18:42                                           ` Christian Brauner
2020-06-15  8:27                                         ` David Laight
2020-06-10  9:30                   ` Christian Brauner
2020-06-04  3:39     ` Sargun Dhillon
2020-06-03  1:10 ` [PATCH v3 2/4] pid: Use file_receive helper to copy FDs Sargun Dhillon
2020-06-03  1:10 ` [PATCH v3 3/4] seccomp: Introduce addfd ioctl to seccomp user notifier Sargun Dhillon
2020-06-03  1:10 ` [PATCH v3 4/4] selftests/seccomp: Test SECCOMP_IOCTL_NOTIF_ADDFD Sargun Dhillon

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