LKML Archive on lore.kernel.org
 help / color / Atom feed
From: Kees Cook <keescook@chromium.org>
To: Tycho Andersen <tycho@tycho.pizza>
Cc: Sargun Dhillon <sargun@sargun.me>,
	Alban Crequy <alban@kinvolk.io>,
	Giuseppe Scrivano <gscrivan@redhat.com>,
	Linux Containers <containers@lists.linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	jannh@google.com
Subject: Re: SECCOMP_IOCTL_NOTIF_ADDFD race condition
Date: Tue, 1 Dec 2020 13:27:05 -0800
Message-ID: <202012011322.26DCBC64F2@keescook> (raw)
In-Reply-To: <20201201131334.GC103125@cisco>

On Tue, Dec 01, 2020 at 08:13:34AM -0500, Tycho Andersen wrote:
> On Tue, Dec 01, 2020 at 01:08:25PM +0000, Sargun Dhillon wrote:
> > On Tue, Dec 01, 2020 at 07:41:05AM -0500, Tycho Andersen wrote:
> > > On Mon, Nov 30, 2020 at 06:20:09PM -0500, Tycho Andersen wrote:
> > > > Idea 1 sounds best to me, but maybe that's because it's the way I
> > > > originally did the fd support that never landed :)
> > > > 
> > > > But here's an Idea 4: we add a way to remotely close an fd (I don't
> > > > see that the current infra can do this, but perhaps I didn't look hard
> > > > enough), and then when you get ENOENT you have to close the fd. Of
> > > > course, this can't be via seccomp, so maybe it's even more racy.
> > > 
> > > Or better yet: what if the kernel closed everything it had added via
> > > ADDFD if it didn't get a valid response from the supervisor? Then
> > > everyone gets this bug fixed for free.
> > > 
> > > Tycho
> > > _______________________________________________
> > > Containers mailing list
> > > Containers@lists.linux-foundation.org
> > > https://lists.linuxfoundation.org/mailman/listinfo/containers
> > 
> > This doesn't solve the problem universally because of the (Go) preemption 
> > problem. Unless we can guarantee that the supervisor can always handle the 
> > request in fewer than 10ms, or if it implements resumption behaviour. I know 
> > that resumption behaviour is a requirement no matter what, but the easier we can 
> > make it to implement resumption, the better chance we are giving users to get 
> > this right.
> 
> Doesn't automatic cleanup of fds make things easier? I'm not sure I
> understand the argument.

I doubt Al would ever allow the "cleanup" approach: his observation was
that the instant a file has been added to the fdtable, it's not possible
to "unwind" that ever, since it could be cloned away, etc, etc.

> I agree it doesn't fix the problem of uncooperative userspace.

IIUC, I see two issues:

- a slow monitor might cause a child to loop forever retrying the same
  interrupted syscall.

- a syscall-interrupted process may have had an fd added that it has no
  idea about.

The former problem seems like a userspace issue. :P But, to help, yeah, is
signal blocking best? Either explicit (at filter apply time) or implicit
(all user_notif-triggering syscalls get all signals blocks automatically)?

For the latter problem, I think we need to get back to Tycho's original
method: add fd and finish syscall in a single action. I can't see any
other way to get around the need for atomicity...

-- 
Kees Cook

      reply index

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-26 13:09 Alban Crequy
2020-11-30 23:20 ` Tycho Andersen
2020-12-01  4:08   ` Sargun Dhillon
2020-12-01 12:41   ` Tycho Andersen
2020-12-01 13:08     ` Sargun Dhillon
2020-12-01 13:13       ` Tycho Andersen
2020-12-01 21:27         ` Kees Cook [this message]

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=202012011322.26DCBC64F2@keescook \
    --to=keescook@chromium.org \
    --cc=alban@kinvolk.io \
    --cc=containers@lists.linux-foundation.org \
    --cc=gscrivan@redhat.com \
    --cc=jannh@google.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=sargun@sargun.me \
    --cc=tycho@tycho.pizza \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lore.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lore.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lore.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lore.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lore.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lore.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lore.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lore.kernel.org/lkml/8 lkml/git/8.git
	git clone --mirror https://lore.kernel.org/lkml/9 lkml/git/9.git
	git clone --mirror https://lore.kernel.org/lkml/10 lkml/git/10.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lore.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git