On Thu, Nov 01, 2018 at 03:48:05PM +0100, Oleg Nesterov wrote:

On 10/30, Tycho Andersen wrote:

quoted

quoted

I am not sure I understand the value of signaled/SECCOMP_NOTIF_FLAG_SIGNALED...
I mean, why it is actually useful?

Sorry if this was already discussed.

:) no problem, many people have complained about this. This is an
implementation of Andy's suggestion here:
https://lkml.org/lkml/2018/3/15/1122

You can see some more detailed discussion here:
https://lkml.org/lkml/2018/9/21/138

Cough, sorry, I simply can't understand what are you talking about ;)
It seems that I need to read all the previous emails... So let me ask
a stupid question below.

quoted

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But my main concern is that either way wait_for_completion_killable() allows
to trivially create a process which doesn't react to SIGSTOP, not good...

Note also that this can happen if, say, both the tracer and tracee run in the
same process group and SIGSTOP is sent to their pgid, if the tracer gets the
signal first the tracee won't stop.

Of freezer. try_to_freeze_tasks() can fail if it freezes the tracer before
it does SECCOMP_IOCTL_NOTIF_SEND.

I think in general the way this is intended to be used these things
wouldn't happen.

Why?

The intent is to run the tracer on the host and have it trace
containers, which would live in a different freezer cgroup, process
group, etc. Of course you could use it in a situation where they would
be, so the concern is still valid, but I'm not sure why you'd do that.

quoted

was malicious and had the ability to create a user namespace to
exhaust pids this way,

Not sure I understand how this connects to my question... nevermind.

quoted

so perhaps we should drop this part of the
patch. I have no real need for it, but perhaps Andy can elaborate?

Yes I think it would be nice to avoid wait_for_completion_killable().

So please help me to understand the problem. Once again, why can not
seccomp_do_user_notification() use wait_for_completion_interruptible() only?

This is called before the task actually starts the syscall, so
-ERESTARTNOINTR if signal_pending() can't hurt.

The idea was that when the tracee gets a signal, it notifies the
tracer exactly once, and then waits for the tracer to decide what to
do. So if we use another wait_for_completion_interruptible(), doesn't
it just get re-woken immediately because the signal is still pending?

...actually I just tested it, and it doesn't. So it seems we could use
_interruptible() here and achieve the same thing.

Now lets suppose seccomp_do_user_notification() simply does

	err = wait_for_completion_interruptible(&n.ready);

	if (err < 0 && state != SECCOMP_NOTIFY_REPLIED) {
		syscall_set_return_value(ERESTARTNOINTR);
		list_del(&n.list);
		return -1;
	}

(I am ignoring the locking/etc). Now the obvious problem is that the listener
doing SECCOMP_IOCTL_NOTIF_SEND can't distinguish -ENOENT from the case when the
tracee was killed, yes?

Is it that important?

The answer to this question depends on how we want the listener to be
able to react. For example, if the listener is in the middle of doing
a mount() on behalf of the task and it gets a signal and we return
immediately, the listener will complete the mount(), try to respond
with success and get -ENOENT. If the task handles the signal and
restarts the mount(), it'll happen twice unless the listener undoes
it when it sees the -ENOENT. If we send another notification with the
SIGNALED flag, the listener has a better picture of what's going on,
which might be nice.

Tycho