[Devel] Re: [RFC][PATCH 3/5] Determine if sender is from ancestor ns

[Sukadev Bhattiprolu]

Roland McGrath [roland at redhat.com] wrote:


Thanks for the review and your comments.
> 
> I don't quarrel with the intent, but I don't like this approach.
> I think we can do it more cleanly.  I don't have it all worked out,
> but a couple of thoughts come to mind.
> 
> Firstly, it's no problem to split SIGNAL_UNKILLABLE out into multiple
> flags.  It's quite noninvasive, only signal.c looks at those flags.  Then
> we can implement the signal magic cleanly keyed on a few separate flags,
> using different flag combinations for global init and container inits.

Ok. Sounds good. Let say for this discussion, container-inits set
SIGNAL_UNKILLABLE_CINIT, global inits continue to set SIGNAL_UNKILLABLE.

> 
> I don't mind a feature that lets an unprivileged container init magically
> ignore normal exit signals.  That just does something it could do itself
> with sigaction, except its /proc/pid/status "SigIgn:" line will lie.

Yes, but after Bastian mentioned it recently, I was wondering if we can
keep "SigIgn:" honest by tweaking procfs code - i.e have procfs check
the SIGNAL_UNKILLABLE flags and include SIG_DFL exit signals in the
SigIgn: line.

We could include blocked SIG_DFL signals in the SigIgn: line ? Hacky ?

> That's OK.  Key that on its own flag and set that in both inits.  Just
> check that flag in prepare_signal() and in get_signal_to_deliver().
> 
> It's a different story for the signals that cannot be caught, blocked, or
> ignored, SIGKILL and SIGSTOP.  At least when sent from outside the
> container, circumventing either of those would be a privilege escalation
> from what's always been understood by admins and so on.  

So if SIGKILL or SIGSTOP are sent from inside the container, we drop them
in prepare_signal() if SIGNAL_UNKILLABLE flags are set. If sent from outside
the container, we queue them and get_signal_to_deliver() delivers these
unless SIGNAL_UNKILLABLE is set (i.e the signals are delivered to cinits but
ignored for global init).

> 
> It's convenient for the implementation that we can treat these differently
> from other signals, precisely because they can never be caught, blocked, or
> ignored.  That is, when we decide in prepare_signal() to queue a SIGKILL or
> SIGSTOP, it can never turn out that we're later going to drop it because it
> went from caught or ignored or blocked to uncaught, unignored, and
> unblocked.  (It's only because of that possibility that there is any need
> to check for a suppressed exit signal in get_signal_to_deliver() rather
> than only in prepare_signal().)  That means that when the decision hinges
> on the namespace correlation of the sender and receiver, you can check that
> when it's handy (current vs p in prepare_signal) rather than trying to
> reconstruct the answer for a queued signal.

Yes. One thing I am not clear on yet is how we decide in prepare_signal()
if it is safe to check the sender's namespace (since caller of send_signal()
could be an interrupt handler or workqueue).

As I was discussing with Bastian Blank, SI_FROMUSER() is not sufficient
since SI_ASYNCIO appears as an user-space signal.

Now is SI_ASYNCIO really supposed to be a user signal ? Or like SI_MESGQ
and SI_POLL can it be a kernel signal ? If we fix SI_ASYNCIO, can we
safely use SI_FROMUSER() for this  or could there be other 'user' signals
from kernel ?

> 
> Finally, one more thought.  This may be moot for the problem at hand if you
> take the approach I just suggested, but probably should be fixed anyway.
> It seems to me that the si_pid and si_uid fields of siginfo_t ought to be
> translated to the namespaces of the receiver.  I think it makes most sense
> to do this on the front end, i.e. in the callers that fill in the siginfo_t
> in the first place (sys_kill et al, or maybe a few layers down?).
> Currently it's inconsistent, but mostly wrong.

Yes, that makes a lot of sense to push that initialization to the callers.
We have been going through and identifying the 'si_pid's that need to be fixed.
Nadia sent out one patch for ipc/mqueue.c

> do_notify_parent() and
> do_notify_parent_cldstop() use the receiver's namespace to compute si_pid,
> but the rest of the signal.c routines do not.  A free side effect of doing
> this is that si_pid for a sender whose PID is not visible to the receiver
> (i.e. outside its container) would be distinctively 0 or -1 or something.
> (-1 might be the best choice, since si_[pu]id=0 already arises now in case
> of signal queue exhaustion and the like.)  Hence, possibly one could simply
> use si_pid>0 as a "sent from inside the container" check on a queued siginfo_t.

Yes, its probably moot with the other approach, but the only drawback with
relying on the "->si_pid > 0" check in get_signal_to_deliver() was that
allocation of siginfo_t could have failed. In which case it would be
unclear whether signal was from parent or same namespace.

But if we fix si_pid problems and correctly pass in siginfo_t, can we
use "si_pid > 0" check in prepare_signal() to decide whether or not
to queue the signal ?

Sukadev
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