Re: [RFC PATCH] Implement /proc/pid/kill

[Joel Fernandes]

On Wed, Oct 31, 2018 at 09:23:39AM +1100, Aleksa Sarai wrote:
> On 2018-10-30, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> > On Wed, Oct 31, 2018 at 07:45:01AM +1100, Aleksa Sarai wrote:
> > [...] 
> > > > > (Unfortunately
> > > > > there are lots of things that make it a bit difficult to use /proc/$pid
> > > > > exclusively for introspection of a process -- especially in the context
> > > > > of containers.)
> > > > 
> > > > Tons of things already break without a working /proc. What do you have in mind?
> > > 
> > > Heh, if only that was the only blocker. :P
> > > 
> > > The basic problem is that currently container runtimes either depend on
> > > some non-transient on-disk state (which becomes invalid on machine
> > > reboots or dead processes and so on), or on long-running processes that
> > > keep file descriptors required for administration of a container alive
> > > (think O_PATH to /dev/pts/ptmx to avoid malicious container filesystem
> > > attacks). Usually both.
> > > 
> > > What would be really useful would be having some way of "hiding away" a
> > > mount namespace (of the pid1 of the container) that has all of the
> > > information and bind-mounts-to-file-descriptors that are necessary for
> > > administration. If the container's pid1 dies all of the transient state
> > > has disappeared automatically -- because the stashed mount namespace has
> > > died. In addition, if this was done the way I'm thinking with (and this
> > > is the contentious bit) hierarchical mount namespaces you could make it
> > > so that the pid1 could not manipulate its current mount namespace to
> > > confuse the administrative process. You would also then create an
> > > intermediate user namespace to help with several race conditions (that
> > > have caused security bugs like CVE-2016-9962) we've seen when joining
> > > containers.
> > > 
> > > Unfortunately this all depends on hierarchical mount namespaces (and
> > > note that this would just be that NS_GET_PARENT gives you the mount
> > > namespace that it was created in -- I'm not suggesting we redesign peers
> > > or anything like that). This makes it basically a non-starter.
> > > 
> > > But if, on top of this ground-work, we then referenced containers
> > > entirely via an fd to /proc/$pid then you could also avoid PID reuse
> > > races (as well as being able to find out implicitly whether a container
> > > has died thanks to the error semantics of /proc/$pid). And that's the
> > > way I would suggest doing it (if we had these other things in place).
> > 
> > I didn't fully follow exactly what you mean. If you can explain for the
> > layman who doesn't know much experience with containers..
> > 
> > Are you saying that keeping open a /proc/$pid directory handle is not
> > sufficient to prevent PID reuse while the proc entries under /proc/$pid are
> > being looked into? If its not sufficient, then isn't that a bug? If it is
> > sufficient, then can we not just keep the handle open while we do whatever we
> > want under /proc/$pid ?
> 
> Sorry, I went on a bit of a tangent about various internals of container
> runtimes. My main point is that I would love to use /proc/$pid because
> it makes reuse handling very trivial and is always correct, but that
> there are things which stop us from being able to use it for everything
> (which is what my incoherent rambling was on about).

Ok thanks. So I am guessing if the following sequence works, then Dan's patch is not
needed.

1. open /proc/<pid> directory
2. inspect /proc/<pid> or do whatever with <pid>
3. Issue the kill on <pid>
4. Close the /proc/<pid> directory opened in step 1.

So unless I missed something, the above sequence will not cause any PID reuse
races.

- Joel