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

[Joel Fernandes]

On Tue, Oct 30, 2018 at 11:10:47PM +0000, Daniel Colascione wrote:
> On Tue, Oct 30, 2018 at 10:33 PM, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> > 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/$pid because
> >> it makes reuse handling very trivial and is always correct, but that
> >> there are things which stop us from being able to use it for everything
> >> (which is what my incoherent rambling was on about).
> >
> > Ok thanks. So I am guessing if the following sequence works, then Dan's patch is not
> > needed.
> >
> > 1. open /proc/<pid> directory
> > 2. inspect /proc/<pid> or do whatever with <pid>
> > 3. Issue the kill on <pid>
> > 4. Close the /proc/<pid> directory opened in step 1.
> >
> > So unless I missed something, the above sequence will not cause any PID reuse
> > races.
> 
> Keeping a /proc/$PID directory file descriptor open does not prevent
> $PID being used to name some other process. If it could, you could
> pretty quickly fill a whole system's process table. See the program
> below, which demonstrates the PID collision.

I know. We both were not sure about that earlier, that's why I requested you
to write the program when we were privately chatting. Now I'm sure because
Aleska answered that and the you program you wrote showed that too.

I wonder if this cannot be plumbed by just making the /proc/$PID directory
opens hold a reference to task_struct (and a reference to whatever else is
supposed to prevent the PID from getting reused), instead of introducing a
brand new API.

> I think Aleksa's larger point is that it's useful to treat processes
> as other file-descriptor-named, poll-able, wait-able resources.
> Consistency is important. A process is just another system resource,
> and like any other system resource, you should be open to hold a file
> descriptor to it and do things to that process via that file
> descriptor. The precise form of this process-handle FD is up for
> debate. The existing /proc/$PID directory FD is a good candidate for a
> process handle FD, since it does almost all of what's needed. But
> regardless of what form a process handle FD takes, we need it. I don't
> see a case for continuing to treat processes in a non-unixy,
> non-file-descriptor-based manner.

So wait, how is that supposed to address what you're now saying above
"quickly fill a whole process table"? You either want this, or you don't :)