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From:   Christian Brauner <christian.brauner@canonical.com>
Date:   Wed, 31 Oct 2018 00:23:16 +0100
Message-ID: <CAPP7u0VQqDvFr=-52+Tmv_K=vT730wG7w9VQqJggsXTPFvaUBQ@mail.gmail.com>
Subject: Re: [RFC PATCH] Implement /proc/pid/kill
To:     Daniel Colascione <dancol@google.com>
Cc:     joel@joelfernandes.org, Aleksa Sarai <cyphar@cyphar.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Tim Murray <timmurray@google.com>,
        Suren Baghdasaryan <surenb@google.com>
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On Wed, Oct 31, 2018 at 12:10 AM Daniel Colascione <dancol@google.com> wrote:
>
> On Tue, Oct 30, 2018 at 10:33 PM, Joel Fernandes <joel@joelfernandes.org> wrote:
> > On Wed, Oct 31, 2018 at 09:23:39AM +1100, Aleksa Sarai wrote:
> >> On 2018-10-30, Joel Fernandes <joel@joelfernandes.org> 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
> $PID being used to name some other process. If it could, you could
> pretty quickly fill a whole system's process table. See the program
> below, which demonstrates the PID collision.
>
> I think Aleksa's larger point is that it's useful to treat processes
> as other file-descriptor-named, poll-able, wait-able resources.
> Consistency is important. A process is just another system resource,
> and like any other system resource, you should be open to hold a file
> descriptor to it and do things to that process via that file
> descriptor. The precise form of this process-handle FD is up for
> debate. The existing /proc/$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.

That's what I'm proposing in the API for which I'm gathering feedback.
I have presented parts of this in various discussions at LSS Europe last week
and will be at LPC.
We don't want to rush an API like this though. It was tried before in
other forms
and these proposals didn't make it.


>
> #include <fcntl.h>
> #include <signal.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <sys/stat.h>
> #include <sys/types.h>
> #include <sys/wait.h>
> #include <unistd.h>
>
> int
> main()
> {
>     int child_pid = fork();
>     if (child_pid < 0)
>         abort();
>
>     char buf[64];
>     int child_procfs_fd;
>
>     if (child_pid == 0) {
>         for (;;)
>             pause();
>         abort();
>     }
>
>     printf("child PID is %d\n", child_pid);
>     sprintf(buf, "/proc/%d", child_pid);
>     child_procfs_fd = open(buf, O_DIRECTORY | O_RDONLY);
>     if (child_procfs_fd < 0)
>         abort();
>     printf("FD# of open /proc/%d is %d\n",
>            child_pid,
>            child_procfs_fd);
>     printf("killing child with SIGKILL\n");
>     kill(child_pid, SIGKILL);
>     if (wait(NULL) != child_pid)
>         abort();
>     printf("child is now dead. PROCFS FD STILL OPEN\n");
>     for (;;) {
>         int new_child_pid = fork();
>         if (new_child_pid < 0)
>             abort();
>         if (new_child_pid == 0)
>             _exit(0);
>         // printf("new child PID: %d\n", new_child_pid);
>         if (wait(NULL) != new_child_pid)
>             abort();
>         if (new_child_pid == child_pid) {
>             printf("FOUND PID COLLISION %d\n", child_pid);
>             printf("old child had pid %d. new, "
>                    "different child has pid %d. "
>                    "procfs directory for old child still open!\n",
>                    child_pid, child_pid);
>             break;
>         }
>     }
>
>     return 0;
> }