Re: [PATCH v4 1/4] seccomp: add a return code to trap to userspace

Tycho Andersen
2018-06-22T15:15:23Z

Hi Jann,

On Fri, Jun 22, 2018 at 04:40:20PM +0200, Jann Horn wrote:
> On Fri, Jun 22, 2018 at 12:05 AM Tycho Andersen <tycho@tycho.ws> wrote:
> > This patch introduces a means for syscalls matched in seccomp to notify
> > some other task that a particular filter has been triggered.
> >
> > The motivation for this is primarily for use with containers. For example,
> > if a container does an init_module(), we obviously don't want to load this
> > untrusted code, which may be compiled for the wrong version of the kernel
> > anyway. Instead, we could parse the module image, figure out which module
> > the container is trying to load and load it on the host.
> >
> > As another example, containers cannot mknod(), since this checks
> > capable(CAP_SYS_ADMIN). However, harmless devices like /dev/null or
> > /dev/zero should be ok for containers to mknod, but we'd like to avoid hard
> > coding some whitelist in the kernel. Another example is mount(), which has
> > many security restrictions for good reason, but configuration or runtime
> > knowledge could potentially be used to relax these restrictions.
> >
> > This patch adds functionality that is already possible via at least two
> > other means that I know about, both of which involve ptrace(): first, one
> > could ptrace attach, and then iterate through syscalls via PTRACE_SYSCALL.
> > Unfortunately this is slow, so a faster version would be to install a
> > filter that does SECCOMP_RET_TRACE, which triggers a PTRACE_EVENT_SECCOMP.
> > Since ptrace allows only one tracer, if the container runtime is that
> > tracer, users inside the container (or outside) trying to debug it will not
> > be able to use ptrace, which is annoying. It also means that older
> > distributions based on Upstart cannot boot inside containers using ptrace,
> > since upstart itself uses ptrace to start services.
> >
> > The actual implementation of this is fairly small, although getting the
> > synchronization right was/is slightly complex.
> >
> > Finally, it's worth noting that the classic seccomp TOCTOU of reading
> > memory data from the task still applies here, but can be avoided with
> > careful design of the userspace handler: if the userspace handler reads all
> > of the task memory that is necessary before applying its security policy,
> > the tracee's subsequent memory edits will not be read by the tracer.
>
> I've been thinking about how one would actually write userspace code
> that uses this API, and whether PID reuse is an issue here. As far as
> I can tell, the following situation can happen:
>
> - seccomped process tries to perform a syscall that gets trapped
> - notification is sent to the supervisor
> - supervisor reads the notification
> - seccomped process gets SIGKILLed
> - new process appears with the PID that the seccomped process had
> - supervisor tries to access memory of the seccomped process via
> process_vm_{read,write}v or /proc/$pid/mem
> - supervisor unintentionally accesses memory of the new process instead
>
> This could have particularly nasty consequences if the supervisor has
> to write to memory of the seccomped process for some reason.
> It might make sense to explicitly document how the API has to be used
> to avoid such a scenario from occuring. AFAICS,
> process_vm_{read,write}v are fundamentally unsafe for this;
> /proc/$pid/mem might be safe if you do the following dance in the
> supervisor to validate that you have a reference to the right struct
> mm before starting to actually access memory:
>
> - supervisor reads a syscall notification for the seccomped process with PID $A
> - supervisor opens /proc/$A/mem [taking a reference on the mm of the
> process that currently has PID $A]
> - supervisor reads all pending events from the notification FD; if
> one of them says that PID $A was signalled, send back -ERESTARTSYS (or
> -ERESTARTNOINTR?) and bail out
> - [at this point, the open FD to /proc/$A/mem is known to actually
> refer to the mm struct of the seccomped process]
> - read and write on the open FD to /proc/$A/mem as necessary
> - send back the syscall result

Yes, this is a nasty problem :(. We have the id in the
request/response structs to avoid this race, so perhaps we can re-use
that? So it would look like:

- supervisor gets syscall notification for $A
- supervisor opens /proc/$A/mem or /proc/$A/map_files/... or a dir fd
to the container's root or whatever
- supervisor calls seccomp(SECCOMP_NOTIFICATION_IS_VALID, req->id, listener_fd)
- supervisor knows that the fds it has open are safe

That way it doesn't have to flush the whole queue? Of course this
makes things a lot slower, but it does enable safety for more than
just memory accesses, and also isn't required for things which
wouldn't read memory.

> It might be nice if the kernel was able to directly give the
> supervisor an FD to /proc/$A/mem that is guaranteed to point to the
> right struct mm, but trying to implement that would probably make this
> patch set significantly larger?

I'll take a look and see how big it is, it doesn't *seem* like it
should be that hard. Famous last words :)

Tycho