[RFC PATCH bpf-next seccomp 10/12] seccomp-ebpf: Add ability to read user memory

[YiFei Zhu]

On Thu, May 13, 2021 at 9:53 AM Andy Lutomirski <luto at amacapital.net> wrote:
> > On May 12, 2021, at 10:26 PM, YiFei Zhu <zhuyifei1999 at gmail.com> wrote:
> >
> > On Wed, May 12, 2021 at 5:36 PM Alexei Starovoitov
> > <alexei.starovoitov at gmail.com> wrote:
> >> Typically the verifier does all the checks at load time to avoid
> >> run-time overhead during program execution. Then at attach time we
> >> check that attach parameters provided at load time match exactly
> >> to those at attach time. ifindex, attach_btf_id, etc fall into this category.
> >> Doing something similar it should be possible to avoid
> >> doing get_dumpable() at run-time.
> >
> > Do you mean to move the check of dumpable to load time instead of
> > runtime? I do not think that makes sense. A process may arbitrarily
> > set its dumpable attribute during execution via prctl. A process could
> > do set itself to non-dumpable, before interacting with sensitive
> > information that would better not be possible to be dumped (eg.
> > ssh-agent does this [1]). Therefore, being dumpable at one point in
> > time does not indicate anything about whether it stays dumpable at a
> > later point in time. Besides, seccomp filters are inherited across
> > clone and exec, attaching to many tasks with no option to detach. What
> > should the load-time check of task dump-ability be against? The
> > current task may only be the tip of an iceburg.
> >
> > [1] https://github.com/openssh/openssh-portable/blob/2dc328023f60212cd29504fc05d849133ae47355/ssh-agent.c#L1398
> >
> >
>
> First things first: why are you checking dumpable at all?  Once you figure out why and whether it’s needed, you may learn something about what task to check.
>
> I don’t think checking dumpable makes any sense.

ptrace. We don't want to extend one's ability to read another
process's memory if they could not read it via ptrace
(process_vm_readv or ptrace(PTRACE_PEEK{TEXT,DATA})). The constraints
for ptrace to access a target's memory I've written down earlier [1],
but tl;dr: to be at least as restrictive as ptrace, a tracer without
CAP_PTRACE cannot trace a non-dumpable process. What's the target
process (i.e. the process whose memory is being read) in the context
of a seccomp filter? The current task. Does that answer your
questions?

[1] https://lore.kernel.org/bpf/CABqSeAT8iz-VhWjWqABqGbF7ydkoT7LmzJ5Do8K1ANQvQK=FJQ@mail.gmail.com/

YiFei Zhu