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

On Thu, May 13, 2021 at 9:53 AM Andy Lutomirski [off-list ref] wrote:

quoted

On May 12, 2021, at 10:26 PM, YiFei Zhu [off-list ref] wrote:

On Wed, May 12, 2021 at 5:36 PM Alexei Starovoitov
[off-list ref] wrote:

quoted

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/ (local)

YiFei Zhu