Re: [PATCH 1/3] bpf: allow zero-initializing hash map seed

On Fri, Oct 5, 2018 at 4:21 PM Lorenz Bauer [off-list ref] wrote:

On Fri, 5 Oct 2018 at 15:12, Jann Horn [off-list ref] wrote:

quoted

On Fri, Oct 5, 2018 at 9:42 AM Lorenz Bauer [off-list ref] wrote:

quoted

On Tue, 2 Oct 2018 at 21:00, Jann Horn [off-list ref] wrote:

quoted

If this is for testing only, you can slap a capable(CAP_SYS_ADMIN)
check in here, right? I doubt it matters, but I don't really like
seeing something like this exposed to unprivileged userspace just
because you need it for kernel testing.

That would mean all tests have to run as root / with CAP_SYS_ADMIN
which isn't ideal.

This patch basically means that it becomes easier for a local user to
construct a BPF hash table that has all of its values stuffed into a
single hash bucket, correct? Which makes it easier to create a BPF
program that generates unusually large RCU stalls by performing ~40000
BPF map lookups, each of which has to walk through the entire linked
list of the hash map bucket? I dislike exposing something like that to
unprivileged userspace.

That's a good point, for which I don't have an answer. You could argue that
this was the status quo until the seed was randomised, so it seems
like this hasn't been a worry so far. Should it be going forward?

I don't think that local DoS bugs, or bugs that locally degrade
performance, are a big deal, but I also think that the kernel should
try to avoid having such issues.

quoted

And if you want to run the whole BPF test suite with all its tests,
don't you already need root privileges? Or is this a different test
suite?

No, I'm thinking about third parties that want to test their own BPF.

Ah. That wasn't clear to me from your patch description.

Can you please describe exactly why something that is not a kernel
unit test needs deterministic BPF hash map behavior?

If you enable unprivileged BPF you can use BPF_PROG_TEST_RUN to
test your programs without root, if I'm not mistaken.