Re: [PATCH v3 bpf-next 13/15] bpf: Prepare bpf_mem_alloc to be used by sleepable bpf programs.

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, Aug 24, 2022 at 12:50 PM Kumar Kartikeya Dwivedi
<memxor@gmail.com> wrote:
>
> On Sat, 20 Aug 2022 at 01:01, Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Fri, Aug 19, 2022 at 3:56 PM Kumar Kartikeya Dwivedi
> > <memxor@gmail.com> wrote:
> > >
> > > On Sat, 20 Aug 2022 at 00:43, Alexei Starovoitov
> > > <alexei.starovoitov@gmail.com> wrote:
> > > >
> > > > On Sat, Aug 20, 2022 at 12:21:46AM +0200, Kumar Kartikeya Dwivedi wrote:
> > > > > On Fri, 19 Aug 2022 at 23:43, Alexei Starovoitov
> > > > > <alexei.starovoitov@gmail.com> wrote:
> > > > > >
> > > > > > From: Alexei Starovoitov <ast@kernel.org>
> > > > > >
> > > > > > Use call_rcu_tasks_trace() to wait for sleepable progs to finish.
> > > > > > Then use call_rcu() to wait for normal progs to finish
> > > > > > and finally do free_one() on each element when freeing objects
> > > > > > into global memory pool.
> > > > > >
> > > > > > Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> > > > > > ---
> > > > >
> > > > > I fear this can make OOM issues very easy to run into, because one
> > > > > sleepable prog that sleeps for a long period of time can hold the
> > > > > freeing of elements from another sleepable prog which either does not
> > > > > sleep often or sleeps for a very short period of time, and has a high
> > > > > update frequency. I'm mostly worried that unrelated sleepable programs
> > > > > not even using the same map will begin to affect each other.
> > > >
> > > > 'sleep for long time'? sleepable bpf prog doesn't mean that they can sleep.
> > > > sleepable progs can copy_from_user, but they're not allowed to waste time.
> > >
> > > It is certainly possible to waste time, but indirectly, not through
> > > the BPF program itself.
> > >
> > > If you have userfaultfd enabled (for unpriv users), an unprivileged
> > > user can trap a sleepable BPF prog (say LSM) using bpf_copy_from_user
> > > for as long as it wants. A similar case can be done using FUSE, IIRC.
> > >
> > > You can then say it's a problem about unprivileged users being able to
> > > use userfaultfd or FUSE, or we could think about fixing
> > > bpf_copy_from_user to return -EFAULT for this case, but it is totally
> > > possible right now for malicious userspace to extend the tasks trace
> > > gp like this for minutes (or even longer) on a system where sleepable
> > > BPF programs are using e.g. bpf_copy_from_user.
> >
> > Well in that sense userfaultfd can keep all sorts of things
> > in the kernel from making progress.
> > But nothing to do with OOM.
> > There is still the max_entries limit.
> > The amount of objects in waiting_for_gp is guaranteed to be less
> > than full prealloc.
>
> My thinking was that once you hold the GP using uffd, we can assume
> you will eventually hit a case where all such maps on the system have
> their max_entries exhausted. So yes, it probably won't OOM, but it
> would be bad regardless.
>
> I think this just begs instead that uffd (and even FUSE) should not be
> available to untrusted processes on the system by default. Both are
> used regularly to widen hard to hit race conditions in the kernel.
>
> But anyway, there's no easy way currently to guarantee the lifetime of
> elements for the sleepable case while being as low overhead as trace
> RCU, so it makes sense to go ahead with this.

Right. We evaluated SRCU for sleepable and it had too much overhead.
That's the reason rcu_tasks_trace was added and sleepable bpf progs
is the only user so far.
The point I'm arguing is that call_rcu_tasks_trace in this patch
doesn't add mm concerns more than the existing call_rcu.
There is CONFIG_PREEMPT_RCU and RT. uffd will cause similar
issues in such configs too.