Re: [PATCH v2 2/2] bpf/selftests: Add selftests for new task kfuncs

[Martin KaFai Lau <martin.lau@linux.dev>]

On 10/3/22 8:56 AM, Kumar Kartikeya Dwivedi wrote:
>>> Also, could you include a test to make sure sleepable programs cannot
>>> call bpf_task_acquire? It seems to assume RCU read lock is held while
>>> that may not be true. If already not possible, maybe a WARN_ON_ONCE
>>> inside the helper to ensure future cases don't creep in.
>>
>> I don't _think_ it's unsafe for a sleepable program to call
>> bpf_task_acquire(). My understanding is that the struct task_struct *
>> parameter to bpf_task_acquire() is not PTR_UNTRUSTED, so it's safe to
>> dereference directly in the kfunc. The implicit assumption here is that
>> the task was either passed to the BPF program (which is calling
>> bpf_task_acquire()) from the main kernel in something like a trace or
>> struct_ops callback, or it was a referenced kptr that was removed from a
>> map with bpf_kptr_xchg(), and is now owned by the BPF program. Given
>> that the ptr type is not PTR_UNTRUSTED, it seemed correct to assume that
>> the task was valid in bpf_task_acquire() regardless of whether we were
>> in an RCU read region or not, but please let me know if I'm wrong about
> 
> I don't think it's correct. You can just walk arbitrary structures and
> obtain a normal PTR_TO_BTF_ID that looks seemingly ok to the verifier
> but has no lifetime guarantees. It's a separate pre-existing problem
> unrelated to your series [0]. PTR_UNTRUSTED is not set for those cases
> currently.
> 
> So the argument to your bpf_task_acquire may already be freed by then.
> This issue would be exacerbated in sleepable BPF programs, where RCU
> read lock is not held, so some cases of pointer walking where it may
> be safe no longer holds.
> 
> I am planning to clean this up, but I'd still prefer if we don't allow
> this helper in sleepable programs, yet. kptr_get is ok to allow.
> Once you have explicit BPF RCU read sections, sleepable programs can
> take it, do loads, and operate on the RCU pointer safely until they
> invalidate it with the outermost bpf_rcu_read_unlock. It's needed for
> local kptrs as well, not just this. I plan to post this very soon, so
> we should be able to fix it up in the current cycle after landing your
> series.
> 
> __rcu tags in the kernel will automatically be understood by the
> verifier and for the majority of the correctly annotated cases this
> will work fine and not break tracing programs.
> 
> [0]: https://lore.kernel.org/bpf/CAADnVQJxe1QT5bvcsrZQCLeZ6kei6WEESP5bDXf_5qcB2Bb6_Q@mail.gmail.com
> 
>> that.  Other kfuncs I saw such as bpf_xdp_ct_lookup() assumed that the
>> parameter passed by the BPF program (which itself was passing on a
>> pointer given to it by the main kernel) is valid as well.
> 
> Yeah, but the CT API doesn't assume validity of random PTR_TO_BTF_ID,
> it requires KF_TRUSTED_ARGS which forces them to have ref_obj_id != 0.

Other than ref_obj_id != 0, could the PTR_TO_BTF_ID obtained through 
btf_ctx_access be marked as trusted (e.g. the ctx[0] in the selftest here)
and bpf_task_acquire() will require KF_TRUSTED_ARGS? would it be safe in general?