On Wed, Nov 02, 2022 at 04:01:11AM +0530, Kumar Kartikeya Dwivedi wrote: > On Wed, 2 Nov 2022 at 03:06, David Vernet <void@xxxxxxxxxxxxx> wrote: > > > > On Tue, Nov 01, 2022 at 01:22:39PM -0700, Alexei Starovoitov wrote: > > > On Tue, Nov 1, 2022 at 11:11 AM David Vernet <void@xxxxxxxxxxxxx> wrote: > > > > > > > > > What kind of bpf prog will be able to pass 'struct nf_conn___init *' into these bpf_ct_* ? > > > > > We've introduced / vs nf_conf specifically to express the relationship > > > > > between allocated nf_conn and other nf_conn-s via different types. > > > > > Why is this not enough? > > > > > > > > Kumar should have more context here (he originally suggested this in > > > > [0]), > > > > > > Quoting: > > > " > > > Unfortunately a side effect of this change is that now since > > > PTR_TO_BTF_ID without ref_obj_id is considered trusted, the bpf_ct_* > > > functions would begin working with tp_btf args. > > > " > > > I couldn't find any tracepoint that has nf_conn___init as an argument. > > > The whole point of that new type was to return it to bpf prog, > > > so the verifier type matches it when it's passed into bpf_ct_* > > > in turn. > > > So I don't see a need for a new OWNED flag still. > > > If nf_conn___init is passed into tracepoint it's a bug and > > > we gotta fix it. > > > > Yep, this is what I'm seeing as well. I think you're right that > > KF_OWNED_ARGS is just strictly unnecessary and that creating wrapper > > types is the way to enable an ownership model like this. > > > > It's not just nf_conn___init. Some CT helpers also take nf_conn. > e.g. bpf_ct_change_timeout, bpf_ct_change_status. > Right now they are only allowed in XDP and TC programs, so the tracing > args part is not a problem _right now_. ... and it will be fine to use bpf_ct_change_timeout from tp_btf as well. > So currently it may not be possible to pass such a trusted but > ref_obj_id == 0 nf_conn to those helpers. > But based on changes unrelated to this, it may become possible in the > future to obtain such a trusted nf_conn pointer. >From kfunc's pov trusted pointer means valid pointer. It doesn't need to be ref_obj_id refcounted from the verifier pov. It can be refcounted on the kernel side and it will be trusted. The code that calls trace_*() passes only trusted pointers into tp-s. If there is a tracepoint somewhere in the kernel that uses a volatile pointer to potentially uaf kernel object it's a bug that should be fixed. > It is a requirement of those kfuncs that the nf_conn has its refcount > held while they are called. and it will be. Just not by the verifier. > KF_TRUSTED_ARGS was encoding this requirement before, but it wouldn't anymore. > It seems better to me to keep that restriction instead of relaxing it, > if it is part of the contract. Disagree as explained above. > It is fine to not require people to dive into these details and just > use KF_TRUSTED_ARGS in general, but we need something to cover special > cases like these where the object is only stable while we hold an > active refcount, RCU protection is not enough against reuse. This is not related to RCU. Let's not mix RCU concerns in here. It's a different topic. > It could be 'expert only' __ref suffix on the nf_conn arg, or > KF_OWNED_ARGS, or something else. I'm still against that. > > > > [...] > > > > > > > > > This PTR_WALKED looks like new thing. > > > > > If we really need it PTR_TO_BTF_ID should be allowlisted instead of denylisted > > > > > as PTR_WALKED is doing. > > > > > I mean we can introduce PTR_TRUSTED and add this flag to return value > > > > > of bpf_get_current_task_btf() and arguments of tracepoints. > > > > > As soon as any ptr walking is done we can clear PTR_TRUSTED to keep > > > > > backward compat behavior of PTR_TO_BTF_ID. > > > > > PTR_WALKED is sort-of doing the same, but not conservative enough. > > > > > Too many things produce PTR_TO_BTF_ID. Auditing it all is challenging. > > > > > > > > I very much prefer the idea of allowlisting instead of denylisting, > > > > though I wish we'd taken that approach from the start rather than going > > > > with PTR_UNTRUSTED. It feels wrong to have both PTR_UNTRUSTED and > > > > PTR_TRUSTED as type modifiers, as the absence of PTR_UNTRUSTED should > > > > (and currently does) imply PTR_TRUSTED. > > > > > > I kind agree, but we gotta have both because of backward compat. > > > We cannot change PTR_TO_BTF_ID as a whole right now. > > > > > > Note PTR_TO_BTF_ID appears in kfuncs too. > > > I'm proposing to use PTR_TO_BTF_ID | PTR_TRUSTED > > > only in tracepoint args and as return value from > > > certain helpers like bpf_get_current_task_btf(). > > > afaik it's all safe. There is no uaf here. > > > uaf is for kfunc. Especially fexit. > > > Those will stay PTR_TO_BTF_ID. Without PTR_TRUSTED. > > > > Ok, this feels like the right approach to me. Unless I'm missing > > something, modulo doing our due diligence and checking if there are any > > existing kfuncs that are relying on different behavior, once this lands > > I think we could maybe even make KF_TRUSTED_ARGS the default for all > > kfuncs? That should probably be done in a separate patch set though. > > > > I do like the allowlist vs denylist point from Alexei. It was also > what I originally suggested in [0], but when I went looking, pointer > walking is really the only case that was problematic, which was being > marked by PTR_WALKED. The other case of handling fexit is unrelated to > both. Args of fentry and fexit will not have PTR_TO_BTF_ID | PTR_TRUSTED. So not an issue. We can allowlist certain hooks. Like all of bpf-lsm hooks and many others. But not all of them.
