On Thu, Nov 03, 2022 at 02:18:35AM +0530, Kumar Kartikeya Dwivedi wrote: > On Wed, Nov 02, 2022 at 08:04:57AM IST, Alexei Starovoitov wrote: > > On Tue, Nov 1, 2022 at 6:01 PM Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> wrote: > > > > > > On Wed, 2 Nov 2022 at 06:02, Alexei Starovoitov > > > <alexei.starovoitov@xxxxxxxxx> wrote: > > > > > > > > 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. > > > > > > > > > > This is all fine. I'm asking you to distinguish between > > > trusted-not-refcounted and trusted-and-refcounted. > > > > That's not what you're asking :) > > > > > It is necessary for nf_conn, since the object can be reused if the > > > refcount is not held. > > > > of course. No one argues the opposite. > > > > > Some other CPU could be reusing the same memory and allocating a new > > > nf_conn on it while we change its status. > > > So it's not ok to call bpf_ct_change_timeout/status on trusted > > > nf_conn, but only on trusted+refcounted nf_conn. > > > > and here we start to disagree. > > > > > Trusted doesn't capture the difference between 'valid' vs 'valid and > > > owned by prog' anymore with the new definition > > > for PTR_TO_BTF_ID. > > > > and here we disagree completely. > > You're asking to distinguish refcnt++ done by the program > > and recognized by the verifier as ref_obj_id > 0 vs > > refcnt++ done by the kernel code before it calls into tracepoint. > > That's odd, right? > > I don't think people adding kfuncs should care what piece > > of code before kfunc did refcnt++. > > > > I think we're talking past each other. Your point is that whenever PTR_TRUSTED > is set (tracepoint args, etc.) the kernel itself should ensure it holds the > refcount on behalf of the program. There shouldn't be a scenario where > PTR_TRUSTED nf_conn is obtained and the program or the kernel isn't holding > an active refcount during its use with helpers/kfuncs. correct. > I was not making that assumption, but it's not wrong. Which is why I wanted to > limit their use to only when verifier is tracking a reference in the program > for the nf_conn returned from an acquire kfunc. limit the use for PTR_TRUSTED to verifier only? and introduce PTR_TRUSTED_BECAUSE_IT_CAME_FROM_KERNEL as well? But the patch we're discussing will be treating them the same way... Or you're proposing the kfunc writer also suppose to add two flags to a kfunc: KF_TRUSTED_ARGS | KF_TRUSTED_BECAUSE_KERNEL_ARGS ? and that's where it comes back to the point I keep stressing in this thread: The kfunc writer should not be exposed to such details. The little people need to understand about the verifier when the write bpf programs or provide kfuncs the better. > > > Yes, in most cases the tracepoints/tracing functions whitelisted will > > > have the caller ensure that, > > > but we should then allow trusted nf_conn in those hooks explicitly, > > > not implicitly by default everywhere. > > > Until then it should be restricted to ref_obj_id > 0 IMO as it is right now. > > > > > > > > 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. > > > > > > > > > > What I meant is that in the normal case, usually objects aren't reused > > > while the RCU read lock is held. > > > In case of nf_conn, the refcount needs to be held to ensure that, > > > since it uses SLAB_TYPESAFE_BY_RCU. > > > This is why bpf_ct_lookup needs to bump the refcount and match the key > > > after that again, and cannot just return the looked up ct directly. > > > > bpf_ct_lookup needs to bump a refcnt?! > > bpf_skb_ct_lookup calls __bpf_nf_ct_lookup > > that calls nf_conntrack_find_get() that does > > the search and incs the refcnt in a generic kernel code. > > There is nothing bpf specific stuff here. bpf kfunc didn't > > add any special refcnt incs. > > > > There are no tracepoints in netfilter, so this discussion > > is all theoretical, but if there was then the code > > should have made sure that refcnt is held before > > passing nf_conn into tracepoint. > > > > Right, if so, it makes sense. Thanks for explaining. > > > > > > It could be 'expert only' __ref suffix on the nf_conn arg, or > > > > > KF_OWNED_ARGS, or something else. > > > > > > > > I'm still against that. > > > > > > > > > > I understand (and agree) that you don't want to complicate things further. > > > It's fine if you want to deal with this later when the above concern > > > materializes. But it will be yet another thing to keep in mind for the > > > future. > > > > I don't share the concern. > > With nf_conn there is none, right? > > But imagine there is only RCU protected pointer that > > is passed into tracepoint somewhere. > > The verifier doesn't recognize refcnt++ on it and ref_obj_id == 0 > > the kernel code doesn't do refcnt++ either. > > But it's still safe and this arg should still be > > PTR_TO_BTF_ID | PTR_TRUSTED. > > The bpf prog can pass it further into kfunc that has KF_TRUSTED_ARGS. > > Since RCU is held before calling into tracepoint the bpf prog > > has to be non sleepable. Additional rcu_read_lock done by > > the prog is redundant, but doesn't hurt. > > When prog is calling kfunc the pointer is still valid and > > kfunc can safely operate on it assuming that object is not going away. > > That is the definition of KF_TRUSTED_ARGS from pov of kfunc. > > You documented it yourself :) > > " > > The KF_TRUSTED_ARGS flag is used for kfuncs taking pointer arguments. It > > indicates that the all pointer arguments will always have a guaranteed lifetime, > > " > > So based on the above, verifier should only mark nf_conn as PTR_TRUSTED in cases > where the kernel already holds the refcount on behalf of the program, yes, but 'holding the reference' is not the only option. Holding rcu_read_lock is fine too. Or any other mechanism that ensures that the pointer has guaranteed lifetime. and back to my earlier point... using your logic we'd need PTR_TRUSTED_BECAUSE_RCU in the verifier and corresponding KF_TRUSTED_ARGS_BECAUSE_RCU flag on kfunc. That just doesn't scale. PTR_RCU flag might actually be needed for some other reason, but KF_TRUSTED_ARGS_BECAUSE_RCU is "too much info" for kfunc providers. > otherwise > for the non-PTR_TRUSTED case program must have ref_obj_id > 0? > > I.e. 'the guaranteed lifetime' for the nf_conn case also means the refcount is > held in context where program can obtain PTR_TRUSTED nf_conn pointer. yes. > It will surely be true in the tracepoint case if there ever is one. Won't work > easily for fentry/fexit but you already said that PTR_TRUSTED shouldn't be set > in that case, atleast by default and without allowlisting those BTF IDs. Correct. allowlisting kernel func's btf_ids is one approach. I might argue that fentry hook on any kernel func that has 'struct nf_conn *' as an argument is also safe (barring recursion into bpf_ct_* kfuncs). Dual compiler of the kernel (native + bpf) will help this kind of safety analysis.
