On Tue, Aug 22, 2023 at 01:47:01AM -0400, David Marchevsky wrote:
> On 8/21/23 10:37 PM, Yonghong Song wrote:
> >
> >
> > On 8/21/23 12:33 PM, Dave Marchevsky wrote:
> >> An earlier patch in the series ensures that the underlying memory of
> >> nodes with bpf_refcount - which can have multiple owners - is not reused
> >> until RCU grace period has elapsed. This prevents
> >> use-after-free with non-owning references that may point to
> >> recently-freed memory. While RCU read lock is held, it's safe to
> >> dereference such a non-owning ref, as by definition RCU GP couldn't have
> >> elapsed and therefore underlying memory couldn't have been reused.
> >>
> >> From the perspective of verifier "trustedness" non-owning refs to
> >> refcounted nodes are now trusted only in RCU CS and therefore should no
> >> longer pass is_trusted_reg, but rather is_rcu_reg. Let's mark them
> >> MEM_RCU in order to reflect this new state.
> >>
> >> Signed-off-by: Dave Marchevsky <davemarchevsky@xxxxxx>
> >> ---
> >> include/linux/bpf.h | 3 ++-
> >> kernel/bpf/verifier.c | 13 ++++++++++++-
> >> 2 files changed, 14 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> >> index eced6400f778..12596af59c00 100644
> >> --- a/include/linux/bpf.h
> >> +++ b/include/linux/bpf.h
> >> @@ -653,7 +653,8 @@ enum bpf_type_flag {
> >> MEM_RCU = BIT(13 + BPF_BASE_TYPE_BITS),
> >> /* Used to tag PTR_TO_BTF_ID | MEM_ALLOC references which are non-owning.
> >> - * Currently only valid for linked-list and rbtree nodes.
> >> + * Currently only valid for linked-list and rbtree nodes. If the nodes
> >> + * have a bpf_refcount_field, they must be tagged MEM_RCU as well.
> >> */
> >> NON_OWN_REF = BIT(14 + BPF_BASE_TYPE_BITS),
> >> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> >> index 8db0afa5985c..55607ab30522 100644
> >> --- a/kernel/bpf/verifier.c
> >> +++ b/kernel/bpf/verifier.c
> >> @@ -8013,6 +8013,7 @@ int check_func_arg_reg_off(struct bpf_verifier_env *env,
> >> case PTR_TO_BTF_ID | PTR_TRUSTED:
> >> case PTR_TO_BTF_ID | MEM_RCU:
> >> case PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF:
> >> + case PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF | MEM_RCU:
> >> /* When referenced PTR_TO_BTF_ID is passed to release function,
> >> * its fixed offset must be 0. In the other cases, fixed offset
> >> * can be non-zero. This was already checked above. So pass
> >> @@ -10479,6 +10480,7 @@ static int process_kf_arg_ptr_to_btf_id(struct bpf_verifier_env *env,
> >> static int ref_set_non_owning(struct bpf_verifier_env *env, struct bpf_reg_state *reg)
> >> {
> >> struct bpf_verifier_state *state = env->cur_state;
> >> + struct btf_record *rec = reg_btf_record(reg);
> >> if (!state->active_lock.ptr) {
> >> verbose(env, "verifier internal error: ref_set_non_owning w/o active lock\n");
> >> @@ -10491,6 +10493,9 @@ static int ref_set_non_owning(struct bpf_verifier_env *env, struct bpf_reg_state
> >> }
> >> reg->type |= NON_OWN_REF;
> >> + if (rec->refcount_off >= 0)
> >> + reg->type |= MEM_RCU;
> >
> > Should the above MEM_RCU marking be done unless reg access is in
> > rcu critical section?
>
> I think it is fine, since non-owning references currently exist only within
> spin_lock CS. Based on Alexei's comments on v1 of this series [0], preemption
> disabled + spin_lock CS should imply RCU CS.
>
> [0]: https://lore.kernel.org/bpf/20230802230715.3ltalexaczbomvbu@MacBook-Pro-8.local/
>
> >
> > I think we still have issues for state resetting
> > with bpf_spin_unlock() and bpf_rcu_read_unlock(), both of which
> > will try to convert the reg state to PTR_UNTRUSTED.
> >
> > Let us say reg state is
> > PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF | MEM_RCU
> >
> > (1). If hitting bpf_spin_unlock(), since MEM_RCU is in
> > the reg state, the state should become
> > PTR_TO_BTF_ID | MEM_ALLOC | MEM_RCU
> > some additional code might be needed so we wont have
> > verifier complaints about ref_obj_id == 0.
> >
> > (2). If hitting bpf_rcu_read_unlock(), the state should become
> > PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF
> > since register access still in bpf_spin_lock() region.
>
> I agree w/ your comment in side reply stating that this
> case isn't possible since bpf_rcu_read_{lock,unlock} in spin_lock CS
> is currently not allowed.
>
> >
> > Does this make sense?
> >
>
>
> IIUC the specific reg state flow you're recommending is based on the convos
> we've had over the past few weeks re: getting rid of special non-owning ref
> lifetime rules, instead using RCU as much as possible. Specifically, this
> recommended change would remove non-owning ref clobbering, instead just removing
> NON_OWN_REF flag on bpf_spin_unlock so that such nodes can no longer be passed
> to collection kfuncs (refcount_acquire, etc).
Overall the patch set makes sense to me, but I want to clarify above.
My understanding that after the patch set applied bpf_spin_unlock()
will invalidate_non_owning_refs(), so what Yonghong is saying in (1)
is not correct.
Instead PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF | MEM_RCU will become mark_reg_invalid().
Re: (2) even if/when bpf_rcu_read_unlock() will allowed inside spinlocked region
it will convert PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF | MEM_RCU to
PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF | PTR_UNTRUSTED
which is a buggy combination which we would need to address if rcu_unlock is allowed eventually.
Did I get it right?
If so I think the whole set is good to do.
