Re: [PATCH bpf-next 2/8] bpf: Allow trusted args to walk struct when checking BTF IDs

[Kumar Kartikeya Dwivedi]

On Fri, 20 Jan 2023 at 11:10, Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Thu, Jan 19, 2023 at 11:23:18PM -0600, David Vernet wrote:
> > On Fri, Jan 20, 2023 at 10:28:15AM +0530, Kumar Kartikeya Dwivedi wrote:
> > > On Fri, Jan 20, 2023 at 05:28:27AM IST, David Vernet wrote:
> > > > When validating BTF types for KF_TRUSTED_ARGS kfuncs, the verifier
> > > > currently enforces that the top-level type must match when calling
> > > > the kfunc. In other words, the verifier does not allow the BPF program
> > > > to pass a bitwise equivalent struct, despite it being functionally safe.
> > > > For example, if you have the following type:
> > > >
> > > > struct  nf_conn___init {
> > > >   struct nf_conn ct;
> > > > };
> > > >
> > > > It would be safe to pass a struct nf_conn___init to a kfunc expecting a
> > > > struct nf_conn.
> > >
> > > Just running bpf_nf selftest would have shown this is false.
> >
> > And I feel silly, because I did run them, and could have sworn they
> > passed...looking now at the change_status_after_alloc testcase I see
> > you're of course correct. Very poor example, thank you for pointing it
> > out.
> >
> > >
> > > > Being able to do this will be useful for certain types
> > > > of kfunc / kptrs enabled by BPF. For example, in a follow-on patch, a
> > > > series of kfuncs will be added which allow programs to do bitwise
> > > > queries on cpumasks that are either allocated by the program (in which
> > > > case they'll be a 'struct bpf_cpumask' type that wraps a cpumask_t as
> > > > its first element), or a cpumask that was allocated by the main kernel
> > > > (in which case it will just be a straight cpumask_t, as in
> > > >  task->cpus_ptr).
> > > >
> > > > Having the two types of cpumasks allows us to distinguish between the
> > > > two for when a cpumask is read-only vs. mutatable. A struct bpf_cpumask
> > > > can be mutated by e.g. bpf_cpumask_clear(), whereas a regular cpumask_t
> > > > cannot be. On the other hand, a struct bpf_cpumask can of course be
> > > > queried in the exact same manner as a cpumask_t, with e.g.
> > > > bpf_cpumask_test_cpu().
> > > >
> > > > If we were to enforce that top level types match, then a user that's
> > > > passing a struct bpf_cpumask to a read-only cpumask_t argument would
> > > > have to cast with something like bpf_cast_to_kern_ctx() (which itself
> > > > would need to be updated to expect the alias, and currently it only
> > > > accommodates a single alias per prog type). Additionally, not specifying
> > > > KF_TRUSTED_ARGS is not an option, as some kfuncs take one argument as a
> > > > struct bpf_cpumask *, and another as a struct cpumask *
> > > > (i.e. cpumask_t).
> > > >
> > > > In order to enable this, this patch relaxes the constraint that a
> > > > KF_TRUSTED_ARGS kfunc must have strict type matching. In order to
> > > > try and be conservative and match existing behavior / expectations, this
> > > > patch also enforces strict type checking for acquire kfuncs. We were
> > > > already enforcing it for release kfuncs, so this should also improve the
> > > > consistency of the semantics for kfuncs.
> > > >
> > >
> > > What you want is to simply follow type at off = 0 (but still enforce the off = 0
> > > requirement). This is something which is currently done for bpf_sk_release (for
> > > struct sk_common) in check_reg_type, but it is not safe in general to just open
> > > this up for all cases. I suggest encoding this particular requirement in the
> > > argument, and simply using triple underscore variant of the type for the special
> > > 'read_only' requirement. This will allow you to use same type in your BPF C
> > > program, while allowing verifier to see them as two different types in kfunc
> > > parameters. Then just relax type following for the particular argument so that
> > > one can pass cpumask_t___ro to kfunc expecting cpumask_t (but only at off = 0,
> > > it just visits first member after failing match on top level type). off = 0
> > > check is still necessary.
> >
> > Sigh, yeah, another ___ workaround but I agree it's probably the best we
> > can do for now, and in general seems pretty useful. Obviously preferable
> > to this patch which just doesn't work. Alexei, are you OK with this? If
> > so, I'll take this approach for v2.
>
> We decided to rely on strict type match when we introduced 'struct nf_conn___init',
> but with that we twisted the C standard to, what looks to be, a wrong direction.
>
> For definition:
> struct nf_conn___init {
>    struct nf_conn ct;
> };
> if a kfunc accepts a pointer to nf_conn it should always accept a pointer to nf_conn__init
> for both read and write, because in C that's valid and safe type cast.
>

The intention of this nf_conn___init was to be invisible to the user.
In selftests there is no trace of nf_conn___init. It is only for
enforcing semantics by virtue of type safety in the verifier.

Allocated but not inserted nf_conn -> nf_conn___init
Inserted/looked up nf_conn -> nf_conn

We can't pass e.g. nf_conn___init * to a function expecting nf_conn *.
The allocated nf_conn may not yet be fully initialized. It is only
after bpf_ct_insert_entry takes the nf_conn___init * and returns
inserted nf_conn * should it be allowed.

But for the user in BPF C it will be the same nf_conn. The verifier
can enforce different semantics on the underlying type's usage in
kfuncs etc, while the user performs normal direct access to the
nf_conn.

It will be the same case here, except you also introduce the case of
kfuncs that are 'polymorphic' and can take both. Relaxing
'strict_type_match' for that arg and placing the type of member you
wish to convert the pointer to gives you such polymorphism. But it's
not correct to do for nf_conn___init to nf_conn, at least not by
default.

In the future we may do:

union bpf_subtype {
  type A;
  type B;
  type C;
};

And using the relaxed rule allows all types at off = 0 to be passed to
kfuncs expecting type A/B/C for bpf_subtype *.
bpf_subtype is a fake type. We're just using the type system to
enforce different API usage for the same underlying kernel type.

> We can fix this design issue by saying that '___init' suffix is special and
> C type casting rules don't apply to it.
> In all other cases bpf_cpumask/cpumask would should allow it.
>

I'm just saying the triple underscore is not visible to the user.
You can declare kfunc that is:
struct foo___x *foo_alloc(void); in the kernel as
struct foo *foo_alloc(void); in BPF program and avoid all the
casting/ugliness and still enforce semantics around use.