[PATCH v16 07/12] bpf: Add bpf_verify_pkcs7_signature() kfunc

[Kumar Kartikeya Dwivedi]

On Tue, 6 Sept 2022 at 10:08, Roberto Sassu
<roberto.sassu at huaweicloud.com> wrote:
>
> On Tue, 2022-09-06 at 04:57 +0200, Kumar Kartikeya Dwivedi wrote:
> > On Mon, 5 Sept 2022 at 16:35, Roberto Sassu
> > <roberto.sassu at huaweicloud.com> wrote:
> > > From: Roberto Sassu <roberto.sassu at huawei.com>
> > >
> > > Add the bpf_verify_pkcs7_signature() kfunc, to give eBPF security
> > > modules
> > > the ability to check the validity of a signature against supplied
> > > data, by
> > > using user-provided or system-provided keys as trust anchor.
> > >
> > > The new kfunc makes it possible to enforce mandatory policies, as
> > > eBPF
> > > programs might be allowed to make security decisions only based on
> > > data
> > > sources the system administrator approves.
> > >
> > > The caller should provide the data to be verified and the signature
> > > as eBPF
> > > dynamic pointers (to minimize the number of parameters) and a
> > > bpf_key
> > > structure containing a reference to the keyring with keys trusted
> > > for
> > > signature verification, obtained from bpf_lookup_user_key() or
> > > bpf_lookup_system_key().
> > >
> > > For bpf_key structures obtained from the former lookup function,
> > > bpf_verify_pkcs7_signature() completes the permission check
> > > deferred by
> > > that function by calling key_validate(). key_task_permission() is
> > > already
> > > called by the PKCS#7 code.
> > >
> > > Signed-off-by: Roberto Sassu <roberto.sassu at huawei.com>
> > > Acked-by: KP Singh <kpsingh at kernel.org>
> > > ---
> > >  kernel/trace/bpf_trace.c | 45
> > > ++++++++++++++++++++++++++++++++++++++++
> > >  1 file changed, 45 insertions(+)
> > >
> > > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > > index 7a7023704ac2..8e2c026b0a58 100644
> > > --- a/kernel/trace/bpf_trace.c
> > > +++ b/kernel/trace/bpf_trace.c
> > > @@ -1294,12 +1294,57 @@ void bpf_key_put(struct bpf_key *bkey)
> > >         kfree(bkey);
> > >  }
> > >
> > > +#ifdef CONFIG_SYSTEM_DATA_VERIFICATION
> > > +/**
> > > + * bpf_verify_pkcs7_signature - verify a PKCS#7 signature
> > > + * @data_ptr: data to verify
> > > + * @sig_ptr: signature of the data
> > > + * @trusted_keyring: keyring with keys trusted for signature
> > > verification
> > > + *
> > > + * Verify the PKCS#7 signature *sig_ptr* against the supplied
> > > *data_ptr*
> > > + * with keys in a keyring referenced by *trusted_keyring*.
> > > + *
> > > + * Return: 0 on success, a negative value on error.
> > > + */
> > > +int bpf_verify_pkcs7_signature(struct bpf_dynptr_kern *data_ptr,
> > > +                              struct bpf_dynptr_kern *sig_ptr,
> > > +                              struct bpf_key *trusted_keyring)
> > > +{
> > > +       int ret;
> > > +
> > > +       if (trusted_keyring->has_ref) {
> > > +               /*
> > > +                * Do the permission check deferred in
> > > bpf_lookup_user_key().
> > > +                * See bpf_lookup_user_key() for more details.
> > > +                *
> > > +                * A call to key_task_permission() here would be
> > > redundant, as
> > > +                * it is already done by keyring_search() called by
> > > +                * find_asymmetric_key().
> > > +                */
> > > +               ret = key_validate(trusted_keyring->key);
> > > +               if (ret < 0)
> > > +                       return ret;
> > > +       }
> > > +
> > > +       return verify_pkcs7_signature(data_ptr->data,
> > > +                                     bpf_dynptr_get_size(data_ptr)
> > > ,
> > > +                                     sig_ptr->data,
> > > +                                     bpf_dynptr_get_size(sig_ptr),
> >
> > MIssing check for data_ptr->data == NULL before making this call?
> > Same
> > for sig_ptr.
>
> Patch 3 requires the dynptrs to be initialized. Isn't enough?
>

No, it seems even initialized dynptr can be NULL at runtime. Look at
both ringbuf_submit_dynptr and ringbuf_discard_dynptr.
The verifier won't know after ringbuf_reserve_dynptr whether it set it
to NULL or some valid pointer.

dynptr_init is basically that stack slot is now STACK_DYNPTR, it says
nothing more about the dynptr.

As far as testing this goes, you can pass invalid parameters to
ringbuf_reserve_dynptr to have it set to NULL, then make sure your
helper returns an error at runtime for it.

> Thanks
>
> Roberto
>