Re: [PATCH v17 07/12] bpf: Add bpf_verify_pkcs7_signature() kfunc

[KP Singh]

On Fri, Sep 9, 2022 at 2:09 PM Roberto Sassu
<roberto.sassu@xxxxxxxxxxxxxxx> wrote:
>
> From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Acked-by: KP Singh <kpsingh@xxxxxxxxxx>
> ---
>  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 ab183dbaa8d1..9df53c40cffd 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),
> +                                     trusted_keyring->key,
> +                                     VERIFYING_UNSPECIFIED_SIGNATURE, NULL,
> +                                     NULL);
> +}

This seems to work if the data that needs to be verified
and the signature is allocated onto the map.

For BPF program signing, the signature will be void * pointer (and length)
in a struct in the kernel

+++ b/include/uapi/linux/bpf.h
@@ -1383,6 +1383,8 @@ union bpf_attr {
                __aligned_u64   fd_array;       /* array of FDs */
                __aligned_u64   core_relos;
                __u32           core_relo_rec_size; /* sizeof(struct
bpf_core_relo) */
+               __aligned_u64   signature;
+               __u32           signature_size;
        };

Something like this in the bpf_prog_aux struct which is passed to
security_bpf_prog_alloc.

Now creating a dynptr to use with this kfunc does not work:

   bpf_dynptr_from_mem(aux->signature, aux->signature_size, 0, &sig_ptr);

So one has to copy kernel data into a map and then create dynptrs.
Would you be able to update
the dynptr logic to handle this case too? (follow up is okay too).

- KP


> +#endif /* CONFIG_SYSTEM_DATA_VERIFICATION */
> +
>  __diag_pop();
>
>  BTF_SET8_START(key_sig_kfunc_set)
>  BTF_ID_FLAGS(func, bpf_lookup_user_key, KF_ACQUIRE | KF_RET_NULL | KF_SLEEPABLE)
>  BTF_ID_FLAGS(func, bpf_lookup_system_key, KF_ACQUIRE | KF_RET_NULL)
>  BTF_ID_FLAGS(func, bpf_key_put, KF_RELEASE)
> +#ifdef CONFIG_SYSTEM_DATA_VERIFICATION
> +BTF_ID_FLAGS(func, bpf_verify_pkcs7_signature, KF_SLEEPABLE)
> +#endif
>  BTF_SET8_END(key_sig_kfunc_set)
>
>  static const struct btf_kfunc_id_set bpf_key_sig_kfunc_set = {
> --
> 2.25.1
>