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
>
