Re: [PATCH v9 07/10] bpf: Add bpf_verify_pkcs7_signature() kfunc

[Alexei Starovoitov]

On Tue, Aug 09, 2022 at 03:46:00PM +0200, Roberto Sassu wrote:
> 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>
> ---
>  kernel/trace/bpf_trace.c | 56 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 56 insertions(+)
> 
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 33ca4cfe6e26..79ba8c96735a 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -21,6 +21,7 @@
>  #include <linux/bsearch.h>
>  #include <linux/sort.h>
>  #include <linux/key.h>
> +#include <linux/verification.h>
>  
>  #include <net/bpf_sk_storage.h>
>  
> @@ -1290,6 +1291,47 @@ noinline __weak 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.
> + */
> +noinline __weak 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->valid_ptr) {
> +		/*
> +		 * Do the permission check deferred in bpf_lookup_user_key().
> +		 *
> +		 * 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);
> +}
> +#endif /* CONFIG_SYSTEM_DATA_VERIFICATION */
> +
>  __diag_pop();
>  
>  BTF_SET8_START(key_kfunc_set)
> @@ -1303,11 +1345,25 @@ static const struct btf_kfunc_id_set bpf_key_kfunc_set = {
>  	.owner = THIS_MODULE,
>  	.set = &key_kfunc_set,
>  };
> +
> +#ifdef CONFIG_SYSTEM_DATA_VERIFICATION
> +BTF_SET8_START(verify_sig_kfunc_set)
> +BTF_ID_FLAGS(func, bpf_verify_pkcs7_signature, KF_SLEEPABLE)
> +BTF_SET8_END(verify_sig_kfunc_set)
> +
> +static const struct btf_kfunc_id_set bpf_verify_sig_kfunc_set = {
> +	.owner = THIS_MODULE,
> +	.set = &verify_sig_kfunc_set,
> +};
> +#endif /* CONFIG_SYSTEM_DATA_VERIFICATION */
>  #endif /* CONFIG_KEYS */
>  
>  const struct btf_kfunc_id_set *kfunc_sets[] = {
>  #ifdef CONFIG_KEYS
>  	&bpf_key_kfunc_set,
> +#ifdef CONFIG_SYSTEM_DATA_VERIFICATION
> +	&bpf_verify_sig_kfunc_set,
> +#endif /* CONFIG_SYSTEM_DATA_VERIFICATION */
>  #endif /* CONFIG_KEYS */
>  };

Why different sets?
The loop over the set from the previous patch can be removed if it's just one set.
Each kfunc can be ifdef-ed independently.