linux-kernel - Re: [PATCH v9 07/10] bpf: Add bpf_verify_pkcs7

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Date:   Wed, 10 Aug 2022 01:09:50 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Roberto Sassu <roberto.sassu@...wei.com>, ast@...nel.org,
        andrii@...nel.org, martin.lau@...ux.dev, song@...nel.org,
        yhs@...com, john.fastabend@...il.com, kpsingh@...nel.org,
        sdf@...gle.com, haoluo@...gle.com, jolsa@...nel.org,
        corbet@....net, dhowells@...hat.com, jarkko@...nel.org,
        rostedt@...dmis.org, mingo@...hat.com, paul@...l-moore.com,
        jmorris@...ei.org, serge@...lyn.com, shuah@...nel.org
Cc:     bpf@...r.kernel.org, linux-doc@...r.kernel.org,
        keyrings@...r.kernel.org, linux-security-module@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 07/10] bpf: Add bpf_verify_pkcs7_signature() kfunc

On 8/9/22 3:46 PM, Roberto Sassu wrote:
[...]
> 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@...wei.com>
> ---
>   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,

nit: see comment on prev patch for noinline and __weak

> +					       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().
> +		 */

In this patch and previous one, you describe that calling key_validate() is
deferred, but you should also provide the actual rationale for readers on
"why" we need to do it.

> +		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,
>   };