Linux pkcs7_parse_message DER解码与signer_info

Linux pkcs7_parse_message DER解码与signer_info

PKCS#7解析器位于crypto/asymmetric_keys/pkcs7_parser.c，负责将DER编码的PKCS#7签名消息解码为struct pkcs7_message。该结构体包含签名者信息、证书链和被签名内容。

入口函数pkcs7_parse_message接收原始DER数据：

struct pkcs7_message *pkcs7_parse_message(const void *data, size_t datalen)
{
struct pkcs7_parse_ctx *ctx;
struct pkcs7_message *msg;
int ret;

msg = kzalloc(sizeof(*msg), GFP_KERNEL);
if (!msg)
return ERR_PTR(-ENOMEM);

ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
if (!ctx) {
pkcs7_free_message(msg);
return ERR_PTR(-ENOMEM);
}

ctx->msg = msg;
ret = asn1_ber_decoder(&pkcs7_decoder, ctx, data, datalen);
if (ret < 0) {
pkcs7_free_message(msg);
msg = ERR_PTR(ret);
}

kfree(ctx);
return msg;
}

asn1_ber_decoder是内核的ASN.1 BER/DER解码器，通过预编译的pkcs7_decoder动作表驱动解析。该动作表由crypto/asymmetric_keys/pkcs7.asn1使用asn1_compiler生成。

解析过程中关键的数据结构struct pkcs7_signed_info存储签名者信息：

struct pkcs7_signed_info {
struct pkcs7_signed_info *next;
struct x509_certificate *signer;
struct asymmetric_key_id *sig_issuer;
struct asymmetric_key_id *sig_key;
unsigned int index;
bool unsupported_crypto;
bool blacklisted;
bool verified;
time64_t signing_time;
enum hash_algo hash_algo;
enum pkey_algo pkey_algo;
enum rsa_padding_type padding_algo;
const void *authattrs;
size_t authattrs_len;
struct public_key_signature *sig;
struct pkcs7_message *msg;
};

PKCS#7的SignedData结构包含多个SignerInfo。解析器处理每个SignerInfo时调用pkcs7_extract_signatory：

static int pkcs7_extract_signatory(struct pkcs7_parse_ctx *ctx, const u8 *data, size_t datalen)
{
struct pkcs7_signed_info *si;
int ret;

si = kzalloc(sizeof(*si), GFP_KERNEL);
if (!si)
return -ENOMEM;

si->msg = ctx->msg;
si->index = ctx->msg->num_signed_info++;

ret = asn1_ber_decoder(&pkcs7_signed_info_decoder, si, data, datalen);
if (ret < 0) {
kfree(si);
return ret;
}

si->next = ctx->msg->signed_infos;
ctx->msg->signed_infos = si;
return 0;
}

签名信息解析的详细动作在pkcs7_signed_info_decoder中处理，包含IssuerAndSerialNumber或SubjectKeyIdentifier的提取：

static int pkcs7_parse_issuer_and_sn(struct pkcs7_parse_ctx *ctx, const u8 *data, size_t datalen)
{
struct asn1_hdr hdr;
const u8 *end;
int ret;

ret = asn1_find_indefinite_length(data, datalen, &hdr);
if (ret < 0)
return ret;

end = hdr.data + hdr.len;
ctx->sinfo->sig_issuer = kzalloc(sizeof(struct asymmetric_key_id), GFP_KERNEL);
if (!ctx->sinfo->sig_issuer)
return -ENOMEM;

ctx->sinfo->sig_issuer->data = kmemdup(hdr.data, hdr.len, GFP_KERNEL);
ctx->sinfo->sig_issuer->datalen = hdr.len;
return 0;
}

digest算法和digest值通过pkcs7_parse_digest提取：

static int pkcs7_parse_digest(struct pkcs7_parse_ctx *ctx, const u8 *data, size_t datalen)
{
struct asn1_hdr hdr;
int ret;

ret = asn1_find_indefinite_length(data, datalen, &hdr);
if (ret < 0)
return ret;

ctx->sinfo->sig->digest = kmemdup(hdr.data, hdr.len, GFP_KERNEL);
ctx->sinfo->sig->digest_size = hdr.len;

switch (hdr.len) {
case 32:
ctx->sinfo->hash_algo = HASH_ALGO_SHA256;
break;
case 48:
ctx->sinfo->hash_algo = HASH_ALGO_SHA384;
break;
case 64:
ctx->sinfo->hash_algo = HASH_ALGO_SHA512;
break;
default:
return -ENOPKG;
}
return 0;
}

签名算法OID到pkey_algo的映射在pkcs7_parse_signing_algo中完成：

static int pkcs7_parse_signing_algo(struct pkcs7_parse_ctx *ctx, const u8 *data, size_t datalen)
{
struct asn1_hdr hdr;
const u8 *end;
const struct oid_lookup *lookup;
int ret;

ret = asn1_find_indefinite_length(data, datalen, &hdr);
if (ret < 0)
return ret;

lookup = oid_lookup(hdr.data, hdr.len);
if (!lookup)
return -ENOPKG;

switch (lookup->oid) {
case OID_rsaEncryption:
case OID_sha1WithRSAEncryption:
case OID_sha256WithRSAEncryption:
case OID_sha384WithRSAEncryption:
case OID_sha512WithRSAEncryption:
ctx->sinfo->pkey_algo = PKEY_ALGO_RSA;
break;
case OID_id_ecdsa_with_sha256:
ctx->sinfo->pkey_algo = PKEY_ALGO_ECDSA;
break;
default:
return -ENOPKG;
}
return 0;
}

解析完成后，pkcs7_verify函数遍历signed_infos链表，对每个签名者执行验证：

int pkcs7_verify(struct pkcs7_message *pkcs7, enum key_being_used_for usage)
{
struct pkcs7_signed_info *sinfo;
int ret;

for (sinfo = pkcs7->signed_infos; sinfo; sinfo = sinfo->next) {
ret = pkcs7_verify_one(pkcs7, sinfo);
if (ret < 0)
return ret;
sinfo->verified = true;
}
return 0;
}

pkcs7_validate_trust负责将每个签名者的证书链锚定到系统信任密钥环，通过keyring_search在".builtin_trusted_keys"中查找匹配的X.509证书。