PKCS#7: Appropriately restrict authenticated attributes and content type

A PKCS#7 or CMS message can have per-signature authenticated attributes
that are digested as a lump and signed by the authorising key for that
signature.  If such attributes exist, the content digest isn't itself
signed, but rather it is included in a special authattr which then
contributes to the signature.

Further, we already require the master message content type to be
pkcs7_signedData - but there's also a separate content type for the data
itself within the SignedData object and this must be repeated inside the
authattrs for each signer [RFC2315 9.2, RFC5652 11.1].

We should really validate the authattrs if they exist or forbid them
entirely as appropriate.  To this end:

 (1) Alter the PKCS#7 parser to reject any message that has more than one
     signature where at least one signature has authattrs and at least one
     that does not.

 (2) Validate authattrs if they are present and strongly restrict them.
     Only the following authattrs are permitted and all others are
     rejected:

     (a) contentType.  This is checked to be an OID that matches the
     	 content type in the SignedData object.

     (b) messageDigest.  This must match the crypto digest of the data.

     (c) signingTime.  If present, we check that this is a valid, parseable
     	 UTCTime or GeneralTime and that the date it encodes fits within
     	 the validity window of the matching X.509 cert.

     (d) S/MIME capabilities.  We don't check the contents.

     (e) Authenticode SP Opus Info.  We don't check the contents.

     (f) Authenticode Statement Type.  We don't check the contents.

     The message is rejected if (a) or (b) are missing.  If the message is
     an Authenticode type, the message is rejected if (e) is missing; if
     not Authenticode, the message is rejected if (d) - (f) are present.

     The S/MIME capabilities authattr (d) unfortunately has to be allowed
     to support kernels already signed by the pesign program.  This only
     affects kexec.  sign-file suppresses them (CMS_NOSMIMECAP).

     The message is also rejected if an authattr is given more than once or
     if it contains more than one element in its set of values.

 (3) Add a parameter to pkcs7_verify() to select one of the following
     restrictions and pass in the appropriate option from the callers:

     (*) VERIFYING_MODULE_SIGNATURE

	 This requires that the SignedData content type be pkcs7-data and
	 forbids authattrs.  sign-file sets CMS_NOATTR.  We could be more
	 flexible and permit authattrs optionally, but only permit minimal
	 content.

     (*) VERIFYING_FIRMWARE_SIGNATURE

	 This requires that the SignedData content type be pkcs7-data and
	 requires authattrs.  In future, this will require an attribute
	 holding the target firmware name in addition to the minimal set.

     (*) VERIFYING_UNSPECIFIED_SIGNATURE

	 This requires that the SignedData content type be pkcs7-data but
	 allows either no authattrs or only permits the minimal set.

     (*) VERIFYING_KEXEC_PE_SIGNATURE

	 This only supports the Authenticode SPC_INDIRECT_DATA content type
	 and requires at least an SpcSpOpusInfo authattr in addition to the
	 minimal set.  It also permits an SPC_STATEMENT_TYPE authattr (and
	 an S/MIME capabilities authattr because the pesign program doesn't
	 remove these).

     (*) VERIFYING_KEY_SIGNATURE
     (*) VERIFYING_KEY_SELF_SIGNATURE

	 These are invalid in this context but are included for later use
	 when limiting the use of X.509 certs.

 (4) The pkcs7_test key type is given a module parameter to select between
     the above options for testing purposes.  For example:

	echo 1 >/sys/module/pkcs7_test_key/parameters/usage
	keyctl padd pkcs7_test foo @s </tmp/stuff.pkcs7

     will attempt to check the signature on stuff.pkcs7 as if it contains a
     firmware blob (1 being VERIFYING_FIRMWARE_SIGNATURE).

Suggested-by: Andy Lutomirski <[email protected]>
Signed-off-by: David Howells <[email protected]>
Reviewed-by: Marcel Holtmann <[email protected]>
Reviewed-by: David Woodhouse <[email protected]>

arch/x86/kernel/kexec-bzimage64.c

@@ -536,7 +536,9 @@ static int bzImage64_verify_sig(const char *kernel, unsigned long kernel_len)
 	int ret;
 
 	ret = verify_pefile_signature(kernel, kernel_len,
-				      system_trusted_keyring, &trusted);
+				      system_trusted_keyring,
+				      VERIFYING_KEXEC_PE_SIGNATURE,
+				      &trusted);
 	if (ret < 0)
 		return ret;
 	if (!trusted)

crypto/asymmetric_keys/asymmetric_type.c

@@ -12,6 +12,7 @@
  */
 #include <keys/asymmetric-subtype.h>
 #include <keys/asymmetric-parser.h>
+#include <crypto/public_key.h>
 #include <linux/seq_file.h>
 #include <linux/module.h>
 #include <linux/slab.h>
@@ -20,6 +21,16 @@
 
 MODULE_LICENSE("GPL");
 
+const char *const key_being_used_for[NR__KEY_BEING_USED_FOR] = {
+	[VERIFYING_MODULE_SIGNATURE]		= "mod sig",
+	[VERIFYING_FIRMWARE_SIGNATURE]		= "firmware sig",
+	[VERIFYING_KEXEC_PE_SIGNATURE]		= "kexec PE sig",
+	[VERIFYING_KEY_SIGNATURE]		= "key sig",
+	[VERIFYING_KEY_SELF_SIGNATURE]		= "key self sig",
+	[VERIFYING_UNSPECIFIED_SIGNATURE]	= "unspec sig",
+};
+EXPORT_SYMBOL_GPL(key_being_used_for);
+
 static LIST_HEAD(asymmetric_key_parsers);
 static DECLARE_RWSEM(asymmetric_key_parsers_sem);
 

crypto/asymmetric_keys/pkcs7.asn1

@@ -8,7 +8,7 @@ ContentType ::= OBJECT IDENTIFIER ({ pkcs7_note_OID })
 SignedData ::= SEQUENCE {
 	version			INTEGER ({ pkcs7_note_signeddata_version }),
 	digestAlgorithms	DigestAlgorithmIdentifiers,
-	contentInfo		ContentInfo,
+	contentInfo		ContentInfo ({ pkcs7_note_content }),
 	certificates		CHOICE {
 		certSet		[0] IMPLICIT ExtendedCertificatesAndCertificates,
 		certSequence	[2] IMPLICIT Certificates
@@ -21,7 +21,7 @@ SignedData ::= SEQUENCE {
 }
 
 ContentInfo ::= SEQUENCE {
-	contentType	ContentType,
+	contentType	ContentType ({ pkcs7_note_OID }),
 	content		[0] EXPLICIT Data OPTIONAL
 }
 
@@ -111,7 +111,7 @@ AuthenticatedAttribute ::= SEQUENCE {
 }
 
 UnauthenticatedAttribute ::= SEQUENCE {
-	type			OBJECT IDENTIFIER ({ pkcs7_note_OID }),
+	type			OBJECT IDENTIFIER,
 	values			SET OF ANY
 }
 

crypto/asymmetric_keys/pkcs7_key_type.c

@@ -14,16 +14,23 @@
 #include <linux/err.h>
 #include <linux/module.h>
 #include <linux/key-type.h>
+#include <keys/asymmetric-type.h>
 #include <crypto/pkcs7.h>
 #include <keys/user-type.h>
 #include <keys/system_keyring.h>
 #include "pkcs7_parser.h"
 
+static unsigned pkcs7_usage;
+module_param_named(usage, pkcs7_usage, uint, S_IWUSR | S_IRUGO);
+MODULE_PARM_DESC(pkcs7_usage,
+		 "Usage to specify when verifying the PKCS#7 message");
+
 /*
  * Preparse a PKCS#7 wrapped and validated data blob.
  */
 static int pkcs7_preparse(struct key_preparsed_payload *prep)
 {
+	enum key_being_used_for usage = pkcs7_usage;
 	struct pkcs7_message *pkcs7;
 	const void *data, *saved_prep_data;
 	size_t datalen, saved_prep_datalen;
@@ -32,6 +39,11 @@ static int pkcs7_preparse(struct key_preparsed_payload *prep)
 
 	kenter("");
 
+	if (usage >= NR__KEY_BEING_USED_FOR) {
+		pr_err("Invalid usage type %d\n", usage);
+		return -EINVAL;
+	}
+
 	saved_prep_data = prep->data;
 	saved_prep_datalen = prep->datalen;
 	pkcs7 = pkcs7_parse_message(saved_prep_data, saved_prep_datalen);
@@ -40,7 +52,7 @@ static int pkcs7_preparse(struct key_preparsed_payload *prep)
 		goto error;
 	}
 
-	ret = pkcs7_verify(pkcs7);
+	ret = pkcs7_verify(pkcs7, usage);
 	if (ret < 0)
 		goto error_free;
 

crypto/asymmetric_keys/pkcs7_parser.c

@@ -81,6 +81,30 @@ void pkcs7_free_message(struct pkcs7_message *pkcs7)
 }
 EXPORT_SYMBOL_GPL(pkcs7_free_message);
 
+/*
+ * Check authenticatedAttributes are provided or not provided consistently.
+ */
+static int pkcs7_check_authattrs(struct pkcs7_message *msg)
+{
+	struct pkcs7_signed_info *sinfo;
+	bool want;
+
+	sinfo = msg->signed_infos;
+	if (sinfo->authattrs) {
+		want = true;
+		msg->have_authattrs = true;
+	}
+
+	for (sinfo = sinfo->next; sinfo; sinfo = sinfo->next)
+		if (!!sinfo->authattrs != want)
+			goto inconsistent;
+	return 0;
+
+inconsistent:
+	pr_warn("Inconsistently supplied authAttrs\n");
+	return -EINVAL;
+}
+
 /**
  * pkcs7_parse_message - Parse a PKCS#7 message
  * @data: The raw binary ASN.1 encoded message to be parsed
@@ -113,6 +137,10 @@ struct pkcs7_message *pkcs7_parse_message(const void *data, size_t datalen)
 		goto out;
 	}
 
+	ret = pkcs7_check_authattrs(ctx->msg);
+	if (ret < 0)
+		goto out;
+
 	msg = ctx->msg;
 	ctx->msg = NULL;
 
@@ -380,6 +408,25 @@ int pkcs7_note_certificate_list(void *context, size_t hdrlen,
 	return 0;
 }
 
+/*
+ * Note the content type.
+ */
+int pkcs7_note_content(void *context, size_t hdrlen,
+		       unsigned char tag,
+		       const void *value, size_t vlen)
+{
+	struct pkcs7_parse_context *ctx = context;
+
+	if (ctx->last_oid != OID_data &&
+	    ctx->last_oid != OID_msIndirectData) {
+		pr_warn("Unsupported data type %d\n", ctx->last_oid);
+		return -EINVAL;
+	}
+
+	ctx->msg->data_type = ctx->last_oid;
+	return 0;
+}
+
 /*
  * Extract the data from the message and store that and its content type OID in
  * the context.
@@ -395,31 +442,90 @@ int pkcs7_note_data(void *context, size_t hdrlen,
 	ctx->msg->data = value;
 	ctx->msg->data_len = vlen;
 	ctx->msg->data_hdrlen = hdrlen;
-	ctx->msg->data_type = ctx->last_oid;
 	return 0;
 }
 
 /*
- * Parse authenticated attributes
+ * Parse authenticated attributes.
  */
 int pkcs7_sig_note_authenticated_attr(void *context, size_t hdrlen,
 				      unsigned char tag,
 				      const void *value, size_t vlen)
 {
 	struct pkcs7_parse_context *ctx = context;
+	struct pkcs7_signed_info *sinfo = ctx->sinfo;
+	enum OID content_type;
 
 	pr_devel("AuthAttr: %02x %zu [%*ph]\n", tag, vlen, (unsigned)vlen, value);
 
 	switch (ctx->last_oid) {
+	case OID_contentType:
+		if (__test_and_set_bit(sinfo_has_content_type, &sinfo->aa_set))
+			goto repeated;
+		content_type = look_up_OID(value, vlen);
+		if (content_type != ctx->msg->data_type) {
+			pr_warn("Mismatch between global data type (%d) and sinfo %u (%d)\n",
+				ctx->msg->data_type, sinfo->index,
+				content_type);
+			return -EBADMSG;
+		}
+		return 0;
+
+	case OID_signingTime:
+		if (__test_and_set_bit(sinfo_has_signing_time, &sinfo->aa_set))
+			goto repeated;
+		/* Should we check that the signing time is consistent
+		 * with the signer's X.509 cert?
+		 */
+		return x509_decode_time(&sinfo->signing_time,
+					hdrlen, tag, value, vlen);
+
 	case OID_messageDigest:
+		if (__test_and_set_bit(sinfo_has_message_digest, &sinfo->aa_set))
+			goto repeated;
 		if (tag != ASN1_OTS)
 			return -EBADMSG;
-		ctx->sinfo->msgdigest = value;
-		ctx->sinfo->msgdigest_len = vlen;
+		sinfo->msgdigest = value;
+		sinfo->msgdigest_len = vlen;
+		return 0;
+
+	case OID_smimeCapabilites:
+		if (__test_and_set_bit(sinfo_has_smime_caps, &sinfo->aa_set))
+			goto repeated;
+		if (ctx->msg->data_type != OID_msIndirectData) {
+			pr_warn("S/MIME Caps only allowed with Authenticode\n");
+			return -EKEYREJECTED;
+		}
+		return 0;
+
+		/* Microsoft SpOpusInfo seems to be contain cont[0] 16-bit BE
+		 * char URLs and cont[1] 8-bit char URLs.
+		 *
+		 * Microsoft StatementType seems to contain a list of OIDs that
+		 * are also used as extendedKeyUsage types in X.509 certs.
+		 */
+	case OID_msSpOpusInfo:
+		if (__test_and_set_bit(sinfo_has_ms_opus_info, &sinfo->aa_set))
+			goto repeated;
+		goto authenticode_check;
+	case OID_msStatementType:
+		if (__test_and_set_bit(sinfo_has_ms_statement_type, &sinfo->aa_set))
+			goto repeated;
+	authenticode_check:
+		if (ctx->msg->data_type != OID_msIndirectData) {
+			pr_warn("Authenticode AuthAttrs only allowed with Authenticode\n");
+			return -EKEYREJECTED;
+		}
+		/* I'm not sure how to validate these */
 		return 0;
 	default:
 		return 0;
 	}
+
+repeated:
+	/* We permit max one item per AuthenticatedAttribute and no repeats */
+	pr_warn("Repeated/multivalue AuthAttrs not permitted\n");
+	return -EKEYREJECTED;
 }
 
 /*
@@ -430,10 +536,25 @@ int pkcs7_sig_note_set_of_authattrs(void *context, size_t hdrlen,
 				    const void *value, size_t vlen)
 {
 	struct pkcs7_parse_context *ctx = context;
+	struct pkcs7_signed_info *sinfo = ctx->sinfo;
+
+	if (!test_bit(sinfo_has_content_type, &sinfo->aa_set) ||
+	    !test_bit(sinfo_has_message_digest, &sinfo->aa_set) ||
+	    (ctx->msg->data_type == OID_msIndirectData &&
+	     !test_bit(sinfo_has_ms_opus_info, &sinfo->aa_set))) {
+		pr_warn("Missing required AuthAttr\n");
+		return -EBADMSG;
+	}
+
+	if (ctx->msg->data_type != OID_msIndirectData &&
+	    test_bit(sinfo_has_ms_opus_info, &sinfo->aa_set)) {
+		pr_warn("Unexpected Authenticode AuthAttr\n");
+		return -EBADMSG;
+	}
 
 	/* We need to switch the 'CONT 0' to a 'SET OF' when we digest */
-	ctx->sinfo->authattrs = value - (hdrlen - 1);
-	ctx->sinfo->authattrs_len = vlen + (hdrlen - 1);
+	sinfo->authattrs = value - (hdrlen - 1);
+	sinfo->authattrs_len = vlen + (hdrlen - 1);
 	return 0;
 }
 
@@ -511,6 +632,11 @@ int pkcs7_note_signed_info(void *context, size_t hdrlen,
 	struct pkcs7_signed_info *sinfo = ctx->sinfo;
 	struct asymmetric_key_id *kid;
 
+	if (ctx->msg->data_type == OID_msIndirectData && !sinfo->authattrs) {
+		pr_warn("Authenticode requires AuthAttrs\n");
+		return -EBADMSG;
+	}
+
 	/* Generate cert issuer + serial number key ID */
 	if (!ctx->expect_skid) {
 		kid = asymmetric_key_generate_id(ctx->raw_serial,

crypto/asymmetric_keys/pkcs7_parser.h

@@ -21,9 +21,9 @@
 struct pkcs7_signed_info {
 	struct pkcs7_signed_info *next;
 	struct x509_certificate *signer; /* Signing certificate (in msg->certs) */
-	unsigned index;
-	bool trusted;
-	bool unsupported_crypto;	/* T if not usable due to missing crypto */
+	unsigned	index;
+	bool		trusted;
+	bool		unsupported_crypto;	/* T if not usable due to missing crypto */
 
 	/* Message digest - the digest of the Content Data (or NULL) */
 	const void	*msgdigest;
@@ -32,6 +32,14 @@ struct pkcs7_signed_info {
 	/* Authenticated Attribute data (or NULL) */
 	unsigned	authattrs_len;
 	const void	*authattrs;
+	unsigned long	aa_set;
+#define	sinfo_has_content_type		0
+#define	sinfo_has_signing_time		1
+#define	sinfo_has_message_digest	2
+#define sinfo_has_smime_caps		3
+#define	sinfo_has_ms_opus_info		4
+#define	sinfo_has_ms_statement_type	5
+	time64_t	signing_time;
 
 	/* Issuing cert serial number and issuer's name [PKCS#7 or CMS ver 1]
 	 * or issuing cert's SKID [CMS ver 3].
@@ -53,6 +61,7 @@ struct pkcs7_message {
 	struct x509_certificate *crl;	/* Revocation list */
 	struct pkcs7_signed_info *signed_infos;
 	u8		version;	/* Version of cert (1 -> PKCS#7 or CMS; 3 -> CMS) */
+	bool		have_authattrs;	/* T if have authattrs */
 
 	/* Content Data (or NULL) */
 	enum OID	data_type;	/* Type of Data */