[PATCH 16/20] PKCS#7: Add an optional authenticated attribute to hold firmware name [ver #5]

From: David Howells
Date: Thu May 28 2015 - 11:50:14 EST

Modify the sign-file program to take a "-F <firmware name>" parameter. The
name is a utf8 string that, if given, is inserted in a PKCS#7 authenticated
attribute from where it can be extracted by the kernel. Authenticated
attributes are added to the signature digest.

If the attribute is present, the signature would be assumed to be for
firmware and would not be permitted with module signing or kexec. The name
associated with the attribute would be compared to the name passed to
request_firmware() and the load request would be denied if they didn't
match.

If not present, the signature would be rejected if used for firmware.

One oddity is that the attribute is per-signature, so if a second signature
was added (which PKCS#7 supports), it would have to have the attribute added
separately to that signature also.

Note that the OID I have selected is an unregistered element of the Red Hat
OID space to serve as an example. An OID would need to be allocated for
this purpose.

The kernel then parses this out, saves the string and makes sure the same
string (or lack thereof) is present from all signers. Then when
system_verify_data() is called, it is passed a NULL if the attribute is
expected not to be present and the name from request_firmware() if it is
expected to be present. Verification is rejected if there's a mismatch.


========================
MARKING KEYS FOR PURPOSE
========================

To mark a key as being for firmware signing only, for example, the openssl
req command can be given an extension specifier to mark the X.509
certificate. Assuming a config script is used, this could be done in one of
a couple of ways:

(1) Add an OID indicating this to the list in the x509v3 extendedKeyUsage
extension, eg:

extendedKeyUsage=critical,1.3.6.1.4.1.2312.99.2

(2) Add our own extension type with our own OID, eg:

1.3.6.1.4.1.2312.99.2=critical,ASN1:NULL

Option (2) is easier to deal with since we examine all the extensions
anyway, but option (1) is probably the correct way.

Note I'm using an unregistered Red Hat space OID here as an example.

We would need to decide how we want to break down the usage space and
register appropriate OIDs. If we went with option (2), we could
parameterise the thing and use that to select either driver class or driver.

It's possible that just declaring that a key is used for firmware signing or
anything but and then including the firmware name in the signature is
sufficient for our purposes, rather than breaking it down per driver class
or per driver.

Not-yet-signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---

crypto/asymmetric_keys/pkcs7_parser.c | 77 +++++++++++++++++++++++++++++++++
crypto/asymmetric_keys/pkcs7_parser.h | 1
include/crypto/pkcs7.h | 1
include/keys/system_keyring.h | 3 +
include/linux/oid_registry.h | 3 +
kernel/module_signing.c | 2 -
kernel/system_keyring.c | 26 +++++++++++
scripts/sign-file.c | 36 ++++++++++++++-
8 files changed, 142 insertions(+), 7 deletions(-)

diff --git a/crypto/asymmetric_keys/pkcs7_parser.c b/crypto/asymmetric_keys/pkcs7_parser.c
index 3bd5a1e4c493..381e3d8bcc8d 100644
--- a/crypto/asymmetric_keys/pkcs7_parser.c
+++ b/crypto/asymmetric_keys/pkcs7_parser.c
@@ -15,6 +15,7 @@
#include <linux/slab.h>
#include <linux/err.h>
#include <linux/oid_registry.h>
+#include <linux/ctype.h>
#include "public_key.h"
#include "pkcs7_parser.h"
#include "pkcs7-asn1.h"
@@ -29,6 +30,13 @@ struct pkcs7_parse_context {
enum OID last_oid; /* Last OID encountered */
unsigned x509_index;
unsigned sinfo_index;
+ unsigned firmware_name_len;
+ enum {
+ maybe_firmware_sig,
+ is_firmware_sig,
+ isnt_firmware_sig
+ } firmware_sig_state : 8;
+ bool signer_has_firmware_name;
const void *raw_serial;
unsigned raw_serial_size;
unsigned raw_issuer_size;
@@ -73,6 +81,7 @@ void pkcs7_free_message(struct pkcs7_message *pkcs7)
pkcs7->signed_infos = sinfo->next;
pkcs7_free_signed_info(sinfo);
}
+ kfree(pkcs7->firmware_name);
kfree(pkcs7);
}
}
@@ -310,6 +319,8 @@ int pkcs7_sig_note_authenticated_attr(void *context, size_t hdrlen,
const void *value, size_t vlen)
{
struct pkcs7_parse_context *ctx = context;
+ char *p;
+ int i;

pr_devel("AuthAttr: %02x %zu [%*ph]\n", tag, vlen, (unsigned)vlen, value);

@@ -320,6 +331,45 @@ int pkcs7_sig_note_authenticated_attr(void *context, size_t hdrlen,
ctx->sinfo->msgdigest = value;
ctx->sinfo->msgdigest_len = vlen;
return 0;
+
+ case OID_firmwareName:
+ if (tag != ASN1_UTF8STR || vlen == 0)
+ return -EBADMSG;
+
+ /* If there's more than one signature, they must have the same
+ * firmware name.
+ */
+ switch (ctx->firmware_sig_state) {
+ case maybe_firmware_sig:
+ for (i = 0; i < vlen; i++)
+ if (!isprint(((const char *)value)[i]))
+ return -EBADMSG;
+ p = kmalloc(vlen + 1, GFP_KERNEL);
+ if (!p)
+ return -ENOMEM;
+ memcpy(p, value, vlen);
+ p[vlen] = 0;
+ ctx->msg->firmware_name = p;
+ ctx->firmware_name_len = vlen;
+ ctx->firmware_sig_state = is_firmware_sig;
+ pr_debug("Found firmware name '%s'\n", p);
+ ctx->signer_has_firmware_name = true;
+ return 0;
+
+ case is_firmware_sig:
+ if (ctx->firmware_name_len != vlen ||
+ memcmp(ctx->msg->firmware_name, value, vlen) != 0) {
+ pr_warn("Mismatched firmware names\n");
+ return -EBADMSG;
+ }
+ ctx->signer_has_firmware_name = true;
+ return 0;
+
+ case isnt_firmware_sig:
+ pr_warn("Mismatched presence of firmware name\n");
+ return -EBADMSG;
+ }
+
default:
return 0;
}
@@ -334,12 +384,39 @@ int pkcs7_sig_note_set_of_authattrs(void *context, size_t hdrlen,
{
struct pkcs7_parse_context *ctx = context;

+ /* Make sure we either have all or no firmware names */
+ switch (ctx->firmware_sig_state) {
+ case maybe_firmware_sig:
+ ctx->firmware_sig_state = isnt_firmware_sig;
+ case isnt_firmware_sig:
+ break;
+ case is_firmware_sig:
+ if (!ctx->signer_has_firmware_name) {
+ pr_warn("Mismatched presence of firmware name\n");
+ return -EBADMSG;
+ }
+ ctx->signer_has_firmware_name = false;
+ break;
+ }
+
/* 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);
return 0;
}

+/**
+ * pkcs7_get_firmware_name - Get firmware name extension value
+ * @pkcs7: The preparsed PKCS#7 message to access
+ *
+ * Get the value of the firmware name extension if there was one or NULL
+ * otherwise.
+ */
+const char *pkcs7_get_firmware_name(const struct pkcs7_message *pkcs7)
+{
+ return pkcs7->firmware_name;
+}
+
/*
* Note the issuing certificate serial number
*/
diff --git a/crypto/asymmetric_keys/pkcs7_parser.h b/crypto/asymmetric_keys/pkcs7_parser.h
index efc7dc9b8f9c..03a30f8578e2 100644
--- a/crypto/asymmetric_keys/pkcs7_parser.h
+++ b/crypto/asymmetric_keys/pkcs7_parser.h
@@ -50,6 +50,7 @@ struct pkcs7_message {
struct x509_certificate *certs; /* Certificate list */
struct x509_certificate *crl; /* Revocation list */
struct pkcs7_signed_info *signed_infos;
+ char *firmware_name; /* Firmware name if present */

/* Content Data (or NULL) */
enum OID data_type; /* Type of Data */
diff --git a/include/crypto/pkcs7.h b/include/crypto/pkcs7.h
index e235ab4957ee..0999eac6313f 100644
--- a/include/crypto/pkcs7.h
+++ b/include/crypto/pkcs7.h
@@ -22,6 +22,7 @@ extern void pkcs7_free_message(struct pkcs7_message *pkcs7);
extern int pkcs7_get_content_data(const struct pkcs7_message *pkcs7,
const void **_data, size_t *_datalen,
bool want_wrapper);
+extern const char *pkcs7_get_firmware_name(const struct pkcs7_message *pkcs7);

/*
* pkcs7_trust.c
diff --git a/include/keys/system_keyring.h b/include/keys/system_keyring.h
index 9791c907cdb7..30303745f845 100644
--- a/include/keys/system_keyring.h
+++ b/include/keys/system_keyring.h
@@ -30,7 +30,8 @@ static inline struct key *get_system_trusted_keyring(void)

#ifdef CONFIG_SYSTEM_DATA_VERIFICATION
extern int system_verify_data(const void *data, unsigned long len,
- const void *raw_pkcs7, size_t pkcs7_len);
+ const void *raw_pkcs7, size_t pkcs7_len,
+ const char *firmware_name);
#endif

#endif /* _KEYS_SYSTEM_KEYRING_H */
diff --git a/include/linux/oid_registry.h b/include/linux/oid_registry.h
index c2bbf672b84e..db8d382699b5 100644
--- a/include/linux/oid_registry.h
+++ b/include/linux/oid_registry.h
@@ -88,6 +88,9 @@ enum OID {
OID_authorityKeyIdentifier, /* 2.5.29.35 */
OID_extKeyUsage, /* 2.5.29.37 */

+ /* Signing */
+ OID_firmwareName, /* 1.3.6.1.4.1.2312.99.1 */
+
OID__NR
};

diff --git a/kernel/module_signing.c b/kernel/module_signing.c
index 70ad463f6df0..9361711897ce 100644
--- a/kernel/module_signing.c
+++ b/kernel/module_signing.c
@@ -72,5 +72,5 @@ int mod_verify_sig(const void *mod, unsigned long *_modlen)
return -EBADMSG;
}

- return system_verify_data(mod, modlen, mod + modlen, sig_len);
+ return system_verify_data(mod, modlen, mod + modlen, sig_len, NULL);
}
diff --git a/kernel/system_keyring.c b/kernel/system_keyring.c
index 95f2dcbc7616..ccb2814f89c1 100644
--- a/kernel/system_keyring.c
+++ b/kernel/system_keyring.c
@@ -113,11 +113,14 @@ late_initcall(load_system_certificate_list);
* @len: Size of @data.
* @raw_pkcs7: The PKCS#7 message that is the signature.
* @pkcs7_len: The size of @raw_pkcs7.
+ * @firmware_name: The required firmware name or NULL.
*/
int system_verify_data(const void *data, unsigned long len,
- const void *raw_pkcs7, size_t pkcs7_len)
+ const void *raw_pkcs7, size_t pkcs7_len,
+ const char *firmware_name)
{
struct pkcs7_message *pkcs7;
+ const char *p7_firmware_name;
bool trusted;
int ret;

@@ -125,6 +128,27 @@ int system_verify_data(const void *data, unsigned long len,
if (IS_ERR(pkcs7))
return PTR_ERR(pkcs7);

+ ret = -EINVAL;
+ p7_firmware_name = pkcs7_get_firmware_name(pkcs7);
+ if (firmware_name) {
+ if (!p7_firmware_name) {
+ pr_err("Expected name '%s' in firmware signature but not present\n",
+ firmware_name);
+ goto error;
+ }
+ if (strcmp(p7_firmware_name, firmware_name) != 0) {
+ pr_err("Expected name '%s' in firmware signature but got '%s'\n",
+ firmware_name, p7_firmware_name);
+ goto error;
+ }
+ } else {
+ if (p7_firmware_name) {
+ pr_err("Unexpected firmware name in signature '%s'\n",
+ p7_firmware_name);
+ goto error;
+ }
+ }
+
/* The data should be detached - so we need to supply it. */
if (pkcs7_supply_detached_data(pkcs7, data, len) < 0) {
pr_err("PKCS#7 signature with non-detached data\n");
diff --git a/scripts/sign-file.c b/scripts/sign-file.c
index ad0aa21bd3ac..94109c9e8ce6 100755
--- a/scripts/sign-file.c
+++ b/scripts/sign-file.c
@@ -43,6 +43,8 @@ void format(void)
{
fprintf(stderr,
"Usage: scripts/sign-file [-dp] <hash algo> <key> <x509> <module> [<dest>]\n");
+ fprintf(stderr,
+ " scripts/sign-file [-dp] -F <firmware name> <hash algo> <key> <x509> <firmware> [<dest>]\n");
exit(2);
}

@@ -105,12 +107,14 @@ static int pem_pw_cb(char *buf, int len, int w, void *v)
int main(int argc, char **argv)
{
struct module_signature sig_info = { .id_type = PKEY_ID_PKCS7 };
+ const char *firmware_name = NULL;
char *hash_algo = NULL;
char *private_key_name, *x509_name, *module_name, *dest_name;
bool save_pkcs7 = false, replace_orig;
bool sign_only = false;
unsigned char buf[4096];
unsigned long module_size, pkcs7_size;
+ PKCS7_SIGNER_INFO *signer;
const EVP_MD *digest_algo;
EVP_PKEY *private_key;
PKCS7 *pkcs7;
@@ -125,10 +129,11 @@ int main(int argc, char **argv)
key_pass = getenv("KBUILD_SIGN_PIN");

do {
- opt = getopt(argc, argv, "dp");
+ opt = getopt(argc, argv, "dpF:");
switch (opt) {
case 'p': save_pkcs7 = true; break;
case 'd': sign_only = true; save_pkcs7 = true; break;
+ case 'F': firmware_name = optarg; break;
case -1: break;
default: format();
}
@@ -210,11 +215,34 @@ int main(int argc, char **argv)

/* Load the PKCS#7 message from the digest buffer. */
pkcs7 = PKCS7_sign(NULL, NULL, NULL, NULL,
- PKCS7_NOCERTS | PKCS7_PARTIAL | PKCS7_BINARY | PKCS7_DETACHED | PKCS7_STREAM);
+ PKCS7_NOCERTS | PKCS7_PARTIAL | PKCS7_BINARY |
+ PKCS7_DETACHED | PKCS7_STREAM);
ERR(!pkcs7, "PKCS7_sign");

- ERR(!PKCS7_sign_add_signer(pkcs7, x509, private_key, digest_algo, PKCS7_NOCERTS | PKCS7_BINARY),
- "PKCS7_sign_add_signer");
+ signer = PKCS7_sign_add_signer(pkcs7, x509, private_key, digest_algo,
+ PKCS7_NOCERTS | PKCS7_BINARY);
+ ERR(!signer, "PKCS7_sign_add_signer");
+
+ if (firmware_name) {
+ /* Add an entry into the authenticated attributes to note the
+ * firmware name if this is a firmware signature.
+ */
+ ASN1_UTF8STRING *str;
+ int nid;
+
+ // As an example, use an OID of redhat.99.1 - which is not assigned
+ nid = OBJ_create("1.3.6.1.4.1.2312.99.1", NULL, NULL);
+ ERR(!nid, "OBJ_create");
+
+ str = M_ASN1_UTF8STRING_new();
+ ERR(!str, "M_ASN1_UTF8STRING_new");
+ ERR(!ASN1_STRING_set(str, firmware_name, strlen(firmware_name)),
+ "ASN1_STRING_set");
+ ERR(!PKCS7_add_signed_attribute(signer, nid,
+ V_ASN1_UTF8STRING, str),
+ "PKCS7_add_signed_attribute");
+ }
+
ERR(PKCS7_final(pkcs7, bm, PKCS7_NOCERTS | PKCS7_BINARY) < 0,
"PKCS7_final");


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