Implement generalised RSA public exponents for verified boot

Remove the verified boot limitation that only allows a single
RSA public exponent of 65537 (F4).  This change allows use with
existing PKI infrastructure and has been tested with HSM-based
PKI.

Change the configuration OF tree format to store the RSA public
exponent as a 64 bit integer and implement backward compatibility
for verified boot configuration trees without this extra field.

Parameterise vboot_test.sh to test different public exponents.

Mathematics and other hard work by Andrew Bott.

Tested with the following public exponents: 3, 5, 17, 257, 39981,
50457, 65537 and 4294967297.

Signed-off-by: Andrew Bott <Andrew.Bott@ipaccess.com>
Signed-off-by: Andrew Wishart <Andrew.Wishart@ipaccess.com>
Signed-off-by: Neil Piercy <Neil.Piercy@ipaccess.com>
Signed-off-by: Michael van der Westhuizen <michael@smart-africa.com>
Cc: Simon Glass <sjg@chromium.org>

1 files changed, 57 insertions, 3 deletions

diff --git a/lib/rsa/rsa-sign.c b/lib/rsa/rsa-sign.c
index 83f5e87..f4d4338 100644
--- a/lib/rsa/rsa-sign.c
+++ b/lib/rsa/rsa-sign.c
@@ -261,10 +261,57 @@ err_priv:
 }
 
 /*
+ * rsa_get_exponent(): - Get the public exponent from an RSA key
+ */
+static int rsa_get_exponent(RSA *key, uint64_t *e)
+{
+	int ret;
+	BIGNUM *bn_te;
+	uint64_t te;
+
+	ret = -EINVAL;
+	bn_te = NULL;
+
+	if (!e)
+		goto cleanup;
+
+	if (BN_num_bits(key->e) > 64)
+		goto cleanup;
+
+	*e = BN_get_word(key->e);
+
+	if (BN_num_bits(key->e) < 33) {
+		ret = 0;
+		goto cleanup;
+	}
+
+	bn_te = BN_dup(key->e);
+	if (!bn_te)
+		goto cleanup;
+
+	if (!BN_rshift(bn_te, bn_te, 32))
+		goto cleanup;
+
+	if (!BN_mask_bits(bn_te, 32))
+		goto cleanup;
+
+	te = BN_get_word(bn_te);
+	te <<= 32;
+	*e |= te;
+	ret = 0;
+
+cleanup:
+	if (bn_te)
+		BN_free(bn_te);
+
+	return ret;
+}
+
+/*
  * rsa_get_params(): - Get the important parameters of an RSA public key
  */
-int rsa_get_params(RSA *key, uint32_t *n0_invp, BIGNUM **modulusp,
-		   BIGNUM **r_squaredp)
+int rsa_get_params(RSA *key, uint64_t *exponent, uint32_t *n0_invp,
+		   BIGNUM **modulusp, BIGNUM **r_squaredp)
 {
 	BIGNUM *big1, *big2, *big32, *big2_32;
 	BIGNUM *n, *r, *r_squared, *tmp;
@@ -286,6 +333,9 @@ int rsa_get_params(RSA *key, uint32_t *n0_invp, BIGNUM **modulusp,
 		return -ENOMEM;
 	}
 
+	if (0 != rsa_get_exponent(key, exponent))
+		ret = -1;
+
 	if (!BN_copy(n, key->n) || !BN_set_word(big1, 1L) ||
 	    !BN_set_word(big2, 2L) || !BN_set_word(big32, 32L))
 		ret = -1;
@@ -386,6 +436,7 @@ static int fdt_add_bignum(void *blob, int noffset, const char *prop_name,
 int rsa_add_verify_data(struct image_sign_info *info, void *keydest)
 {
 	BIGNUM *modulus, *r_squared;
+	uint64_t exponent;
 	uint32_t n0_inv;
 	int parent, node;
 	char name[100];
@@ -397,7 +448,7 @@ int rsa_add_verify_data(struct image_sign_info *info, void *keydest)
 	ret = rsa_get_pub_key(info->keydir, info->keyname, &rsa);
 	if (ret)
 		return ret;
-	ret = rsa_get_params(rsa, &n0_inv, &modulus, &r_squared);
+	ret = rsa_get_params(rsa, &exponent, &n0_inv, &modulus, &r_squared);
 	if (ret)
 		return ret;
 	bits = BN_num_bits(modulus);
@@ -442,6 +493,9 @@ int rsa_add_verify_data(struct image_sign_info *info, void *keydest)
 	if (!ret)
 		ret = fdt_setprop_u32(keydest, node, "rsa,n0-inverse", n0_inv);
 	if (!ret) {
+		ret = fdt_setprop_u64(keydest, node, "rsa,exponent", exponent);
+	}
+	if (!ret) {
 		ret = fdt_add_bignum(keydest, node, "rsa,modulus", modulus,
 				     bits);
 	}