lib: Add ASN.1 encoder

We have a need in the TPM2 trusted keys to return the ASN.1 form of the TPM
key blob so it can be operated on by tools outside of the kernel.  The
specific tools are the openssl_tpm2_engine, openconnect and the Intel
tpm2-tss-engine.  To do that, we have to be able to read and write the same
binary key format the tools use.  The current ASN.1 decoder does fine for
reading, but we need pieces of an ASN.1 encoder to write the key blob in
binary compatible form.

For backwards compatibility, the trusted key reader code will still accept
the two TPM2B quantities that it uses today, but the writer will only
output the ASN.1 form.

The current implementation only encodes the ASN.1 bits we actually need.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

1 files changed, 454 insertions, 0 deletions

diff --git a/lib/asn1_encoder.c b/lib/asn1_encoder.c
new file mode 100644
index 000000000000..41e71aae3ef6
--- /dev/null
+++ b/lib/asn1_encoder.c
@@ -0,0 +1,454 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Simple encoder primitives for ASN.1 BER/DER/CER
+ *
+ * Copyright (C) 2019 James.Bottomley@HansenPartnership.com
+ */
+
+#include <linux/asn1_encoder.h>
+#include <linux/bug.h>
+#include <linux/string.h>
+#include <linux/module.h>
+
+/**
+ * asn1_encode_integer() - encode positive integer to ASN.1
+ * @data:	pointer to the pointer to the data
+ * @end_data:	end of data pointer, points one beyond last usable byte in @data
+ * @integer:	integer to be encoded
+ *
+ * This is a simplified encoder: it only currently does
+ * positive integers, but it should be simple enough to add the
+ * negative case if a use comes along.
+ */
+unsigned char *
+asn1_encode_integer(unsigned char *data, const unsigned char *end_data,
+		    s64 integer)
+{
+	int data_len = end_data - data;
+	unsigned char *d = &data[2];
+	bool found = false;
+	int i;
+
+	if (WARN(integer < 0,
+		 "BUG: integer encode only supports positive integers"))
+		return ERR_PTR(-EINVAL);
+
+	if (IS_ERR(data))
+		return data;
+
+	/* need at least 3 bytes for tag, length and integer encoding */
+	if (data_len < 3)
+		return ERR_PTR(-EINVAL);
+
+	/* remaining length where at d (the start of the integer encoding) */
+	data_len -= 2;
+
+	data[0] = _tag(UNIV, PRIM, INT);
+	if (integer == 0) {
+		*d++ = 0;
+		goto out;
+	}
+
+	for (i = sizeof(integer); i > 0 ; i--) {
+		int byte = integer >> (8 * (i - 1));
+
+		if (!found && byte == 0)
+			continue;
+
+		/*
+		 * for a positive number the first byte must have bit
+		 * 7 clear in two's complement (otherwise it's a
+		 * negative number) so prepend a leading zero if
+		 * that's not the case
+		 */
+		if (!found && (byte & 0x80)) {
+			/*
+			 * no check needed here, we already know we
+			 * have len >= 1
+			 */
+			*d++ = 0;
+			data_len--;
+		}
+
+		found = true;
+		if (data_len == 0)
+			return ERR_PTR(-EINVAL);
+
+		*d++ = byte;
+		data_len--;
+	}
+
+ out:
+	data[1] = d - data - 2;
+
+	return d;
+}
+EXPORT_SYMBOL_GPL(asn1_encode_integer);
+
+/* calculate the base 128 digit values setting the top bit of the first octet */
+static int asn1_encode_oid_digit(unsigned char **_data, int *data_len, u32 oid)
+{
+	unsigned char *data = *_data;
+	int start = 7 + 7 + 7 + 7;
+	int ret = 0;
+
+	if (*data_len < 1)
+		return -EINVAL;
+
+	/* quick case */
+	if (oid == 0) {
+		*data++ = 0x80;
+		(*data_len)--;
+		goto out;
+	}
+
+	while (oid >> start == 0)
+		start -= 7;
+
+	while (start > 0 && *data_len > 0) {
+		u8 byte;
+
+		byte = oid >> start;
+		oid = oid - (byte << start);
+		start -= 7;
+		byte |= 0x80;
+		*data++ = byte;
+		(*data_len)--;
+	}
+
+	if (*data_len > 0) {
+		*data++ = oid;
+		(*data_len)--;
+	} else {
+		ret = -EINVAL;
+	}
+
+ out:
+	*_data = data;
+	return ret;
+}
+
+/**
+ * asn1_encode_oid() - encode an oid to ASN.1
+ * @data:	position to begin encoding at
+ * @end_data:	end of data pointer, points one beyond last usable byte in @data
+ * @oid:	array of oids
+ * @oid_len:	length of oid array
+ *
+ * this encodes an OID up to ASN.1 when presented as an array of OID values
+ */
+unsigned char *
+asn1_encode_oid(unsigned char *data, const unsigned char *end_data,
+		u32 oid[], int oid_len)
+{
+	int data_len = end_data - data;
+	unsigned char *d = data + 2;
+	int i, ret;
+
+	if (WARN(oid_len < 2, "OID must have at least two elements"))
+		return ERR_PTR(-EINVAL);
+
+	if (WARN(oid_len > 32, "OID is too large"))
+		return ERR_PTR(-EINVAL);
+
+	if (IS_ERR(data))
+		return data;
+
+
+	/* need at least 3 bytes for tag, length and OID encoding */
+	if (data_len < 3)
+		return ERR_PTR(-EINVAL);
+
+	data[0] = _tag(UNIV, PRIM, OID);
+	*d++ = oid[0] * 40 + oid[1];
+
+	data_len -= 3;
+
+	ret = 0;
+
+	for (i = 2; i < oid_len; i++) {
+		ret = asn1_encode_oid_digit(&d, &data_len, oid[i]);
+		if (ret < 0)
+			return ERR_PTR(ret);
+	}
+
+	data[1] = d - data - 2;
+
+	return d;
+}
+EXPORT_SYMBOL_GPL(asn1_encode_oid);
+
+/**
+ * asn1_encode_length() - encode a length to follow an ASN.1 tag
+ * @data: pointer to encode at
+ * @data_len: pointer to remaning length (adjusted by routine)
+ * @len: length to encode
+ *
+ * This routine can encode lengths up to 65535 using the ASN.1 rules.
+ * It will accept a negative length and place a zero length tag
+ * instead (to keep the ASN.1 valid).  This convention allows other
+ * encoder primitives to accept negative lengths as singalling the
+ * sequence will be re-encoded when the length is known.
+ */
+static int asn1_encode_length(unsigned char **data, int *data_len, int len)
+{
+	if (*data_len < 1)
+		return -EINVAL;
+
+	if (len < 0) {
+		*((*data)++) = 0;
+		(*data_len)--;
+		return 0;
+	}
+
+	if (len <= 0x7f) {
+		*((*data)++) = len;
+		(*data_len)--;
+		return 0;
+	}
+
+	if (*data_len < 2)
+		return -EINVAL;
+
+	if (len <= 0xff) {
+		*((*data)++) = 0x81;
+		*((*data)++) = len & 0xff;
+		*data_len -= 2;
+		return 0;
+	}
+
+	if (*data_len < 3)
+		return -EINVAL;
+
+	if (len <= 0xffff) {
+		*((*data)++) = 0x82;
+		*((*data)++) = (len >> 8) & 0xff;
+		*((*data)++) = len & 0xff;
+		*data_len -= 3;
+		return 0;
+	}
+
+	if (WARN(len > 0xffffff, "ASN.1 length can't be > 0xffffff"))
+		return -EINVAL;
+
+	if (*data_len < 4)
+		return -EINVAL;
+	*((*data)++) = 0x83;
+	*((*data)++) = (len >> 16) & 0xff;
+	*((*data)++) = (len >> 8) & 0xff;
+	*((*data)++) = len & 0xff;
+	*data_len -= 4;
+
+	return 0;
+}
+
+/**
+ * asn1_encode_tag() - add a tag for optional or explicit value
+ * @data:	pointer to place tag at
+ * @end_data:	end of data pointer, points one beyond last usable byte in @data
+ * @tag:	tag to be placed
+ * @string:	the data to be tagged
+ * @len:	the length of the data to be tagged
+ *
+ * Note this currently only handles short form tags < 31.
+ *
+ * Standard usage is to pass in a @tag, @string and @length and the
+ * @string will be ASN.1 encoded with @tag and placed into @data.  If
+ * the encoding would put data past @end_data then an error is
+ * returned, otherwise a pointer to a position one beyond the encoding
+ * is returned.
+ *
+ * To encode in place pass a NULL @string and -1 for @len and the
+ * maximum allowable beginning and end of the data; all this will do
+ * is add the current maximum length and update the data pointer to
+ * the place where the tag contents should be placed is returned.  The
+ * data should be copied in by the calling routine which should then
+ * repeat the prior statement but now with the known length.  In order
+ * to avoid having to keep both before and after pointers, the repeat
+ * expects to be called with @data pointing to where the first encode
+ * returned it and still NULL for @string but the real length in @len.
+ */
+unsigned char *
+asn1_encode_tag(unsigned char *data, const unsigned char *end_data,
+		u32 tag, const unsigned char *string, int len)
+{
+	int data_len = end_data - data;
+	int ret;
+
+	if (WARN(tag > 30, "ASN.1 tag can't be > 30"))
+		return ERR_PTR(-EINVAL);
+
+	if (!string && WARN(len > 127,
+			    "BUG: recode tag is too big (>127)"))
+		return ERR_PTR(-EINVAL);
+
+	if (IS_ERR(data))
+		return data;
+
+	if (!string && len > 0) {
+		/*
+		 * we're recoding, so move back to the start of the
+		 * tag and install a dummy length because the real
+		 * data_len should be NULL
+		 */
+		data -= 2;
+		data_len = 2;
+	}
+
+	if (data_len < 2)
+		return ERR_PTR(-EINVAL);
+
+	*(data++) = _tagn(CONT, CONS, tag);
+	data_len--;
+	ret = asn1_encode_length(&data, &data_len, len);
+	if (ret < 0)
+		return ERR_PTR(ret);
+
+	if (!string)
+		return data;
+
+	if (data_len < len)
+		return ERR_PTR(-EINVAL);
+
+	memcpy(data, string, len);
+	data += len;
+
+	return data;
+}
+EXPORT_SYMBOL_GPL(asn1_encode_tag);
+
+/**
+ * asn1_encode_octet_string() - encode an ASN.1 OCTET STRING
+ * @data:	pointer to encode at
+ * @end_data:	end of data pointer, points one beyond last usable byte in @data
+ * @string:	string to be encoded
+ * @len:	length of string
+ *
+ * Note ASN.1 octet strings may contain zeros, so the length is obligatory.
+ */
+unsigned char *
+asn1_encode_octet_string(unsigned char *data,
+			 const unsigned char *end_data,
+			 const unsigned char *string, u32 len)
+{
+	int data_len = end_data - data;
+	int ret;
+
+	if (IS_ERR(data))
+		return data;
+
+	/* need minimum of 2 bytes for tag and length of zero length string */
+	if (data_len < 2)
+		return ERR_PTR(-EINVAL);
+
+	*(data++) = _tag(UNIV, PRIM, OTS);
+	data_len--;
+
+	ret = asn1_encode_length(&data, &data_len, len);
+	if (ret)
+		return ERR_PTR(ret);
+
+	if (data_len < len)
+		return ERR_PTR(-EINVAL);
+
+	memcpy(data, string, len);
+	data += len;
+
+	return data;
+}
+EXPORT_SYMBOL_GPL(asn1_encode_octet_string);
+
+/**
+ * asn1_encode_sequence() - wrap a byte stream in an ASN.1 SEQUENCE
+ * @data:	pointer to encode at
+ * @end_data:	end of data pointer, points one beyond last usable byte in @data
+ * @seq:	data to be encoded as a sequence
+ * @len:	length of the data to be encoded as a sequence
+ *
+ * Fill in a sequence.  To encode in place, pass NULL for @seq and -1
+ * for @len; then call again once the length is known (still with NULL
+ * for @seq). In order to avoid having to keep both before and after
+ * pointers, the repeat expects to be called with @data pointing to
+ * where the first encode placed it.
+ */
+unsigned char *
+asn1_encode_sequence(unsigned char *data, const unsigned char *end_data,
+		     const unsigned char *seq, int len)
+{
+	int data_len = end_data - data;
+	int ret;
+
+	if (!seq && WARN(len > 127,
+			 "BUG: recode sequence is too big (>127)"))
+		return ERR_PTR(-EINVAL);
+
+	if (IS_ERR(data))
+		return data;
+
+	if (!seq && len >= 0) {
+		/*
+		 * we're recoding, so move back to the start of the
+		 * sequence and install a dummy length because the
+		 * real length should be NULL
+		 */
+		data -= 2;
+		data_len = 2;
+	}
+
+	if (data_len < 2)
+		return ERR_PTR(-EINVAL);
+
+	*(data++) = _tag(UNIV, CONS, SEQ);
+	data_len--;
+
+	ret = asn1_encode_length(&data, &data_len, len);
+	if (ret)
+		return ERR_PTR(ret);
+
+	if (!seq)
+		return data;
+
+	if (data_len < len)
+		return ERR_PTR(-EINVAL);
+
+	memcpy(data, seq, len);
+	data += len;
+
+	return data;
+}
+EXPORT_SYMBOL_GPL(asn1_encode_sequence);
+
+/**
+ * asn1_encode_boolean() - encode a boolean value to ASN.1
+ * @data:	pointer to encode at
+ * @end_data:	end of data pointer, points one beyond last usable byte in @data
+ * @val:	the boolean true/false value
+ */
+unsigned char *
+asn1_encode_boolean(unsigned char *data, const unsigned char *end_data,
+		    bool val)
+{
+	int data_len = end_data - data;
+
+	if (IS_ERR(data))
+		return data;
+
+	/* booleans are 3 bytes: tag, length == 1 and value == 0 or 1 */
+	if (data_len < 3)
+		return ERR_PTR(-EINVAL);
+
+	*(data++) = _tag(UNIV, PRIM, BOOL);
+	data_len--;
+
+	asn1_encode_length(&data, &data_len, 1);
+
+	if (val)
+		*(data++) = 1;
+	else
+		*(data++) = 0;
+
+	return data;
+}
+EXPORT_SYMBOL_GPL(asn1_encode_boolean);
+
+MODULE_LICENSE("GPL");