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Subject[PATCHv2 6/7] tools: add dmesg decryption program
Date
Example execution:

dmesg | dmesg-decipher <private-key.pem>

Signed-off-by: Dan Aloni <dan@kernelim.com>
---
tools/Makefile | 9 +-
tools/kmsg/.gitignore | 1 +
tools/kmsg/Makefile | 14 ++
tools/kmsg/dmesg-decipher.c | 354 ++++++++++++++++++++++++++++++++++++++++++++
4 files changed, 377 insertions(+), 1 deletion(-)
create mode 100644 tools/kmsg/.gitignore
create mode 100644 tools/kmsg/Makefile
create mode 100644 tools/kmsg/dmesg-decipher.c

diff --git a/tools/Makefile b/tools/Makefile
index be02c8b904db..5a661e4c9012 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -18,6 +18,7 @@ help:
@echo ' hv - tools used when in Hyper-V clients'
@echo ' iio - IIO tools'
@echo ' kvm_stat - top-like utility for displaying kvm statistics'
+ @echo ' kmsg - A tool for decrypting a dmesg using a private key'
@echo ' leds - LEDs tools'
@echo ' liblockdep - user-space wrapper for kernel locking-validator'
@echo ' bpf - misc BPF tools'
@@ -91,6 +92,9 @@ freefall: FORCE
kvm_stat: FORCE
$(call descend,kvm/$@)

+kmsg: FORCE
+ $(call descend,kmsg)
+
all: acpi cgroup cpupower gpio hv firewire liblockdep \
perf selftests spi turbostat usb \
virtio vm bpf x86_energy_perf_policy \
@@ -167,6 +171,9 @@ tmon_clean:
freefall_clean:
$(call descend,laptop/freefall,clean)

+kmsg_clean:
+ $(call descend,kmsg,clean)
+
build_clean:
$(call descend,build,clean)

@@ -174,6 +181,6 @@ clean: acpi_clean cgroup_clean cpupower_clean hv_clean firewire_clean \
perf_clean selftests_clean turbostat_clean spi_clean usb_clean virtio_clean \
vm_clean bpf_clean iio_clean x86_energy_perf_policy_clean tmon_clean \
freefall_clean build_clean libbpf_clean libsubcmd_clean liblockdep_clean \
- gpio_clean objtool_clean leds_clean wmi_clean
+ gpio_clean objtool_clean leds_clean wmi_clean kmsg_clean

.PHONY: FORCE
diff --git a/tools/kmsg/.gitignore b/tools/kmsg/.gitignore
new file mode 100644
index 000000000000..a5b4e26b8d0b
--- /dev/null
+++ b/tools/kmsg/.gitignore
@@ -0,0 +1 @@
+dmesg-decipher
diff --git a/tools/kmsg/Makefile b/tools/kmsg/Makefile
new file mode 100644
index 000000000000..9f4ef7b11798
--- /dev/null
+++ b/tools/kmsg/Makefile
@@ -0,0 +1,14 @@
+# SPDX-License-Identifier: GPL-2.0
+CC := $(CROSS_COMPILE)gcc
+
+CFLAGS := -O2 -Wall $$(pkg-config --libs openssl)
+
+PROGS := dmesg-decipher
+
+%: %.c
+ $(CC) $(CFLAGS) -o $@ $^
+
+all: $(PROGS)
+
+clean:
+ rm -fr $(PROGS)
diff --git a/tools/kmsg/dmesg-decipher.c b/tools/kmsg/dmesg-decipher.c
new file mode 100644
index 000000000000..1ad2b0a27402
--- /dev/null
+++ b/tools/kmsg/dmesg-decipher.c
@@ -0,0 +1,354 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * dmesg-decipher.c
+ *
+ * A sample utility to decrypt an encrypted dmesg output, for
+ * development with kernels having kmsg encryption enabled.
+ *
+ * base64 decoding code taken from lib/base64-armor.c
+ *
+ * Copyright (c) Dan Aloni, 2017
+ *
+ * Compile with:
+ *
+ * gcc -O2 -Wall $(pkg-config --libs openssl) \
+ * dmesg-decipher -o dmesg-decipher
+ */
+
+#include <openssl/err.h>
+#include <openssl/pem.h>
+#include <openssl/pkcs7.h>
+
+#include <regex.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <string.h>
+
+/*
+ * The following is based on code from:
+ *
+ * https://wiki.openssl.org/index.php/EVP_Authenticated_Encryption_and_Decryption
+ */
+static int aes_256_gcm_decrypt(unsigned char *ciphertext, size_t ciphertext_len,
+ unsigned char *aad, size_t aad_len,
+ unsigned char *tag, unsigned char *key,
+ unsigned char *iv, size_t iv_len,
+ unsigned char *plaintext)
+{
+ EVP_CIPHER_CTX *ctx;
+ int len;
+ int plaintext_len;
+ int ret = -1;
+
+ /* Create and initialise the context */
+ ctx = EVP_CIPHER_CTX_new();
+ if (!ctx)
+ return -1;
+
+ /* Initialise the decryption operation. */
+ if (!EVP_DecryptInit_ex(ctx, EVP_aes_128_gcm(), NULL, NULL, NULL))
+ goto free;
+
+ /* Set IV length. Not necessary if this is 12 bytes (96 bits) */
+ if (!EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_IVLEN, iv_len, NULL))
+ goto free;
+
+ /* Initialise key and IV */
+ if (!EVP_DecryptInit_ex(ctx, NULL, NULL, key, iv))
+ goto free;
+
+ /* Provide any AAD data. This can be called zero or more times as
+ * required
+ */
+ if (aad_len != 0)
+ if (!EVP_DecryptUpdate(ctx, NULL, &len, aad, aad_len))
+ goto free;
+
+ /* Provide the message to be decrypted, and obtain the plaintext output.
+ * EVP_DecryptUpdate can be called multiple times if necessary
+ */
+ if (!EVP_DecryptUpdate(ctx, plaintext, &len, ciphertext,
+ ciphertext_len))
+ goto free;
+ plaintext_len = len;
+
+ /* Set expected tag value. Works in OpenSSL 1.0.1d and later */
+ if (!EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_TAG, 16, tag))
+ goto free;
+
+ /* Finalise the decryption. A positive return value indicates success,
+ * anything else is a failure - the plaintext is not trustworthy.
+ */
+ ret = EVP_DecryptFinal_ex(ctx, plaintext + len, &len);
+
+free:
+ /* Clean up */
+ EVP_CIPHER_CTX_free(ctx);
+
+ if (ret > 0) {
+ /* Success */
+ plaintext_len += len;
+ return plaintext_len;
+ }
+
+ /* Verify failed */
+ return -1;
+}
+
+static int decode_bits(char c)
+{
+ if (c >= 'A' && c <= 'Z')
+ return c - 'A';
+ if (c >= 'a' && c <= 'z')
+ return c - 'a' + 26;
+ if (c >= '0' && c <= '9')
+ return c - '0' + 52;
+ if (c == '+')
+ return 62;
+ if (c == '/')
+ return 63;
+ if (c == '=')
+ return 0; /* just non-negative, please */
+ return -EINVAL;
+}
+
+static int base64_unarmor(char *dst, int dst_max, const char *src,
+ const char *end)
+{
+ int olen = 0;
+
+ while (src < end) {
+ int a, b, c, d;
+
+ if (src[0] == '\n') {
+ src++;
+ continue;
+ }
+ if (src + 4 > end)
+ return -EINVAL;
+ a = decode_bits(src[0]);
+ b = decode_bits(src[1]);
+ c = decode_bits(src[2]);
+ d = decode_bits(src[3]);
+ if (a < 0 || b < 0 || c < 0 || d < 0)
+ return -EINVAL;
+
+ if (dst_max < 1)
+ return -ENOSPC;
+ *dst++ = (a << 2) | (b >> 4);
+ dst_max--;
+ if (src[2] == '=')
+ return olen + 1;
+ if (dst_max < 1)
+ return -ENOSPC;
+ *dst++ = ((b & 15) << 4) | (c >> 2);
+ dst_max--;
+ if (src[3] == '=')
+ return olen + 2;
+ if (dst_max < 1)
+ return -ENOSPC;
+ *dst++ = ((c & 3) << 6) | d;
+ dst_max--;
+ olen += 3;
+ src += 4;
+ }
+ return olen;
+}
+
+static int parse_int_regex_match(const char *source, regmatch_t match,
+ size_t *output)
+{
+ char decimal_number[0x10] = {
+ 0,
+ };
+ size_t len = match.rm_eo - match.rm_so;
+
+ if (len >= sizeof(decimal_number))
+ return -1;
+
+ memcpy(&decimal_number[0], &source[match.rm_so], len);
+
+ *output = atoi(decimal_number);
+ return 0;
+}
+
+static const char session_key_pattern[] = "(.*)K:([0-9a-zA-Z~+/=]+)";
+static const char message_pattern[] =
+ "(.*)M:([0-9a-zA-Z~+/=]+),([0-9]+),([0-9]+)";
+
+static int decrypt_message(char *line, regmatch_t *matches, uint8_t *sess_key)
+{
+ char plain_text[0x1000], *enc;
+ uint8_t cipher_msg_bin[0x1000];
+ size_t cipher_msg_size = sizeof(cipher_msg_bin);
+ size_t cipher_size;
+ const regmatch_t prefix = matches[1];
+ const regmatch_t ciphermsg = matches[2];
+ const regmatch_t auth_str_len = matches[3];
+ const regmatch_t iv_str_len = matches[4];
+ size_t auth_len;
+ size_t iv_len;
+ int ret;
+
+ ret = parse_int_regex_match(line, auth_str_len, &auth_len);
+ if (ret)
+ return -1;
+
+ ret = parse_int_regex_match(line, iv_str_len, &iv_len);
+ if (ret)
+ return -1;
+
+ for (enc = &line[ciphermsg.rm_so]; enc < &line[ciphermsg.rm_eo]; enc++)
+ if (*enc == '~')
+ *enc = '\n';
+
+ ret = base64_unarmor((char *)cipher_msg_bin, cipher_msg_size,
+ &line[ciphermsg.rm_so], &line[ciphermsg.rm_eo]);
+ if (ret < 0) {
+ fprintf(stderr, "error decoding base64 message (code = %d)\n",
+ ret);
+ return -1;
+ }
+
+ cipher_msg_size = ret;
+
+ if (iv_len >= cipher_msg_size || auth_len >= cipher_msg_size
+ || auth_len + iv_len > cipher_msg_size) {
+ return -1;
+ }
+
+ cipher_size = cipher_msg_size - auth_len - iv_len;
+
+ ret = aes_256_gcm_decrypt(/* Ciphertext */
+ (uint8_t *)cipher_msg_bin, cipher_size,
+
+ /* AAD */
+ NULL, 0,
+
+ /* tag */
+ (uint8_t *)&cipher_msg_bin[cipher_size],
+
+ /* key */
+ sess_key,
+
+ /* IV */
+ (uint8_t *)&cipher_msg_bin[cipher_size
+ + auth_len],
+ iv_len,
+
+ /* Plain text */
+ (uint8_t *)plain_text);
+ if (ret > 0) {
+ fwrite(line, prefix.rm_eo, 1, stdout);
+ fwrite(plain_text, ret, 1, stdout);
+ fwrite("\n", 1, 1, stdout);
+ }
+
+ return ret;
+}
+
+int main(int argc, char **argv)
+{
+ BIO *tbio = NULL;
+ RSA *rsa;
+ int ret = 1;
+ char line[0x1000];
+ uint8_t enc_sess_key[0x200];
+ uint8_t sess_key[0x200] = {
+ 0,
+ };
+ bool got_key = false;
+
+ OpenSSL_add_all_algorithms();
+ ERR_load_crypto_strings();
+
+ regex_t session_key_regex;
+ regex_t message_regex;
+
+ ret = regcomp(&session_key_regex, session_key_pattern, REG_EXTENDED);
+ if (ret)
+ goto err;
+
+ ret = regcomp(&message_regex, message_pattern, REG_EXTENDED);
+ if (ret)
+ goto err;
+
+ if (argc < 2) {
+ fprintf(stderr, "not enough parameters\n");
+ return -1;
+ }
+
+ /* Read in recipient certificate and private key */
+ tbio = BIO_new_file(argv[1], "r");
+ if (!tbio) {
+ fprintf(stderr, "BIO_new_file - error\n");
+ goto err;
+ }
+
+ rsa = PEM_read_bio_RSAPrivateKey(tbio, NULL, NULL, NULL);
+ if (!rsa)
+ goto err;
+
+ while (true) {
+ regmatch_t matches[5];
+
+ if (!fgets(line, sizeof(line), stdin))
+ break;
+
+ if (!got_key
+ && !regexec(&session_key_regex, line, 5, matches, 0)) {
+ const regmatch_t match = matches[2];
+ size_t enc_sess_key_size = sizeof(enc_sess_key);
+ char *enc;
+
+ for (enc = &line[match.rm_so]; enc < &line[match.rm_eo];
+ enc++)
+ if (*enc == '~')
+ *enc = '\n';
+
+ ret = base64_unarmor(
+ (char *)&enc_sess_key, enc_sess_key_size,
+ &line[match.rm_so], &line[match.rm_eo]);
+ if (ret < 0) {
+ fprintf(stderr,
+ "error decoding session key"
+ " (code = %d)\n",
+ ret);
+ return -1;
+ }
+
+ enc_sess_key_size = ret;
+
+ ret = RSA_private_decrypt(enc_sess_key_size,
+ enc_sess_key, sess_key, rsa,
+ RSA_PKCS1_PADDING);
+ if (ret < 0)
+ goto err;
+
+ got_key = true;
+ }
+
+ if (!regexec(&message_regex, line, 5, matches, 0)) {
+ if (!got_key) {
+ fprintf(stderr,
+ "session key must precede messages\n");
+ break;
+ }
+
+ ret = decrypt_message(line, matches, sess_key);
+ if (ret < 0) {
+ fprintf(stderr,
+ "error decrypting message"
+ " (code = %d)\n",
+ ret);
+ break;
+ }
+ }
+ }
+
+ regfree(&session_key_regex);
+ regfree(&message_regex);
+
+err:
+ return -1;
+}
--
2.14.3
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