[PATCH v8 09/22] crypto: lib - implement library version of AES in CFB mode

James Bottomley
2024-04-29T20:29:43Z

From: Ard Biesheuvel <ardb@kernel.org>

Implement AES in CFB mode using the existing, mostly constant-time
generic AES library implementation. This will be used by the TPM code
to encrypt communications with TPM hardware, which is often a discrete
component connected using sniffable wires or traces.

While a CFB template does exist, using a skcipher is a major pain for
non-performance critical synchronous crypto where the algorithm is known
at compile time and the data is in contiguous buffers with valid kernel
virtual addresses.

Tested-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Reviewed-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/all/20230216201410.15010-1-James.Bottomley@HansenPartnership.com/
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
---
include/crypto/aes.h | 5 +
lib/crypto/Kconfig | 5 +
lib/crypto/Makefile | 3 +
lib/crypto/aescfb.c | 257 +++++++++++++++++++++++++++++++++++++++++++
4 files changed, 270 insertions(+)
create mode 100644 lib/crypto/aescfb.c

diff --git a/include/crypto/aes.h b/include/crypto/aes.h
index 2090729701ab..9339da7c20a8 100644
--- a/include/crypto/aes.h
+++ b/include/crypto/aes.h
@@ -87,4 +87,9 @@ void aes_decrypt(const struct crypto_aes_ctx *ctx, u8 *out, const u8 *in);
extern const u8 crypto_aes_sbox[];
extern const u8 crypto_aes_inv_sbox[];

+void aescfb_encrypt(const struct crypto_aes_ctx *ctx, u8 *dst, const u8 *src,
+ int len, const u8 iv[AES_BLOCK_SIZE]);
+void aescfb_decrypt(const struct crypto_aes_ctx *ctx, u8 *dst, const u8 *src,
+ int len, const u8 iv[AES_BLOCK_SIZE]);
+
#endif
diff --git a/lib/crypto/Kconfig b/lib/crypto/Kconfig
index 45436bfc6dff..b01253cac70a 100644
--- a/lib/crypto/Kconfig
+++ b/lib/crypto/Kconfig
@@ -8,6 +8,11 @@ config CRYPTO_LIB_UTILS
config CRYPTO_LIB_AES
tristate

+config CRYPTO_LIB_AESCFB
+ tristate
+ select CRYPTO_LIB_AES
+ select CRYPTO_LIB_UTILS
+
config CRYPTO_LIB_AESGCM
tristate
select CRYPTO_LIB_AES
diff --git a/lib/crypto/Makefile b/lib/crypto/Makefile
index 8d1446c2be71..969baab8c805 100644
--- a/lib/crypto/Makefile
+++ b/lib/crypto/Makefile
@@ -10,6 +10,9 @@ obj-$(CONFIG_CRYPTO_LIB_CHACHA_GENERIC) += libchacha.o
obj-$(CONFIG_CRYPTO_LIB_AES) += libaes.o
libaes-y := aes.o

+obj-$(CONFIG_CRYPTO_LIB_AESCFB) += libaescfb.o
+libaescfb-y := aescfb.o
+
obj-$(CONFIG_CRYPTO_LIB_AESGCM) += libaesgcm.o
libaesgcm-y := aesgcm.o

diff --git a/lib/crypto/aescfb.c b/lib/crypto/aescfb.c
new file mode 100644
index 000000000000..749dc1258a44
--- /dev/null
+++ b/lib/crypto/aescfb.c
@@ -0,0 +1,257 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Minimal library implementation of AES in CFB mode
+ *
+ * Copyright 2023 Google LLC
+ */
+
+#include <linux/module.h>
+
+#include <crypto/algapi.h>
+#include <crypto/aes.h>
+
+#include <asm/irqflags.h>
+
+static void aescfb_encrypt_block(const struct crypto_aes_ctx *ctx, void *dst,
+ const void *src)
+{
+ unsigned long flags;
+
+ /*
+ * In AES-CFB, the AES encryption operates on known 'plaintext' (the IV
+ * and ciphertext), making it susceptible to timing attacks on the
+ * encryption key. The AES library already mitigates this risk to some
+ * extent by pulling the entire S-box into the caches before doing any
+ * substitutions, but this strategy is more effective when running with
+ * interrupts disabled.
+ */
+ local_irq_save(flags);
+ aes_encrypt(ctx, dst, src);
+ local_irq_restore(flags);
+}
+
+/**
+ * aescfb_encrypt - Perform AES-CFB encryption on a block of data
+ *
+ * @ctx: The AES-CFB key schedule
+ * @dst: Pointer to the ciphertext output buffer
+ * @src: Pointer the plaintext (may equal @dst for encryption in place)
+ * @len: The size in bytes of the plaintext and ciphertext.
+ * @iv: The initialization vector (IV) to use for this block of data
+ */
+void aescfb_encrypt(const struct crypto_aes_ctx *ctx, u8 *dst, const u8 *src,
+ int len, const u8 iv[AES_BLOCK_SIZE])
+{
+ u8 ks[AES_BLOCK_SIZE];
+ const u8 *v = iv;
+
+ while (len > 0) {
+ aescfb_encrypt_block(ctx, ks, v);
+ crypto_xor_cpy(dst, src, ks, min(len, AES_BLOCK_SIZE));
+ v = dst;
+
+ dst += AES_BLOCK_SIZE;
+ src += AES_BLOCK_SIZE;
+ len -= AES_BLOCK_SIZE;
+ }
+
+ memzero_explicit(ks, sizeof(ks));
+}
+EXPORT_SYMBOL(aescfb_encrypt);
+
+/**
+ * aescfb_decrypt - Perform AES-CFB decryption on a block of data
+ *
+ * @ctx: The AES-CFB key schedule
+ * @dst: Pointer to the plaintext output buffer
+ * @src: Pointer the ciphertext (may equal @dst for decryption in place)
+ * @len: The size in bytes of the plaintext and ciphertext.
+ * @iv: The initialization vector (IV) to use for this block of data
+ */
+void aescfb_decrypt(const struct crypto_aes_ctx *ctx, u8 *dst, const u8 *src,
+ int len, const u8 iv[AES_BLOCK_SIZE])
+{
+ u8 ks[2][AES_BLOCK_SIZE];
+
+ aescfb_encrypt_block(ctx, ks[0], iv);
+
+ for (int i = 0; len > 0; i ^= 1) {
+ if (len > AES_BLOCK_SIZE)
+ /*
+ * Generate the keystream for the next block before
+ * performing the XOR, as that may update in place and
+ * overwrite the ciphertext.
+ */
+ aescfb_encrypt_block(ctx, ks[!i], src);
+
+ crypto_xor_cpy(dst, src, ks[i], min(len, AES_BLOCK_SIZE));
+
+ dst += AES_BLOCK_SIZE;
+ src += AES_BLOCK_SIZE;
+ len -= AES_BLOCK_SIZE;
+ }
+
+ memzero_explicit(ks, sizeof(ks));
+}
+EXPORT_SYMBOL(aescfb_decrypt);
+
+MODULE_DESCRIPTION("Generic AES-CFB library");
+MODULE_AUTHOR("Ard Biesheuvel <ardb@kernel.org>");
+MODULE_LICENSE("GPL");
+
+#ifndef CONFIG_CRYPTO_MANAGER_DISABLE_TESTS
+
+/*
+ * Test code below. Vectors taken from crypto/testmgr.h
+ */
+
+static struct {
+ u8 ptext[64];
+ u8 ctext[64];
+
+ u8 key[AES_MAX_KEY_SIZE];
+ u8 iv[AES_BLOCK_SIZE];
+
+ int klen;
+ int len;
+} const aescfb_tv[] __initconst = {
+ { /* From NIST SP800-38A */
+ .key = "\x2b\x7e\x15\x16\x28\xae\xd2\xa6"
+ "\xab\xf7\x15\x88\x09\xcf\x4f\x3c",
+ .klen = 16,
+ .iv = "\x00\x01\x02\x03\x04\x05\x06\x07"
+ "\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f",
+ .ptext = "\x6b\xc1\xbe\xe2\x2e\x40\x9f\x96"
+ "\xe9\x3d\x7e\x11\x73\x93\x17\x2a"
+ "\xae\x2d\x8a\x57\x1e\x03\xac\x9c"
+ "\x9e\xb7\x6f\xac\x45\xaf\x8e\x51"
+ "\x30\xc8\x1c\x46\xa3\x5c\xe4\x11"
+ "\xe5\xfb\xc1\x19\x1a\x0a\x52\xef"
+ "\xf6\x9f\x24\x45\xdf\x4f\x9b\x17"
+ "\xad\x2b\x41\x7b\xe6\x6c\x37\x10",
+ .ctext = "\x3b\x3f\xd9\x2e\xb7\x2d\xad\x20"
+ "\x33\x34\x49\xf8\xe8\x3c\xfb\x4a"
+ "\xc8\xa6\x45\x37\xa0\xb3\xa9\x3f"
+ "\xcd\xe3\xcd\xad\x9f\x1c\xe5\x8b"
+ "\x26\x75\x1f\x67\xa3\xcb\xb1\x40"
+ "\xb1\x80\x8c\xf1\x87\xa4\xf4\xdf"
+ "\xc0\x4b\x05\x35\x7c\x5d\x1c\x0e"
+ "\xea\xc4\xc6\x6f\x9f\xf7\xf2\xe6",
+ .len = 64,
+ }, {
+ .key = "\x8e\x73\xb0\xf7\xda\x0e\x64\x52"
+ "\xc8\x10\xf3\x2b\x80\x90\x79\xe5"
+ "\x62\xf8\xea\xd2\x52\x2c\x6b\x7b",
+ .klen = 24,
+ .iv = "\x00\x01\x02\x03\x04\x05\x06\x07"
+ "\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f",
+ .ptext = "\x6b\xc1\xbe\xe2\x2e\x40\x9f\x96"
+ "\xe9\x3d\x7e\x11\x73\x93\x17\x2a"
+ "\xae\x2d\x8a\x57\x1e\x03\xac\x9c"
+ "\x9e\xb7\x6f\xac\x45\xaf\x8e\x51"
+ "\x30\xc8\x1c\x46\xa3\x5c\xe4\x11"
+ "\xe5\xfb\xc1\x19\x1a\x0a\x52\xef"
+ "\xf6\x9f\x24\x45\xdf\x4f\x9b\x17"
+ "\xad\x2b\x41\x7b\xe6\x6c\x37\x10",
+ .ctext = "\xcd\xc8\x0d\x6f\xdd\xf1\x8c\xab"
+ "\x34\xc2\x59\x09\xc9\x9a\x41\x74"
+ "\x67\xce\x7f\x7f\x81\x17\x36\x21"
+ "\x96\x1a\x2b\x70\x17\x1d\x3d\x7a"
+ "\x2e\x1e\x8a\x1d\xd5\x9b\x88\xb1"
+ "\xc8\xe6\x0f\xed\x1e\xfa\xc4\xc9"
+ "\xc0\x5f\x9f\x9c\xa9\x83\x4f\xa0"
+ "\x42\xae\x8f\xba\x58\x4b\x09\xff",
+ .len = 64,
+ }, {
+ .key = "\x60\x3d\xeb\x10\x15\xca\x71\xbe"
+ "\x2b\x73\xae\xf0\x85\x7d\x77\x81"
+ "\x1f\x35\x2c\x07\x3b\x61\x08\xd7"
+ "\x2d\x98\x10\xa3\x09\x14\xdf\xf4",
+ .klen = 32,
+ .iv = "\x00\x01\x02\x03\x04\x05\x06\x07"
+ "\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f",
+ .ptext = "\x6b\xc1\xbe\xe2\x2e\x40\x9f\x96"
+ "\xe9\x3d\x7e\x11\x73\x93\x17\x2a"
+ "\xae\x2d\x8a\x57\x1e\x03\xac\x9c"
+ "\x9e\xb7\x6f\xac\x45\xaf\x8e\x51"
+ "\x30\xc8\x1c\x46\xa3\x5c\xe4\x11"
+ "\xe5\xfb\xc1\x19\x1a\x0a\x52\xef"
+ "\xf6\x9f\x24\x45\xdf\x4f\x9b\x17"
+ "\xad\x2b\x41\x7b\xe6\x6c\x37\x10",
+ .ctext = "\xdc\x7e\x84\xbf\xda\x79\x16\x4b"
+ "\x7e\xcd\x84\x86\x98\x5d\x38\x60"
+ "\x39\xff\xed\x14\x3b\x28\xb1\xc8"
+ "\x32\x11\x3c\x63\x31\xe5\x40\x7b"
+ "\xdf\x10\x13\x24\x15\xe5\x4b\x92"
+ "\xa1\x3e\xd0\xa8\x26\x7a\xe2\xf9"
+ "\x75\xa3\x85\x74\x1a\xb9\xce\xf8"
+ "\x20\x31\x62\x3d\x55\xb1\xe4\x71",
+ .len = 64,
+ }, { /* > 16 bytes, not a multiple of 16 bytes */
+ .key = "\x2b\x7e\x15\x16\x28\xae\xd2\xa6"
+ "\xab\xf7\x15\x88\x09\xcf\x4f\x3c",
+ .klen = 16,
+ .iv = "\x00\x01\x02\x03\x04\x05\x06\x07"
+ "\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f",
+ .ptext = "\x6b\xc1\xbe\xe2\x2e\x40\x9f\x96"
+ "\xe9\x3d\x7e\x11\x73\x93\x17\x2a"
+ "\xae",
+ .ctext = "\x3b\x3f\xd9\x2e\xb7\x2d\xad\x20"
+ "\x33\x34\x49\xf8\xe8\x3c\xfb\x4a"
+ "\xc8",
+ .len = 17,
+ }, { /* < 16 bytes */
+ .key = "\x2b\x7e\x15\x16\x28\xae\xd2\xa6"
+ "\xab\xf7\x15\x88\x09\xcf\x4f\x3c",
+ .klen = 16,
+ .iv = "\x00\x01\x02\x03\x04\x05\x06\x07"
+ "\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f",
+ .ptext = "\x6b\xc1\xbe\xe2\x2e\x40\x9f",
+ .ctext = "\x3b\x3f\xd9\x2e\xb7\x2d\xad",
+ .len = 7,
+ },
+};
+
+static int __init libaescfb_init(void)
+{
+ for (int i = 0; i < ARRAY_SIZE(aescfb_tv); i++) {
+ struct crypto_aes_ctx ctx;
+ u8 buf[64];
+
+ if (aes_expandkey(&ctx, aescfb_tv[i].key, aescfb_tv[i].klen)) {
+ pr_err("aes_expandkey() failed on vector %d\n", i);
+ return -ENODEV;
+ }
+
+ aescfb_encrypt(&ctx, buf, aescfb_tv[i].ptext, aescfb_tv[i].len,
+ aescfb_tv[i].iv);
+ if (memcmp(buf, aescfb_tv[i].ctext, aescfb_tv[i].len)) {
+ pr_err("aescfb_encrypt() #1 failed on vector %d\n", i);
+ return -ENODEV;
+ }
+
+ /* decrypt in place */
+ aescfb_decrypt(&ctx, buf, buf, aescfb_tv[i].len, aescfb_tv[i].iv);
+ if (memcmp(buf, aescfb_tv[i].ptext, aescfb_tv[i].len)) {
+ pr_err("aescfb_decrypt() failed on vector %d\n", i);
+ return -ENODEV;
+ }
+
+ /* encrypt in place */
+ aescfb_encrypt(&ctx, buf, buf, aescfb_tv[i].len, aescfb_tv[i].iv);
+ if (memcmp(buf, aescfb_tv[i].ctext, aescfb_tv[i].len)) {
+ pr_err("aescfb_encrypt() #2 failed on vector %d\n", i);
+
+ return -ENODEV;
+ }
+
+ }
+ return 0;
+}
+module_init(libaescfb_init);
+
+static void __exit libaescfb_exit(void)
+{
+}
+module_exit(libaescfb_exit);
+#endif
--
2.35.3