/* * Copyright 1995-2021 The OpenSSL Project Authors. All Rights Reserved. * * Licensed under the Apache License 2.0 (the "License"). You may not use * this file except in compliance with the License. You can obtain a copy * in the file LICENSE in the source distribution or at * https://www.openssl.org/source/license.html */ #include #include #include "internal/constant_time.h" #include "internal/cryptlib.h" /* * This file has no dependencies on the rest of libssl because it is shared * with the providers. It contains functions for low level CBC TLS padding * removal. Responsibility for this lies with the cipher implementations in the * providers. However there are legacy code paths in libssl which also need to * do this. In time those legacy code paths can be removed and this file can be * moved out of libssl. */ static int ssl3_cbc_copy_mac(size_t *reclen, size_t origreclen, unsigned char *recdata, unsigned char **mac, int *alloced, size_t block_size, size_t mac_size, size_t good, OSSL_LIB_CTX *libctx); int ssl3_cbc_remove_padding_and_mac(size_t *reclen, size_t origreclen, unsigned char *recdata, unsigned char **mac, int *alloced, size_t block_size, size_t mac_size, OSSL_LIB_CTX *libctx); int tls1_cbc_remove_padding_and_mac(size_t *reclen, size_t origreclen, unsigned char *recdata, unsigned char **mac, int *alloced, size_t block_size, size_t mac_size, int aead, OSSL_LIB_CTX *libctx); /*- * ssl3_cbc_remove_padding removes padding from the decrypted, SSLv3, CBC * record in |recdata| by updating |reclen| in constant time. It also extracts * the MAC from the underlying record and places a pointer to it in |mac|. The * MAC data can either be newly allocated memory, or a pointer inside the * |recdata| buffer. If allocated then |*alloced| is set to 1, otherwise it is * set to 0. * * origreclen: the original record length before any changes were made * block_size: the block size of the cipher used to encrypt the record. * mac_size: the size of the MAC to be extracted * aead: 1 if an AEAD cipher is in use, or 0 otherwise * returns: * 0: if the record is publicly invalid. * 1: if the record is publicly valid. If the padding removal fails then the * MAC returned is random. */ int ssl3_cbc_remove_padding_and_mac(size_t *reclen, size_t origreclen, unsigned char *recdata, unsigned char **mac, int *alloced, size_t block_size, size_t mac_size, OSSL_LIB_CTX *libctx) { size_t padding_length; size_t good; const size_t overhead = 1 /* padding length byte */ + mac_size; /* * These lengths are all public so we can test them in non-constant time. */ if (overhead > *reclen) return 0; padding_length = recdata[*reclen - 1]; good = constant_time_ge_s(*reclen, padding_length + overhead); /* SSLv3 requires that the padding is minimal. */ good &= constant_time_ge_s(block_size, padding_length + 1); *reclen -= good & (padding_length + 1); return ssl3_cbc_copy_mac(reclen, origreclen, recdata, mac, alloced, block_size, mac_size, good, libctx); } /*- * tls1_cbc_remove_padding_and_mac removes padding from the decrypted, TLS, CBC * record in |recdata| by updating |reclen| in constant time. It also extracts * the MAC from the underlying record and places a pointer to it in |mac|. The * MAC data can either be newly allocated memory, or a pointer inside the * |recdata| buffer. If allocated then |*alloced| is set to 1, otherwise it is * set to 0. * * origreclen: the original record length before any changes were made * block_size: the block size of the cipher used to encrypt the record. * mac_size: the size of the MAC to be extracted * aead: 1 if an AEAD cipher is in use, or 0 otherwise * returns: * 0: if the record is publicly invalid. * 1: if the record is publicly valid. If the padding removal fails then the * MAC returned is random. */ int tls1_cbc_remove_padding_and_mac(size_t *reclen, size_t origreclen, unsigned char *recdata, unsigned char **mac, int *alloced, size_t block_size, size_t mac_size, int aead, OSSL_LIB_CTX *libctx) { size_t good = -1; size_t padding_length, to_check, i; size_t overhead = ((block_size == 1) ? 0 : 1) /* padding length byte */ + mac_size; /* * These lengths are all public so we can test them in non-constant * time. */ if (overhead > *reclen) return 0; if (block_size != 1) { padding_length = recdata[*reclen - 1]; if (aead) { /* padding is already verified and we don't need to check the MAC */ *reclen -= padding_length + 1 + mac_size; return 1; } good = constant_time_ge_s(*reclen, overhead + padding_length); /* * The padding consists of a length byte at the end of the record and * then that many bytes of padding, all with the same value as the * length byte. Thus, with the length byte included, there are i+1 bytes * of padding. We can't check just |padding_length+1| bytes because that * leaks decrypted information. Therefore we always have to check the * maximum amount of padding possible. (Again, the length of the record * is public information so we can use it.) */ to_check = 256; /* maximum amount of padding, inc length byte. */ if (to_check > *reclen) to_check = *reclen; for (i = 0; i < to_check; i++) { unsigned char mask = constant_time_ge_8_s(padding_length, i); unsigned char b = recdata[*reclen - 1 - i]; /* * The final |padding_length+1| bytes should all have the value * |padding_length|. Therefore the XOR should be zero. */ good &= ~(mask & (padding_length ^ b)); } /* * If any of the final |padding_length+1| bytes had the wrong value, one * or more of the lower eight bits of |good| will be cleared. */ good = constant_time_eq_s(0xff, good & 0xff); *reclen -= good & (padding_length + 1); } return ssl3_cbc_copy_mac(reclen, origreclen, recdata, mac, alloced, block_size, mac_size, good, libctx); } /*- * ssl3_cbc_copy_mac copies |md_size| bytes from the end of the record in * |recdata| to |*mac| in constant time (independent of the concrete value of * the record length |reclen|, which may vary within a 256-byte window). * * On entry: * origreclen >= mac_size * mac_size <= EVP_MAX_MD_SIZE * * If CBC_MAC_ROTATE_IN_PLACE is defined then the rotation is performed with * variable accesses in a 64-byte-aligned buffer. Assuming that this fits into * a single or pair of cache-lines, then the variable memory accesses don't * actually affect the timing. CPUs with smaller cache-lines [if any] are * not multi-core and are not considered vulnerable to cache-timing attacks. */ #define CBC_MAC_ROTATE_IN_PLACE static int ssl3_cbc_copy_mac(size_t *reclen, size_t origreclen, unsigned char *recdata, unsigned char **mac, int *alloced, size_t block_size, size_t mac_size, size_t good, OSSL_LIB_CTX *libctx) { #if defined(CBC_MAC_ROTATE_IN_PLACE) unsigned char rotated_mac_buf[64 + EVP_MAX_MD_SIZE]; unsigned char *rotated_mac; #else unsigned char rotated_mac[EVP_MAX_MD_SIZE]; #endif unsigned char randmac[EVP_MAX_MD_SIZE]; unsigned char *out; /* * mac_end is the index of |recdata| just after the end of the MAC. */ size_t mac_end = *reclen; size_t mac_start = mac_end - mac_size; size_t in_mac; /* * scan_start contains the number of bytes that we can ignore because the * MAC's position can only vary by 255 bytes. */ size_t scan_start = 0; size_t i, j; size_t rotate_offset; if (!ossl_assert(origreclen >= mac_size && mac_size <= EVP_MAX_MD_SIZE)) return 0; /* If no MAC then nothing to be done */ if (mac_size == 0) { /* No MAC so we can do this in non-constant time */ if (good == 0) return 0; return 1; } *reclen -= mac_size; if (block_size == 1) { /* There's no padding so the position of the MAC is fixed */ if (mac != NULL) *mac = &recdata[*reclen]; if (alloced != NULL) *alloced = 0; return 1; } /* Create the random MAC we will emit if padding is bad */ if (RAND_bytes_ex(libctx, randmac, mac_size, 0) <= 0) return 0; if (!ossl_assert(mac != NULL && alloced != NULL)) return 0; *mac = out = OPENSSL_malloc(mac_size); if (*mac == NULL) return 0; *alloced = 1; #if defined(CBC_MAC_ROTATE_IN_PLACE) rotated_mac = rotated_mac_buf + ((0 - (size_t)rotated_mac_buf) & 63); #endif /* This information is public so it's safe to branch based on it. */ if (origreclen > mac_size + 255 + 1) scan_start = origreclen - (mac_size + 255 + 1); in_mac = 0; rotate_offset = 0; memset(rotated_mac, 0, mac_size); for (i = scan_start, j = 0; i < origreclen; i++) { size_t mac_started = constant_time_eq_s(i, mac_start); size_t mac_ended = constant_time_lt_s(i, mac_end); unsigned char b = recdata[i]; in_mac |= mac_started; in_mac &= mac_ended; rotate_offset |= j & mac_started; rotated_mac[j++] |= b & in_mac; j &= constant_time_lt_s(j, mac_size); } /* Now rotate the MAC */ #if defined(CBC_MAC_ROTATE_IN_PLACE) j = 0; for (i = 0; i < mac_size; i++) { /* in case cache-line is 32 bytes, touch second line */ ((volatile unsigned char *)rotated_mac)[rotate_offset ^ 32]; /* If the padding wasn't good we emit a random MAC */ out[j++] = constant_time_select_8((unsigned char)(good & 0xff), rotated_mac[rotate_offset++], randmac[i]); rotate_offset &= constant_time_lt_s(rotate_offset, mac_size); } #else memset(out, 0, mac_size); rotate_offset = mac_size - rotate_offset; rotate_offset &= constant_time_lt_s(rotate_offset, mac_size); for (i = 0; i < mac_size; i++) { for (j = 0; j < mac_size; j++) out[j] |= rotated_mac[i] & constant_time_eq_8_s(j, rotate_offset); rotate_offset++; rotate_offset &= constant_time_lt_s(rotate_offset, mac_size); /* If the padding wasn't good we emit a random MAC */ out[i] = constant_time_select_8((unsigned char)(good & 0xff), out[i], randmac[i]); } #endif return 1; }