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openssl/ssl/record/tls_pad.c

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/*
* Copyright 1995-2022 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 <openssl/rand.h>
#include <openssl/evp.h>
#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;
char aux1, aux2, aux3, mask;
#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,
* load from both lines and select appropriately
*/
aux1 = rotated_mac[rotate_offset & ~32];
aux2 = rotated_mac[rotate_offset | 32];
mask = constant_time_eq_8(rotate_offset & ~32, rotate_offset);
aux3 = constant_time_select_8(mask, aux1, aux2);
rotate_offset++;
/* If the padding wasn't good we emit a random MAC */
out[j++] = constant_time_select_8((unsigned char)(good & 0xff),
aux3,
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;
}
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