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Add mbedtls_ssl_cf_memcpy_offset() with tests

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The tests are supposed to be failing now (in all.sh component
test_memsan_constant_flow), but they don't as apparently MemSan doesn't
complain when the src argument of memcpy() is uninitialized, see
https://github.com/google/sanitizers/issues/1296

The next commit will add an option to test constant flow with valgrind, which
will hopefully correctly flag the current non-constant-flow implementation.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
This commit is contained in:
Manuel Pégourié-Gonnard 2020-08-18 12:02:54 +02:00
parent 3c31afaca6
commit 7fe2c5f086
4 changed files with 92 additions and 8 deletions
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24
library/ssl_invasive.h
View file

@ -73,6 +73,30 @@ int mbedtls_ssl_cf_hmac(
const unsigned char *data, size_t data_len_secret,
size_t min_data_len, size_t max_data_len,
unsigned char *output );
/** \brief Copy data from a secret position with constant flow.
*
* This function copies \p len bytes from \p src_base + \p offset_secret to \p
* dst, with a code flow and memory access pattern that does not depend on \p
* offset_secret, but only on \p offset_min, \p offset_max and \p len.
*
* \param dst The destination buffer. This must point to a writable
* buffer of at least \p len bytes.
* \param src_base The base of the source buffer. This must point to a
* readable buffer of at least \p offset_max + \p len
* bytes.
* \param offset_secret The offset in the source buffer from which to copy.
* This must be no less than \p offset_min and no greater
* than \p offset_max.
* \param offset_min The minimal value of \p offset_secret.
* \param offset_max The maximal value of \p offset_secret.
* \param len The number of bytes to copy.
*/
void mbedtls_ssl_cf_memcpy_offset( unsigned char *dst,
const unsigned char *src_base,
size_t offset_secret,
size_t offset_min, size_t offset_max,
size_t len );
#endif /* MBEDTLS_TEST_HOOKS && MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC */
#endif /* MBEDTLS_SSL_INVASIVE_H */

31
library/ssl_msg.c
View file

@ -1193,6 +1193,24 @@ cleanup:
mbedtls_md_free( &aux );
return( ret );
}
/*
* Constant-flow memcpy from variable position in buffer.
* - functionally equivalent to memcpy(dst, src + offset_secret, len)
* - but with execution flow independant from the value of offset_secret.
*/
MBEDTLS_STATIC_TESTABLE void mbedtls_ssl_cf_memcpy_offset(
unsigned char *dst,
const unsigned char *src_base,
size_t offset_secret,
size_t offset_min, size_t offset_max,
size_t len )
{
/* WIP - THIS IS NOT ACTUALLY CONSTANT-FLOW!
* This is just to be able to write tests and check they work. */
ssl_read_memory( src_base + offset_min, offset_max - offset_min + len );
memcpy( dst, src_base + offset_secret, len );
}
#endif /* MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC */
int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context const *ssl,
@ -1674,7 +1692,7 @@ int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context const *ssl,
{
/*
* The next two sizes are the minimum and maximum values of
* in_msglen over all padlen values.
* data_len over all padlen values.
*
* They're independent of padlen, since we previously did
* data_len -= padlen.
@ -1695,13 +1713,10 @@ int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context const *ssl,
return( ret );
}
/* Make sure we access all the memory that could contain the MAC,
* before we check it in the next code block. This makes the
* synchronisation requirements for just-in-time Prime+Probe
* attacks much tighter and hopefully impractical. */
ssl_read_memory( data + min_len,
max_len - min_len + transform->maclen );
memcpy( mac_peer, data + rec->data_len, transform->maclen );
mbedtls_ssl_cf_memcpy_offset( mac_peer, data,
rec->data_len,
min_len, max_len,
transform->maclen );
}
else
#endif /* MBEDTLS_SSL_PROTO_TLS1 || MBEDTLS_SSL_PROTO_TLS1_1 || \

12
tests/suites/test_suite_ssl.data
View file

@ -10545,3 +10545,15 @@ ssl_cf_hmac:MBEDTLS_MD_SHA256
Constant-flow HMAC: SHA384
depends_on:MBEDTLS_SHA512_C:!MBEDTLS_SHA512_NO_SHA384
ssl_cf_hmac:MBEDTLS_MD_SHA384
# these are the numbers we'd get with an empty plaintext and truncated HMAC
Constant-flow memcpy from offset: small
ssl_cf_memcpy_offset:0:5:10
# we could get this with 255-bytes plaintext and untruncated SHA-256
Constant-flow memcpy from offset: medium
ssl_cf_memcpy_offset:0:255:32
# we could get this with 255-bytes plaintext and untruncated SHA-384
Constant-flow memcpy from offset: large
ssl_cf_memcpy_offset:100:339:48

33
tests/suites/test_suite_ssl.function
View file

@ -4361,3 +4361,36 @@ exit:
mbedtls_free( out );
}
/* END_CASE */
/* BEGIN_CASE depends_on:MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC:MBEDTLS_TEST_HOOKS */
void ssl_cf_memcpy_offset( int offset_min, int offset_max, int len )
{
unsigned char *dst = NULL;
unsigned char *src = NULL;
size_t src_len = offset_max + len;
size_t secret;
ASSERT_ALLOC( dst, len );
ASSERT_ALLOC( src, src_len );
/* Fill src in a way that we can detect if we copied the right bytes */
mbedtls_test_rnd_std_rand( NULL, src, src_len );
for( secret = offset_min; secret <= (size_t) offset_max; secret++ )
{
test_set_step( (int) secret );
TEST_CF_SECRET( &secret, sizeof( secret ) );
mbedtls_ssl_cf_memcpy_offset( dst, src, secret,
offset_min, offset_max, len );
TEST_CF_PUBLIC( &secret, sizeof( secret ) );
TEST_CF_PUBLIC( dst, len );
ASSERT_COMPARE( dst, len, src + secret, len );
}
exit:
mbedtls_free( dst );
mbedtls_free( src );
}
/* END_CASE */