Merge pull request #3638 from ARMmbed/better-cf-padding-checks
Better constant-flow idioms for TLS-CBC padding checks
diff --git a/library/ssl_msg.c b/library/ssl_msg.c
index 2ea3580..981d94e 100644
--- a/library/ssl_msg.c
+++ b/library/ssl_msg.c
@@ -1045,21 +1045,86 @@
#if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC)
/*
- * Constant-flow conditional memcpy:
- * - if c1 == c2, equivalent to memcpy(dst, src, len),
- * - otherwise, a no-op,
- * but with execution flow independent of the values of c1 and c2.
+ * Turn a bit into a mask:
+ * - if bit == 1, return the all-bits 1 mask, aka (size_t) -1
+ * - if bit == 0, return the all-bits 0 mask, aka 0
*
- * Use only bit operations to avoid branches that could be used by some
- * compilers on some platforms to translate comparison operators.
+ * This function can be used to write constant-time code by replacing branches
+ * with bit operations using masks.
+ *
+ * This function is implemented without using comparison operators, as those
+ * might be translated to branches by some compilers on some platforms.
*/
-static void mbedtls_ssl_cf_memcpy_if_eq( unsigned char *dst,
- const unsigned char *src,
- size_t len,
- size_t c1, size_t c2 )
+static size_t mbedtls_ssl_cf_mask_from_bit( size_t bit )
{
- /* diff = 0 if c1 == c2, non-zero otherwise */
- const size_t diff = c1 ^ c2;
+ /* MSVC has a warning about unary minus on unsigned integer types,
+ * but this is well-defined and precisely what we want to do here. */
+#if defined(_MSC_VER)
+#pragma warning( push )
+#pragma warning( disable : 4146 )
+#endif
+ return -bit;
+#if defined(_MSC_VER)
+#pragma warning( pop )
+#endif
+}
+
+/*
+ * Constant-flow mask generation for "less than" comparison:
+ * - if x < y, return all bits 1, that is (size_t) -1
+ * - otherwise, return all bits 0, that is 0
+ *
+ * This function can be used to write constant-time code by replacing branches
+ * with bit operations using masks.
+ *
+ * This function is implemented without using comparison operators, as those
+ * might be translated to branches by some compilers on some platforms.
+ */
+static size_t mbedtls_ssl_cf_mask_lt( size_t x, size_t y )
+{
+ /* This has the most significant bit set if and only if x < y */
+ const size_t sub = x - y;
+
+ /* sub1 = (x < y) ? 1 : 0 */
+ const size_t sub1 = sub >> ( sizeof( sub ) * 8 - 1 );
+
+ /* mask = (x < y) ? 0xff... : 0x00... */
+ const size_t mask = mbedtls_ssl_cf_mask_from_bit( sub1 );
+
+ return( mask );
+}
+
+/*
+ * Constant-flow mask generation for "greater or equal" comparison:
+ * - if x >= y, return all bits 1, that is (size_t) -1
+ * - otherwise, return all bits 0, that is 0
+ *
+ * This function can be used to write constant-time code by replacing branches
+ * with bit operations using masks.
+ *
+ * This function is implemented without using comparison operators, as those
+ * might be translated to branches by some compilers on some platforms.
+ */
+static size_t mbedtls_ssl_cf_mask_ge( size_t x, size_t y )
+{
+ return( ~mbedtls_ssl_cf_mask_lt( x, y ) );
+}
+
+/*
+ * Constant-flow boolean "equal" comparison:
+ * return x == y
+ *
+ * This function can be used to write constant-time code by replacing branches
+ * with bit operations - it can be used in conjunction with
+ * mbedtls_ssl_cf_mask_from_bit().
+ *
+ * This function is implemented without using comparison operators, as those
+ * might be translated to branches by some compilers on some platforms.
+ */
+static size_t mbedtls_ssl_cf_bool_eq( size_t x, size_t y )
+{
+ /* diff = 0 if x == y, non-zero otherwise */
+ const size_t diff = x ^ y;
/* MSVC has a warning about unary minus on unsigned integer types,
* but this is well-defined and precisely what we want to do here. */
@@ -1068,22 +1133,40 @@
#pragma warning( disable : 4146 )
#endif
- /* diff_msb's most significant bit is equal to c1 != c2 */
+ /* diff_msb's most significant bit is equal to x != y */
const size_t diff_msb = ( diff | -diff );
- /* diff1 = c1 != c2 */
- const size_t diff1 = diff_msb >> ( sizeof( diff_msb ) * 8 - 1 );
-
- /* mask = c1 != c2 ? 0xff : 0x00 */
- const unsigned char mask = (unsigned char) -diff1;
-
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
- /* dst[i] = c1 != c2 ? dst[i] : src[i] */
+ /* diff1 = (x != y) ? 1 : 0 */
+ const size_t diff1 = diff_msb >> ( sizeof( diff_msb ) * 8 - 1 );
+
+ return( 1 ^ diff1 );
+}
+
+/*
+ * Constant-flow conditional memcpy:
+ * - if c1 == c2, equivalent to memcpy(dst, src, len),
+ * - otherwise, a no-op,
+ * but with execution flow independent of the values of c1 and c2.
+ *
+ * This function is implemented without using comparison operators, as those
+ * might be translated to branches by some compilers on some platforms.
+ */
+static void mbedtls_ssl_cf_memcpy_if_eq( unsigned char *dst,
+ const unsigned char *src,
+ size_t len,
+ size_t c1, size_t c2 )
+{
+ /* mask = c1 == c2 ? 0xff : 0x00 */
+ const size_t equal = mbedtls_ssl_cf_bool_eq( c1, c2 );
+ const unsigned char mask = (unsigned char) mbedtls_ssl_cf_mask_from_bit( equal );
+
+ /* dst[i] = c1 == c2 ? src[i] : dst[i] */
for( size_t i = 0; i < len; i++ )
- dst[i] = ( dst[i] & mask ) | ( src[i] & ~mask );
+ dst[i] = ( src[i] & mask ) | ( dst[i] & ~mask );
}
/*
@@ -1528,8 +1611,11 @@
if( auth_done == 1 )
{
- correct *= ( rec->data_len >= padlen + 1 );
- padlen *= ( rec->data_len >= padlen + 1 );
+ const size_t mask = mbedtls_ssl_cf_mask_ge(
+ rec->data_len,
+ padlen + 1 );
+ correct &= mask;
+ padlen &= mask;
}
else
{
@@ -1543,8 +1629,11 @@
}
#endif
- correct *= ( rec->data_len >= transform->maclen + padlen + 1 );
- padlen *= ( rec->data_len >= transform->maclen + padlen + 1 );
+ const size_t mask = mbedtls_ssl_cf_mask_ge(
+ rec->data_len,
+ transform->maclen + padlen + 1 );
+ correct &= mask;
+ padlen &= mask;
}
padlen++;
@@ -1555,6 +1644,10 @@
#if defined(MBEDTLS_SSL_PROTO_SSL3)
if( transform->minor_ver == MBEDTLS_SSL_MINOR_VERSION_0 )
{
+ /* This is the SSL 3.0 path, we don't have to worry about Lucky
+ * 13, because there's a strictly worse padding attack built in
+ * the protocol (known as part of POODLE), so we don't care if the
+ * code is not constant-time, in particular branches are OK. */
if( padlen > transform->ivlen )
{
#if defined(MBEDTLS_SSL_DEBUG_ALL)
@@ -1578,7 +1671,6 @@
* `min(256,plaintext_len)` reads (but take into account
* only the last `padlen` bytes for the padding check). */
size_t pad_count = 0;
- size_t real_count = 0;
volatile unsigned char* const check = data;
/* Index of first padding byte; it has been ensured above
@@ -1590,16 +1682,21 @@
for( idx = start_idx; idx < rec->data_len; idx++ )
{
- real_count |= ( idx >= padding_idx );
- pad_count += real_count * ( check[idx] == padlen - 1 );
+ /* pad_count += (idx >= padding_idx) &&
+ * (check[idx] == padlen - 1);
+ */
+ const size_t mask = mbedtls_ssl_cf_mask_ge( idx, padding_idx );
+ const size_t equal = mbedtls_ssl_cf_bool_eq( check[idx],
+ padlen - 1 );
+ pad_count += mask & equal;
}
- correct &= ( pad_count == padlen );
+ correct &= mbedtls_ssl_cf_bool_eq( pad_count, padlen );
#if defined(MBEDTLS_SSL_DEBUG_ALL)
if( padlen > 0 && correct == 0 )
MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad padding byte detected" ) );
#endif
- padlen &= correct * 0x1FF;
+ padlen &= mbedtls_ssl_cf_mask_from_bit( correct );
}
else
#endif /* MBEDTLS_SSL_PROTO_TLS1 || MBEDTLS_SSL_PROTO_TLS1_1 || \