Merge remote-tracking branch 'restricted/pr/495' into mbedtls-2.1

ChangeLog

@@ -2,6 +2,39 @@ mbed TLS ChangeLog (Sorted per branch, date)
 
 = mbed TLS x.x.x branch released xxxx-xx-xx
 
+Security
+   * Fix a vulnerability in TLS ciphersuites based on CBC and using SHA-384,
+     in (D)TLS 1.0 to 1.2, that allowed an active network attacker to
+     partially recover the plaintext of messages under some conditions by
+     exploiting timing measurements. With DTLS, the attacker could perform
+     this recovery by sending many messages in the same connection. With TLS
+     or if mbedtls_ssl_conf_dtls_badmac_limit() was used, the attack only
+     worked if the same secret (for example a HTTP Cookie) has been repeatedly
+     sent over connections manipulated by the attacker. Connections using GCM
+     or CCM instead of CBC, using hash sizes other than SHA-384, or using
+     Encrypt-then-Mac (RFC 7366) were not affected. The vulnerability was
+     caused by a miscalculation (for SHA-384) in a countermeasure to the
+     original Lucky 13 attack. Found by Kenny Paterson, Eyal Ronen and Adi
+     Shamir.
+   * Fix a vulnerability in TLS ciphersuites based on CBC, in (D)TLS 1.0 to
+     1.2, that allowed a local attacker, able to execute code on the local
+     machine as well as manipulate network packets, to partially recover the
+     plaintext of messages under some conditions by using a cache attack
+     targetting an internal MD/SHA buffer. With TLS or if
+     mbedtls_ssl_conf_dtls_badmac_limit() was used, the attack only worked if
+     the same secret (for example a HTTP Cookie) has been repeatedly sent over
+     connections manipulated by the attacker. Connections using GCM or CCM
+     instead of CBC or using Encrypt-then-Mac (RFC 7366) were not affected.
+     Found by Kenny Paterson, Eyal Ronen and Adi Shamir.
+   * Add a counter-measure against a vulnerability in TLS ciphersuites based
+     on CBC, in (D)TLS 1.0 to 1.2, that allowed a local attacker, able to
+     execute code on the local machine as well as manipulate network packets,
+     to partially recover the plaintext of messages under some conditions (see
+     previous entry) by using a cache attack targeting the SSL input record
+     buffer. Connections using GCM or CCM instead of CBC or using
+     Encrypt-then-Mac (RFC 7366) were not affected. Found by Kenny Paterson,
+     Eyal Ronen and Adi Shamir.
+
 Bugfix
    * Fix compilation error on C++, because of a variable named new.
      Found and fixed by Hirotaka Niisato in #1783.

library/md5.c

@@ -275,36 +275,51 @@ void mbedtls_md5_update( mbedtls_md5_context *ctx, const unsigned char *input, s
     }
 }
 
-static const unsigned char md5_padding[64] =
-{
- 0x80, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
-};
-
 /*
  * MD5 final digest
  */
 void mbedtls_md5_finish( mbedtls_md5_context *ctx, unsigned char output[16] )
 {
-    uint32_t last, padn;
+    uint32_t used;
     uint32_t high, low;
-    unsigned char msglen[8];
 
+    /*
+     * Add padding: 0x80 then 0x00 until 8 bytes remain for the length
+     */
+    used = ctx->total[0] & 0x3F;
+
+    ctx->buffer[used++] = 0x80;
+
+    if( used <= 56 )
+    {
+        /* Enough room for padding + length in current block */
+        memset( ctx->buffer + used, 0, 56 - used );
+    }
+    else
+    {
+        /* We'll need an extra block */
+        memset( ctx->buffer + used, 0, 64 - used );
+
+        mbedtls_md5_process( ctx, ctx->buffer );
+
+        memset( ctx->buffer, 0, 56 );
+    }
+
+    /*
+     * Add message length
+     */
     high = ( ctx->total[0] >> 29 )
          | ( ctx->total[1] <<  3 );
     low  = ( ctx->total[0] <<  3 );
 
-    PUT_UINT32_LE( low,  msglen, 0 );
+    PUT_UINT32_LE( low,  ctx->buffer, 56 );
-    PUT_UINT32_LE( high, msglen, 4 );
+    PUT_UINT32_LE( high, ctx->buffer, 60 );
 
-    last = ctx->total[0] & 0x3F;
+    mbedtls_md5_process( ctx, ctx->buffer );
-    padn = ( last < 56 ) ? ( 56 - last ) : ( 120 - last );
-
-    mbedtls_md5_update( ctx, md5_padding, padn );
-    mbedtls_md5_update( ctx, msglen, 8 );
 
+    /*
+     * Output final state
+     */
     PUT_UINT32_LE( ctx->state[0], output,  0 );
     PUT_UINT32_LE( ctx->state[1], output,  4 );
     PUT_UINT32_LE( ctx->state[2], output,  8 );

library/sha1.c

@@ -307,36 +307,51 @@ void mbedtls_sha1_update( mbedtls_sha1_context *ctx, const unsigned char *input,
         memcpy( (void *) (ctx->buffer + left), input, ilen );
 }
 
-static const unsigned char sha1_padding[64] =
-{
- 0x80, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
-};
-
 /*
  * SHA-1 final digest
  */
 void mbedtls_sha1_finish( mbedtls_sha1_context *ctx, unsigned char output[20] )
 {
-    uint32_t last, padn;
+    uint32_t used;
     uint32_t high, low;
-    unsigned char msglen[8];
 
+    /*
+     * Add padding: 0x80 then 0x00 until 8 bytes remain for the length
+     */
+    used = ctx->total[0] & 0x3F;
+
+    ctx->buffer[used++] = 0x80;
+
+    if( used <= 56 )
+    {
+        /* Enough room for padding + length in current block */
+        memset( ctx->buffer + used, 0, 56 - used );
+    }
+    else
+    {
+        /* We'll need an extra block */
+        memset( ctx->buffer + used, 0, 64 - used );
+
+        mbedtls_sha1_process( ctx, ctx->buffer );
+
+        memset( ctx->buffer, 0, 56 );
+    }
+
+    /*
+     * Add message length
+     */
     high = ( ctx->total[0] >> 29 )
          | ( ctx->total[1] <<  3 );
     low  = ( ctx->total[0] <<  3 );
 
-    PUT_UINT32_BE( high, msglen, 0 );
+    PUT_UINT32_BE( high, ctx->buffer, 56 );
-    PUT_UINT32_BE( low,  msglen, 4 );
+    PUT_UINT32_BE( low,  ctx->buffer, 60 );
 
-    last = ctx->total[0] & 0x3F;
+    mbedtls_sha1_process( ctx, ctx->buffer );
-    padn = ( last < 56 ) ? ( 56 - last ) : ( 120 - last );
-
-    mbedtls_sha1_update( ctx, sha1_padding, padn );
-    mbedtls_sha1_update( ctx, msglen, 8 );
 
+    /*
+     * Output final state
+     */
     PUT_UINT32_BE( ctx->state[0], output,  0 );
     PUT_UINT32_BE( ctx->state[1], output,  4 );
     PUT_UINT32_BE( ctx->state[2], output,  8 );

library/sha256.c

@@ -273,36 +273,51 @@ void mbedtls_sha256_update( mbedtls_sha256_context *ctx, const unsigned char *in
         memcpy( (void *) (ctx->buffer + left), input, ilen );
 }
 
-static const unsigned char sha256_padding[64] =
-{
- 0x80, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
-};
-
 /*
  * SHA-256 final digest
  */
 void mbedtls_sha256_finish( mbedtls_sha256_context *ctx, unsigned char output[32] )
 {
-    uint32_t last, padn;
+    uint32_t used;
     uint32_t high, low;
-    unsigned char msglen[8];
 
+    /*
+     * Add padding: 0x80 then 0x00 until 8 bytes remain for the length
+     */
+    used = ctx->total[0] & 0x3F;
+
+    ctx->buffer[used++] = 0x80;
+
+    if( used <= 56 )
+    {
+        /* Enough room for padding + length in current block */
+        memset( ctx->buffer + used, 0, 56 - used );
+    }
+    else
+    {
+        /* We'll need an extra block */
+        memset( ctx->buffer + used, 0, 64 - used );
+
+        mbedtls_sha256_process( ctx, ctx->buffer );
+
+        memset( ctx->buffer, 0, 56 );
+    }
+
+    /*
+     * Add message length
+     */
     high = ( ctx->total[0] >> 29 )
          | ( ctx->total[1] <<  3 );
     low  = ( ctx->total[0] <<  3 );
 
-    PUT_UINT32_BE( high, msglen, 0 );
+    PUT_UINT32_BE( high, ctx->buffer, 56 );
-    PUT_UINT32_BE( low,  msglen, 4 );
+    PUT_UINT32_BE( low,  ctx->buffer, 60 );
 
-    last = ctx->total[0] & 0x3F;
+    mbedtls_sha256_process( ctx, ctx->buffer );
-    padn = ( last < 56 ) ? ( 56 - last ) : ( 120 - last );
-
-    mbedtls_sha256_update( ctx, sha256_padding, padn );
-    mbedtls_sha256_update( ctx, msglen, 8 );
 
+    /*
+     * Output final state
+     */
     PUT_UINT32_BE( ctx->state[0], output,  0 );
     PUT_UINT32_BE( ctx->state[1], output,  4 );
     PUT_UINT32_BE( ctx->state[2], output,  8 );

library/sha512.c

@@ -302,40 +302,51 @@ void mbedtls_sha512_update( mbedtls_sha512_context *ctx, const unsigned char *in
         memcpy( (void *) (ctx->buffer + left), input, ilen );
 }
 
-static const unsigned char sha512_padding[128] =
-{
- 0x80, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
-};
-
 /*
  * SHA-512 final digest
  */
 void mbedtls_sha512_finish( mbedtls_sha512_context *ctx, unsigned char output[64] )
 {
-    size_t last, padn;
+    unsigned used;
     uint64_t high, low;
-    unsigned char msglen[16];
 
+    /*
+     * Add padding: 0x80 then 0x00 until 16 bytes remain for the length
+     */
+    used = ctx->total[0] & 0x7F;
+
+    ctx->buffer[used++] = 0x80;
+
+    if( used <= 112 )
+    {
+        /* Enough room for padding + length in current block */
+        memset( ctx->buffer + used, 0, 112 - used );
+    }
+    else
+    {
+        /* We'll need an extra block */
+        memset( ctx->buffer + used, 0, 128 - used );
+
+        mbedtls_sha512_process( ctx, ctx->buffer );
+
+        memset( ctx->buffer, 0, 112 );
+    }
+
+    /*
+     * Add message length
+     */
     high = ( ctx->total[0] >> 61 )
          | ( ctx->total[1] <<  3 );
     low  = ( ctx->total[0] <<  3 );
 
-    PUT_UINT64_BE( high, msglen, 0 );
+    PUT_UINT64_BE( high, ctx->buffer, 112 );
-    PUT_UINT64_BE( low,  msglen, 8 );
+    PUT_UINT64_BE( low,  ctx->buffer, 120 );
 
-    last = (size_t)( ctx->total[0] & 0x7F );
+    mbedtls_sha512_process( ctx, ctx->buffer );
-    padn = ( last < 112 ) ? ( 112 - last ) : ( 240 - last );
-
-    mbedtls_sha512_update( ctx, sha512_padding, padn );
-    mbedtls_sha512_update( ctx, msglen, 16 );
 
+    /*
+     * Output final state
+     */
     PUT_UINT64_BE( ctx->state[0], output,  0 );
     PUT_UINT64_BE( ctx->state[1], output,  8 );
     PUT_UINT64_BE( ctx->state[2], output, 16 );

library/ssl_tls.c

@@ -1260,6 +1260,27 @@ static void ssl_mac( mbedtls_md_context_t *md_ctx,
 #define SSL_SOME_MODES_USE_MAC
 #endif
 
+/* The function below is only used in the Lucky 13 counter-measure in
+ * ssl_decrypt_buf(). These are the defines that guard the call site. */
+#if defined(SSL_SOME_MODES_USE_MAC) && \
+    ( defined(MBEDTLS_SSL_PROTO_TLS1) || \
+      defined(MBEDTLS_SSL_PROTO_TLS1_1) || \
+      defined(MBEDTLS_SSL_PROTO_TLS1_2) )
+/* This function makes sure every byte in the memory region is accessed
+ * (in ascending addresses order) */
+static void ssl_read_memory( unsigned char *p, size_t len )
+{
+    unsigned char acc = 0;
+    volatile unsigned char force;
+
+    for( ; len != 0; p++, len-- )
+        acc ^= *p;
+
+    force = acc;
+    (void) force;
+}
+#endif /* SSL_SOME_MODES_USE_MAC && ( TLS1 || TLS1_1 || TLS1_2 ) */
+
 /*
  * Encryption/decryption functions
  */
@@ -1935,8 +1956,10 @@ static int ssl_decrypt_buf( mbedtls_ssl_context *ssl )
         return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
     }
 
+#if defined(MBEDTLS_SSL_DEBUG_ALL)
     MBEDTLS_SSL_DEBUG_BUF( 4, "raw buffer after decryption",
                    ssl->in_msg, ssl->in_msglen );
+#endif
 
     /*
      * Authenticate if not done yet.
@@ -1969,20 +1992,69 @@ static int ssl_decrypt_buf( mbedtls_ssl_context *ssl )
         {
             /*
              * Process MAC and always update for padlen afterwards to make
-             * total time independent of padlen
+             * total time independent of padlen.
-             *
-             * extra_run compensates MAC check for padlen
              *
              * Known timing attacks:
              *  - Lucky Thirteen (http://www.isg.rhul.ac.uk/tls/TLStiming.pdf)
              *
+             * To compensate for different timings for the MAC calculation
+             * depending on how much padding was removed (which is determined
+             * by padlen), process extra_run more blocks through the hash
+             * function.
+             *
+             * The formula in the paper is
+             *   extra_run = ceil( (L1-55) / 64 ) - ceil( (L2-55) / 64 )
+             * where L1 is the size of the header plus the decrypted message
+             * plus CBC padding and L2 is the size of the header plus the
+             * decrypted message. This is for an underlying hash function
+             * with 64-byte blocks.
              * We use ( (Lx+8) / 64 ) to handle 'negative Lx' values
-             * correctly. (We round down instead of up, so -56 is the correct
+             * correctly. We round down instead of up, so -56 is the correct
-             * value for our calculations instead of -55)
+             * value for our calculations instead of -55.
+             *
+             * Repeat the formula rather than defining a block_size variable.
+             * This avoids requiring division by a variable at runtime
+             * (which would be marginally less efficient and would require
+             * linking an extra division function in some builds).
              */
             size_t j, extra_run = 0;
+
+            /*
+             * The next two sizes are the minimum and maximum values of
+             * in_msglen over all padlen values.
+             *
+             * They're independent of padlen, since we previously did
+             * in_msglen -= padlen.
+             *
+             * Note that max_len + maclen is never more than the buffer
+             * length, as we previously did in_msglen -= maclen too.
+             */
+            const size_t max_len = ssl->in_msglen + padlen;
+            const size_t min_len = ( max_len > 256 ) ? max_len - 256 : 0;
+
+            switch( ssl->transform_in->ciphersuite_info->mac )
+            {
+#if defined(MBEDTLS_MD5_C) || defined(MBEDTLS_SHA1_C) || \
+    defined(MBEDTLS_SHA256_C)
+                case MBEDTLS_MD_MD5:
+                case MBEDTLS_MD_SHA1:
+                case MBEDTLS_MD_SHA256:
+                    /* 8 bytes of message size, 64-byte compression blocks */
                     extra_run = ( 13 + ssl->in_msglen + padlen + 8 ) / 64 -
                                 ( 13 + ssl->in_msglen          + 8 ) / 64;
+                    break;
+#endif
+#if defined(MBEDTLS_SHA512_C)
+                case MBEDTLS_MD_SHA384:
+                    /* 16 bytes of message size, 128-byte compression blocks */
+                    extra_run = ( 13 + ssl->in_msglen + padlen + 16 ) / 128 -
+                                ( 13 + ssl->in_msglen          + 16 ) / 128;
+                    break;
+#endif
+                default:
+                    MBEDTLS_SSL_DEBUG_MSG( 1, ( "should never happen" ) );
+                    return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
+            }
 
             extra_run &= correct * 0xFF;
 
@@ -1991,12 +2063,25 @@ static int ssl_decrypt_buf( mbedtls_ssl_context *ssl )
             mbedtls_md_hmac_update( &ssl->transform_in->md_ctx_dec, ssl->in_len, 2 );
             mbedtls_md_hmac_update( &ssl->transform_in->md_ctx_dec, ssl->in_msg,
                              ssl->in_msglen );
+            /* Make sure we access everything even when padlen > 0. This
+             * makes the synchronisation requirements for just-in-time
+             * Prime+Probe attacks much tighter and hopefully impractical. */
+            ssl_read_memory( ssl->in_msg + ssl->in_msglen, padlen );
             mbedtls_md_hmac_finish( &ssl->transform_in->md_ctx_dec, mac_expect );
-            /* Call mbedtls_md_process at least once due to cache attacks */
+
+            /* Call mbedtls_md_process at least once due to cache attacks
+             * that observe whether md_process() was called of not */
             for( j = 0; j < extra_run + 1; j++ )
                 mbedtls_md_process( &ssl->transform_in->md_ctx_dec, ssl->in_msg );
 
             mbedtls_md_hmac_reset( &ssl->transform_in->md_ctx_dec );
+
+            /* 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( ssl->in_msg + min_len,
+                                 max_len - min_len + ssl->transform_in->maclen );
         }
         else
 #endif /* MBEDTLS_SSL_PROTO_TLS1 || MBEDTLS_SSL_PROTO_TLS1_1 || \
@@ -2006,9 +2091,11 @@ static int ssl_decrypt_buf( mbedtls_ssl_context *ssl )
             return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
         }
 
+#if defined(MBEDTLS_SSL_DEBUG_ALL)
         MBEDTLS_SSL_DEBUG_BUF( 4, "expected mac", mac_expect, ssl->transform_in->maclen );
         MBEDTLS_SSL_DEBUG_BUF( 4, "message  mac", ssl->in_msg + ssl->in_msglen,
                                ssl->transform_in->maclen );
+#endif
 
         if( mbedtls_ssl_safer_memcmp( ssl->in_msg + ssl->in_msglen, mac_expect,
                                       ssl->transform_in->maclen ) != 0 )