Merge remote-tracking branch 'restricted/pr/537' into mbedtls-2.1
diff --git a/ChangeLog b/ChangeLog
index 0936755..951c134 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -5,9 +5,17 @@
Security
* Fix timing variations and memory access variations in RSA PKCS#1 v1.5
decryption that could lead to a Bleichenbacher-style padding oracle
- attack. In TLS, this affects RSA-based ciphersuites without DHE or
- ECDHE. Reported by Yuval Yarom, Eyal Ronen, Adi Shamir, David Wong and
- Daniel Genkin.
+ attack. In TLS, this affects servers that accept ciphersuites based on
+ RSA decryption (i.e. ciphersuites whose name contains RSA but not
+ (EC)DH(E)). Reported by Eyal Ronen, Robert Gillham, Daniel Genkin, Adi
+ Shamir, David Wong and Yuval Yarom. CVE-2018-19608
+ * In mbedtls_mpi_write_binary(), don't leak the exact size of the number
+ via branching and memory access patterns. An attacker who could submit
+ a plaintext for RSA PKCS#1 v1.5 decryption but only observe the timing
+ of the decryption and not its result could nonetheless decrypt RSA
+ plaintexts and forge RSA signatures. Other asymmetric algorithms may
+ have been similarly vulnerable. Reported by Eyal Ronen, Robert Gillham,
+ Daniel Genkin, Adi Shamir, David Wong and Yuval Yarom.
= mbed TLS 2.1.16 branch released 2018-11-19
diff --git a/library/bignum.c b/library/bignum.c
index 61aa961..5014662 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -313,6 +313,10 @@
return( ( X->p[pos / biL] >> ( pos % biL ) ) & 0x01 );
}
+/* Get a specific byte, without range checks. */
+#define GET_BYTE( X, i ) \
+ ( ( ( X )->p[( i ) / ciL] >> ( ( ( i ) % ciL ) * 8 ) ) & 0xff )
+
/*
* Set a bit to a specific value of 0 or 1
*/
@@ -696,19 +700,40 @@
/*
* Export X into unsigned binary data, big endian
*/
-int mbedtls_mpi_write_binary( const mbedtls_mpi *X, unsigned char *buf, size_t buflen )
+int mbedtls_mpi_write_binary( const mbedtls_mpi *X,
+ unsigned char *buf, size_t buflen )
{
- size_t i, j, n;
+ size_t stored_bytes = X->n * ciL;
+ size_t bytes_to_copy;
+ unsigned char *p;
+ size_t i;
- n = mbedtls_mpi_size( X );
+ if( stored_bytes < buflen )
+ {
+ /* There is enough space in the output buffer. Write initial
+ * null bytes and record the position at which to start
+ * writing the significant bytes. In this case, the execution
+ * trace of this function does not depend on the value of the
+ * number. */
+ bytes_to_copy = stored_bytes;
+ p = buf + buflen - stored_bytes;
+ memset( buf, 0, buflen - stored_bytes );
+ }
+ else
+ {
+ /* The output buffer is smaller than the allocated size of X.
+ * However X may fit if its leading bytes are zero. */
+ bytes_to_copy = buflen;
+ p = buf;
+ for( i = bytes_to_copy; i < stored_bytes; i++ )
+ {
+ if( GET_BYTE( X, i ) != 0 )
+ return( MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL );
+ }
+ }
- if( buflen < n )
- return( MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL );
-
- memset( buf, 0, buflen );
-
- for( i = buflen - 1, j = 0; n > 0; i--, j++, n-- )
- buf[i] = (unsigned char)( X->p[j / ciL] >> ((j % ciL) << 3) );
+ for( i = 0; i < bytes_to_copy; i++ )
+ p[bytes_to_copy - i - 1] = GET_BYTE( X, i );
return( 0 );
}