Introduce function checking sanity of the DTLS HS header
This commit introduces helper functions
- ssl_get_hs_frag_len()
- ssl_get_hs_frag_off()
to parse the fragment length resp. fragment offset fields
in the handshake header.
Moreover, building on these helper functions, it adds a
function ssl_check_hs_header() checking the validity of
a DTLS handshake header with respect to the specification,
i.e. the indicated fragment must be a subrange of the total
handshake message, and the total handshake fragment length
(including header) must not exceed the record content size.
These checks were previously performed at a later stage during
ssl_reassemble_dtls_handshake().
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index d7c6165..a321eaf 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -3426,6 +3426,41 @@
}
return( 0 );
}
+
+static uint32_t ssl_get_hs_frag_len( mbedtls_ssl_context *ssl )
+{
+ return( ( ssl->in_msg[9] << 16 ) |
+ ( ssl->in_msg[10] << 8 ) |
+ ssl->in_msg[11] );
+}
+
+static uint32_t ssl_get_hs_frag_off( mbedtls_ssl_context *ssl )
+{
+ return( ( ssl->in_msg[6] << 16 ) |
+ ( ssl->in_msg[7] << 8 ) |
+ ssl->in_msg[8] );
+}
+
+static int ssl_check_hs_header( mbedtls_ssl_context *ssl )
+{
+ uint32_t msg_len, frag_off, frag_len;
+
+ msg_len = ssl_get_hs_total_len( ssl );
+ frag_off = ssl_get_hs_frag_off( ssl );
+ frag_len = ssl_get_hs_frag_len( ssl );
+
+ if( frag_off > msg_len )
+ return( -1 );
+
+ if( frag_len > msg_len - frag_off )
+ return( -1 );
+
+ if( frag_len + 12 > ssl->in_msglen )
+ return( -1 );
+
+ return( 0 );
+}
+
/*
* Mark bits in bitmask (used for DTLS HS reassembly)
*/
@@ -3688,6 +3723,12 @@
int ret;
unsigned int recv_msg_seq = ( ssl->in_msg[4] << 8 ) | ssl->in_msg[5];
+ if( ssl_check_hs_header( ssl ) != 0 )
+ {
+ MBEDTLS_SSL_DEBUG_MSG( 1, ( "invalid handshake header" ) );
+ return( MBEDTLS_ERR_SSL_INVALID_RECORD );
+ }
+
if( ssl->handshake != NULL &&
( ( ssl->state != MBEDTLS_SSL_HANDSHAKE_OVER &&
recv_msg_seq != ssl->handshake->in_msg_seq ) ||