From 444678ea8b7c9a2a38af2723ff103c87f6d9252a Mon Sep 17 00:00:00 2001
From: Krzysztof Stachowiak <krzysiek.stachowiak@gmail.com>
Date: Tue, 20 Mar 2018 14:09:53 +0100
Subject: [PATCH] Add a missing buffer size check

---
 library/ssl_cli.c | 35 ++++++++++++++++++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index 5a49d5367..a66cbb0db 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -2511,9 +2511,42 @@ static int ssl_parse_certificate_request( mbedtls_ssl_context *ssl )
         // TODO: should check the signature part against our pk_key though
         size_t sig_alg_len = ( ( buf[mbedtls_ssl_hs_hdr_len( ssl ) + 1 + n] <<  8 )
                              | ( buf[mbedtls_ssl_hs_hdr_len( ssl ) + 2 + n]       ) );
+#if defined(MBEDTLS_DEBUG_C)
+        unsigned char* sig_alg;
+        size_t i;
+#endif
 
+        /*
+         * The farthes access in buf is in the loop few lines below:
+         *     sig_alg[i + 1],
+         * where:
+         *     sig_alg = buf + ...hdr_len + 3 + n,
+         *     max(i) = sig_alg_len - 1.
+         * Therefore the farthest access is:
+         *     buf[...hdr_len + 3 + n + sig_alg_len - 1 + 1],
+         * which reduces to:
+         *     buf[...hdr_len + 3 + n + sig_alg_len],
+         * which is one less than we need the buf to be.
+         */
+        if( ssl->in_hslen <= mbedtls_ssl_hs_hdr_len( ssl ) + 3 + n + sig_alg_len )
+        {
+            MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate request message" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
+            return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE_REQUEST );
+        }
+
+#if defined(MBEDTLS_DEBUG_C)
+        sig_alg = buf + mbedtls_ssl_hs_hdr_len( ssl ) + 3 + n;
+        for( i = 0; i < sig_alg_len; i += 2 )
+        {
+            MBEDTLS_SSL_DEBUG_MSG( 3, ( "Supported Signature Algorithm found: %d"
+                                        ",%d", sig_alg[i], sig_alg[i + 1]  ) );
+        }
+#endif
+
+        n += 2 + sig_alg_len;
         m += 2;
-        n += sig_alg_len;
 
         if( ssl->in_hslen < mbedtls_ssl_hs_hdr_len( ssl ) + 2 + n )
         {