commit fd8f79d89f47f661e9c5fb0627d07599d4c722e3
author Gilles Peskine <Gilles.Peskine@arm.com> Wed May 03 18:32:21 2017 +0200
committer Gilles Peskine <Gilles.Peskine@arm.com> Wed May 03 18:45:15 2017 +0200
tree 3434c22188c1d2c14a7f1bf0849a309350d4d9ec
parent 63a48d10e94557cb1e226c37face768c5d149f76

More length checks in RSA PKCS1v15 verify

Tighten ASN.1 parsing of RSA PKCS#1 v1.5 signatures, to avoid a
potential Bleichenbacher-style attack.

Backport to 2.1.x

library/rsa.c

diff --git a/library/rsa.c b/library/rsa.c
index e831875..520f1a2 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -1343,7 +1343,7 @@
 {
     int ret;
     size_t len, siglen, asn1_len;
-    unsigned char *p, *end;
+    unsigned char *p, *p0, *end;
     unsigned char buf[MBEDTLS_MPI_MAX_SIZE];
     mbedtls_md_type_t msg_md_alg;
     const mbedtls_md_info_t *md_info;
@@ -1396,23 +1396,27 @@
 
     // Parse the ASN.1 structure inside the PKCS#1 v1.5 structure
     //
+    p0 = p;
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &asn1_len,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
         return( MBEDTLS_ERR_RSA_VERIFY_FAILED );
 
-    if( asn1_len + 2 != len )
+    if( p != p0 + 2 || asn1_len + 2 != len )
         return( MBEDTLS_ERR_RSA_VERIFY_FAILED );
 
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &asn1_len,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
         return( MBEDTLS_ERR_RSA_VERIFY_FAILED );
 
-    if( asn1_len + 6 + hashlen != len )
+    if( p != p0 + 4 || asn1_len + 6 + hashlen != len )
         return( MBEDTLS_ERR_RSA_VERIFY_FAILED );
 
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &oid.len, MBEDTLS_ASN1_OID ) ) != 0 )
         return( MBEDTLS_ERR_RSA_VERIFY_FAILED );
 
+    if( p != p0 + 6 )
+        return( MBEDTLS_ERR_RSA_VERIFY_FAILED );
+
     oid.p = p;
     p += oid.len;
 
@@ -1428,10 +1432,11 @@
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &asn1_len, MBEDTLS_ASN1_NULL ) ) != 0 )
         return( MBEDTLS_ERR_RSA_VERIFY_FAILED );
 
+    p0 = p;
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &asn1_len, MBEDTLS_ASN1_OCTET_STRING ) ) != 0 )
         return( MBEDTLS_ERR_RSA_VERIFY_FAILED );
 
-    if( asn1_len != hashlen )
+    if( p != p0 + 2 || asn1_len != hashlen )
         return( MBEDTLS_ERR_RSA_VERIFY_FAILED );
 
     if( memcmp( p, hash, hashlen ) != 0 )