commit 9e2058281daa77d88cdf17d63cf30dbeaf39905b
author Gilles Peskine <Gilles.Peskine@arm.com> Wed Oct 18 19:03:42 2017 +0200
committer Gilles Peskine <Gilles.Peskine@arm.com> Wed Oct 18 19:06:50 2017 +0200
tree 96bdc815b0b00e145dfa5eea7f55e727fa8d11f9
parent d0cd855145bd6f582947114b47f12502ba3ef4d8

RSA PSS: fix minimum length check for keys of size 8N+1

The check introduced by the previous security fix was off by one. It
fixed the buffer overflow but was not compliant with the definition of
PSS which technically led to accepting some invalid signatures (but
not signatures made without the private key).

library/rsa.c

diff --git a/library/rsa.c b/library/rsa.c
index 9e05b4b..8933aa1 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -1370,9 +1370,6 @@
         return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA );
 
     hlen = mbedtls_md_get_size( md_info );
-    if( siglen < hlen + 2 )
-        return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA );
-    hash_start = buf + siglen - hlen - 1;
 
     memset( zeros, 0, 8 );
 
@@ -1391,6 +1388,10 @@
     if( buf[0] >> ( 8 - siglen * 8 + msb ) )
         return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA );
 
+    if( siglen < hlen + 2 )
+        return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA );
+    hash_start = p + siglen - hlen - 1;
+
     mbedtls_md_init( &md_ctx );
     if( ( ret = mbedtls_md_setup( &md_ctx, md_info, 0 ) ) != 0 )
     {