Add additional DigestInfo checks.

Reencode DigestInto in DER and check against the original: this
will reject any improperly encoded DigestInfo structures.

Note: this is a precautionary measure, there is no known attack
which can exploit this.

Thanks to Brian Smith for reporting this issue.
Reviewed-by: Tim Hudson <tjh@openssl.org>

2 files changed, 29 insertions, 2 deletions

diff --git a/CHANGES b/CHANGES
index 98a7204da8..b1739f60bb 100644
--- a/CHANGES
+++ b/CHANGES
@@ -4,7 +4,15 @@
 
  Changes between 0.9.8zb and 0.9.8zc [xx XXX xxxx]
 
-  *)
+  *) Add additional DigestInfo checks.
+ 
+     Reencode DigestInto in DER and check against the original when
+     verifying RSA signature: this will reject any improperly encoded
+     DigestInfo structures.
+
+     Note: this is a precautionary measure and no attacks are currently known.
+
+     [Steve Henson]
 
  Changes between 0.9.8za and 0.9.8zb [6 Aug 2014]
 
diff --git a/crypto/rsa/rsa_sign.c b/crypto/rsa/rsa_sign.c
index 743dfd7650..de6ceeb0be 100644
--- a/crypto/rsa/rsa_sign.c
+++ b/crypto/rsa/rsa_sign.c
@@ -155,6 +155,25 @@ int RSA_sign(int type, const unsigned char *m, unsigned int m_len,
 	return(ret);
 	}
 
+/*
+ * Check DigestInfo structure does not contain extraneous data by reencoding
+ * using DER and checking encoding against original. 
+ */
+static int rsa_check_digestinfo(X509_SIG *sig, const unsigned char *dinfo, int dinfolen)
+	{
+	unsigned char *der = NULL;
+	int derlen;
+	int ret = 0;
+	derlen = i2d_X509_SIG(sig, &der);
+	if (derlen <= 0)
+		return 0;
+	if (derlen == dinfolen && !memcmp(dinfo, der, derlen))
+		ret = 1;
+	OPENSSL_cleanse(der, derlen);
+	OPENSSL_free(der);
+	return ret;
+	}
+
 int RSA_verify(int dtype, const unsigned char *m, unsigned int m_len,
 	     unsigned char *sigbuf, unsigned int siglen, RSA *rsa)
 	{
@@ -215,7 +234,7 @@ int RSA_verify(int dtype, const unsigned char *m, unsigned int m_len,
 		if (sig == NULL) goto err;
 
 		/* Excess data can be used to create forgeries */
-		if(p != s+i)
+		if(p != s+i || !rsa_check_digestinfo(sig, s, i))
 			{
 			RSAerr(RSA_F_RSA_VERIFY,RSA_R_BAD_SIGNATURE);
 			goto err;