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libsec: check if modulus is too small for message in pkcs1padbuf()

--- a/sys/src/libsec/port/x509.c

+++ b/sys/src/libsec/port/x509.c

@@ -2143,6 +2143,10 @@

 	mpint *mp;

 	pm1 = n - 1 - len;

+	if(pm1 <= 2){

+		werrstr("pkcs1padbuf: modulus too small");

+		return nil;

+	}

 	p = (uchar*)emalloc(n);

 	p[0] = 0;

 	p[1] = 1;

@@ -2827,6 +2831,8 @@

 		goto errret;

 	pkcs1 = pkcs1pad(sigbytes, pk->n);

 	freebytes(sigbytes);

+	if(pkcs1 == nil)

+		goto errret;

 	rsadecrypt(priv, pkcs1, pkcs1);

 	buflen = mptobe(pkcs1, nil, 0, &buf);

@@ -2894,6 +2900,8 @@

 		goto errret;

 	pkcs1 = pkcs1pad(sigbytes, pk->n);

 	freebytes(sigbytes);

+	if(pkcs1 == nil)

+		goto errret;

 	rsadecrypt(priv, pkcs1, pkcs1);

 	buflen = mptobe(pkcs1, nil, 0, &buf);