KEYS: Add placeholder for KDF usage with DH

The values computed during Diffie-Hellman key exchange are often used
in combination with key derivation functions to create cryptographic
keys. Add a placeholder for a later implementation to configure a
key derivation function that will transform the Diffie-Hellman
result returned by the KEYCTL_DH_COMPUTE command.

[This patch was stripped down from a patch produced by Mat Martineau that
had a bug in the compat code - so for the moment Stephan's patch simply
requires that the placeholder argument must be NULL]

Original-signed-off-by: Mat Martineau <[e-mail]>
Signed-off-by: Stephan Mueller <[e-mail]>
Signed-off-by: David Howells <[e-mail]>
Signed-off-by: James Morris <[e-mail]>

Documentation/security/keys.txt

···
  (*) Compute a Diffie-Hellman shared secret or public key
 
        long keyctl(KEYCTL_DH_COMPUTE, struct keyctl_dh_params *params,
-		   char *buffer, size_t buflen);
+		   char *buffer, size_t buflen,
+		   void *reserved);
 
      The params struct contains serial numbers for three keys:
 
···
      If the base is the shared generator, the result is the local
      public key.  If the base is the remote public key, the result is
      the shared secret.
+
+     The reserved argument must be set to NULL.
 
      The buffer length must be at least the length of the prime, or zero.
 

security/keys/compat.c

···
 
 	case KEYCTL_DH_COMPUTE:
 		return keyctl_dh_compute(compat_ptr(arg2), compat_ptr(arg3),
-					 arg4);
+					 arg4, compat_ptr(arg5));
 
 	default:
 		return -EOPNOTSUPP;

security/keys/dh.c

···
 }
 
 long keyctl_dh_compute(struct keyctl_dh_params __user *params,
-		       char __user *buffer, size_t buflen)
+		       char __user *buffer, size_t buflen,
+		       void __user *reserved)
 {
 	long ret;
 	MPI base, private, prime, result;
···
 	}
 	if (copy_from_user(&pcopy, params, sizeof(pcopy)) != 0) {
 		ret = -EFAULT;
+		goto out;
+	}
+
+	if (reserved) {
+		ret = -EINVAL;
 		goto out;
 	}
 

security/keys/internal.h

···
 
 #ifdef CONFIG_KEY_DH_OPERATIONS
 extern long keyctl_dh_compute(struct keyctl_dh_params __user *, char __user *,
-			      size_t);
+			      size_t, void __user *);
 #else
 static inline long keyctl_dh_compute(struct keyctl_dh_params __user *params,
-				     char __user *buffer, size_t buflen)
+				     char __user *buffer, size_t buflen,
+				     void __user *reserved)
 {
 	return -EOPNOTSUPP;
 }

security/keys/keyctl.c

···
 
 	case KEYCTL_DH_COMPUTE:
 		return keyctl_dh_compute((struct keyctl_dh_params __user *) arg2,
-					 (char __user *) arg3,
-					 (size_t) arg4);
+					 (char __user *) arg3, (size_t) arg4,
+					 (void __user *) arg5);
 
 	default:
 		return -EOPNOTSUPP;