KEYS: Add KEYCTL_DH_COMPUTE command

This adds userspace access to Diffie-Hellman computations through a
new keyctl() syscall command to calculate shared secrets or public
keys using input parameters stored in the keyring.

Input key ids are provided in a struct due to the current 5-arg limit
for the keyctl syscall. Only user keys are supported in order to avoid
exposing the content of logon or encrypted keys.

The output is written to the provided buffer, based on the assumption
that the values are only needed in userspace.

Future support for other types of key derivation would involve a new
command, like KEYCTL_ECDH_COMPUTE.

Once Diffie-Hellman support is included in the crypto API, this code
can be converted to use the crypto API to take advantage of possible
hardware acceleration and reduce redundant code.

Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David Howells <dhowells@redhat.com>

include/uapi/linux/keyctl.h

@@ -12,6 +12,8 @@
 #ifndef _LINUX_KEYCTL_H
 #define _LINUX_KEYCTL_H
 
+#include <linux/types.h>
+
 /* special process keyring shortcut IDs */
 #define KEY_SPEC_THREAD_KEYRING		-1	/* - key ID for thread-specific keyring */
 #define KEY_SPEC_PROCESS_KEYRING	-2	/* - key ID for process-specific keyring */
@@ -57,5 +59,13 @@
 #define KEYCTL_INSTANTIATE_IOV		20	/* instantiate a partially constructed key */
 #define KEYCTL_INVALIDATE		21	/* invalidate a key */
 #define KEYCTL_GET_PERSISTENT		22	/* get a user's persistent keyring */
+#define KEYCTL_DH_COMPUTE		23	/* Compute Diffie-Hellman values */
+
+/* keyctl structures */
+struct keyctl_dh_params {
+	__s32 private;
+	__s32 prime;
+	__s32 base;
+};
 
 #endif /*  _LINUX_KEYCTL_H */