KEYS: Convert KEYCTL_DH_COMPUTE to use the crypto KPP API

The initial Diffie-Hellman computation made direct use of the MPI
library because the crypto module did not support DH at the time. Now
that KPP is implemented, KEYCTL_DH_COMPUTE should use it to get rid of
duplicate code and leverage possible hardware acceleration.

This fixes an issue whereby the input to the KDF computation would
include additional uninitialized memory when the result of the
Diffie-Hellman computation was shorter than the input prime number.

Signed-off-by: Mat Martineau <[email protected]>
Signed-off-by: David Howells <[email protected]>
Signed-off-by: James Morris <[email protected]>

Author: mjmartineau

security/keys/Kconfig

@@ -93,9 +93,9 @@ config ENCRYPTED_KEYS
 config KEY_DH_OPERATIONS
        bool "Diffie-Hellman operations on retained keys"
        depends on KEYS
-       select MPILIB
        select CRYPTO
        select CRYPTO_HASH
+       select CRYPTO_DH
        help
 	 This option provides support for calculating Diffie-Hellman
 	 public keys and shared secrets using values stored as keys