crypto: virtio - Drop sign/verify operations

crypto: virtio - Drop sign/verify operations

The virtio crypto driver exposes akcipher sign/verify operations in a
user space ABI.  This blocks removal of sign/verify from akcipher_alg.

Herbert opines:

   "I would say that this is something that we can break.  Breaking it
    is no different to running virtio on a host that does not support
    these algorithms.  After all, a software implementation must always
    be present.

    I deliberately left akcipher out of crypto_user because the API
    is still in flux.  We should not let virtio constrain ourselves."
    https://lore.kernel.org/all/ZtqoNAgcnXnrYhZZ@gondor.apana.org.au/

   "I would remove virtio akcipher support in its entirety.  This API
    was never meant to be exposed outside of the kernel."
    https://lore.kernel.org/all/Ztqql_gqgZiMW8zz@gondor.apana.org.au/

Drop sign/verify support from virtio crypto.  There's no strong reason
to also remove encrypt/decrypt support, so keep it.

A key selling point of virtio crypto is to allow guest access to crypto
accelerators on the host.  So far the only akcipher algorithm supported
by virtio crypto is RSA.  Dropping sign/verify merely means that the
PKCS#1 padding is now always generated or verified inside the guest,
but the actual signature generation/verification (which is an RSA
decrypt/encrypt operation) may still use an accelerator on the host.

Generating or verifying the PKCS#1 padding is cheap, so a hardware
accelerator won't be of much help there.  Which begs the question
whether virtio crypto support for sign/verify makes sense at all.

It would make sense for the sign operation if the host has a security
chip to store asymmetric private keys.  But the kernel doesn't even
have an asymmetric_key_subtype yet for hardware-based private keys.
There's at least one rudimentary driver for such chips (atmel-ecc.c for
ATECC508A), but it doesn't implement the sign operation.  The kernel
would first have to grow support for a hardware asymmetric_key_subtype
and at least one driver implementing the sign operation before exposure
to guests via virtio makes sense.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>