[PATCH v2] crypto: akcipher - default implementation for setting a private key

[Ignat Korchagin <ignat@cloudflare.com>]

Changes from v1:
  * removed the default implementation from set_pub_key: it is assumed that
    an implementation must always have this callback defined as there are
    no use case for an algorithm, which doesn't need a public key

Many akcipher implementations (like ECDSA) support only signature
verifications, so they don't have all callbacks defined.

Commit 78a0324f4a53 ("crypto: akcipher - default implementations for
request callbacks") introduced default callbacks for sign/verify
operations, which just return an error code.

However, these are not enough, because before calling sign the caller would
likely call set_priv_key first on the instantiated transform (as the
in-kernel testmgr does). This function does not have a default stub, so the
kernel crashes, when trying to set a private key on an akcipher, which
doesn't support signature generation.

I've noticed this, when trying to add a KAT vector for ECDSA signature to
the testmgr.

With this patch the testmgr returns an error in dmesg (as it should)
instead of crashing the kernel NULL ptr dereference.

Fixes: 78a0324f4a53 ("crypto: akcipher - default implementations for request callbacks")
Signed-off-by: Ignat Korchagin <ignat@cloudflare.com>
---
 crypto/akcipher.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/crypto/akcipher.c b/crypto/akcipher.c
index f866085c8a4a..ab975a420e1e 100644
--- a/crypto/akcipher.c
+++ b/crypto/akcipher.c
@@ -120,6 +120,12 @@ static int akcipher_default_op(struct akcipher_request *req)
 	return -ENOSYS;
 }

+static int akcipher_default_set_key(struct crypto_akcipher *tfm,
+				     const void *key, unsigned int keylen)
+{
+	return -ENOSYS;
+}
+
 int crypto_register_akcipher(struct akcipher_alg *alg)
 {
 	struct crypto_alg *base = &alg->base;
@@ -132,6 +138,8 @@ int crypto_register_akcipher(struct akcipher_alg *alg)
 		alg->encrypt = akcipher_default_op;
 	if (!alg->decrypt)
 		alg->decrypt = akcipher_default_op;
+	if (!alg->set_priv_key)
+		alg->set_priv_key = akcipher_default_set_key;

 	akcipher_prepare_alg(alg);
 	return crypto_register_alg(base);
--
2.36.1