Re: [PATCH 10/18] crypto: dh - introduce support for ephemeral key generation to dh-generic

Stephan Müller [off-list ref] writes:

Am Mittwoch, 1. Dezember 2021, 01:48:50 CET schrieb Nicolai Stange:

Hi Nicolai,

quoted

The support for NVME in-band authentication currently in the works ([1])
needs to generate ephemeral DH keys. Make dh-generic's ->set_secret()
to generate an ephemeral key via the recently added crypto_dh_gen_privkey()
in case the input ->key_size is zero. Note that this behaviour is in
analogy to ecdh's ->set_secret().

[1] https://lkml.kernel.org/r/20211122074727.25988-4-hare@suse.de

Signed-off-by: Nicolai Stange <redacted>
---
 crypto/dh.c | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/crypto/dh.c b/crypto/dh.c
index 131b80064cb1..2e49b114e038 100644
--- a/crypto/dh.c
+++ b/crypto/dh.c

@@ -71,25 +71,41 @@ static int dh_set_secret(struct crypto_kpp *tfm, const

void *buf, {
 	struct dh_ctx *ctx = dh_get_ctx(tfm);
 	struct dh params;
+	char key[CRYPTO_DH_MAX_PRIVKEY_SIZE];
+	int err;

 	/* Free the old MPI key if any */
 	dh_clear_ctx(ctx);

-	if (crypto_dh_decode_key(buf, len, &params) < 0)
+	err = crypto_dh_decode_key(buf, len, &params);
+	if (err)
 		goto err_clear_ctx;

-	if (dh_set_params(ctx, &params) < 0)
+	if (!params.key_size) {

As this params data may come from user space, shouldn't we use the same logic 
as in ecdh's set_key function:

if (!params.key || !params.key_size)

crypto_dh_decode_key() always leaves params.key set even for
!params.key_size, so checking for !params.key wouldn't buy anything
here. FWIW, it seems like it's actually the same for
crypto_ecdh_decode_key().

I'd personally prefer to not add the !params.key check, because it would
suggest that there are code paths which can lead to the condition
params.key_size && !params.key. I would find this confusing when reading
the code, but OTOH I don't have strong objections, so if you insist on
adding the !params.key check, I'd be Ok with it.

Thanks,

Nicolai

?

quoted

+		err = crypto_dh_gen_privkey(params.group_id, key,
+					    &params.key_size);
+		if (err)
+			goto err_clear_ctx;
+		params.key = key;
+	}
+
+	err = dh_set_params(ctx, &params);
+	if (err)
 		goto err_clear_ctx;

 	ctx->xa = mpi_read_raw_data(params.key, params.key_size);
-	if (!ctx->xa)
+	if (!ctx->xa) {
+		err = -EINVAL;
 		goto err_clear_ctx;
+	}
+
+	memzero_explicit(key, sizeof(key));

 	return 0;

 err_clear_ctx:
 	dh_clear_ctx(ctx);
-	return -EINVAL;
+	return err;
 }

 /*

Ciao
Stephan

-- 
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg), GF: Ivo Totev