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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     herbert@gondor.apana.org.au, linux-crypto@vger.kernel.org
Cc:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Andrej Shadura <andrew.shadura@collabora.co.uk>,
        Jiri Kosina <jkosina@suse.cz>
Subject: [PATCH] hwrng: u2fzero - account for high quality RNG
Date:   Sat, 19 Nov 2022 14:42:59 +0100
Message-Id: <20221119134259.2969204-1-Jason@zx2c4.com>
MIME-Version: 1.0
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Precedence: bulk

The U2F zero apparently has a real TRNG in it with maximum quality, not
one with quality of "1", which was likely a misinterpretation of the
field as a boolean. So remove the assignment entirely, so that we get
the default quality setting.

In the u2f-zero firmware, the 0x21 RNG command used by this driver is
handled as such [1]:

  case U2F_CUSTOM_GET_RNG:
    if (atecc_send_recv(ATECC_CMD_RNG,ATECC_RNG_P1,ATECC_RNG_P2,
      NULL, 0,
      appdata.tmp,
      sizeof(appdata.tmp), &res) == 0 )
    {
      memmove(msg->pkt.init.payload, res.buf, 32);
      U2FHID_SET_LEN(msg, 32);
      usb_write((uint8_t*)msg, 64);
    }
    else
    {
      U2FHID_SET_LEN(msg, 0);
      usb_write((uint8_t*)msg, 64);
    }

This same call to `atecc_send_recv(ATECC_CMD_RNG,ATECC_RNG_P1,
ATECC_RNG_P2,...)` is then also used in the token's cryptographically
critical "u2f_new_keypair" function, as its rather straightforward
source of random bytes [2]:

  int8_t u2f_new_keypair(uint8_t * handle, uint8_t * appid, uint8_t * pubkey)
  {
    struct atecc_response res;
    uint8_t private_key[36];
    int i;

    watchdog();

    if (atecc_send_recv(ATECC_CMD_RNG,ATECC_RNG_P1,ATECC_RNG_P2,
      NULL, 0,
      appdata.tmp,
      sizeof(appdata.tmp), &res) != 0 )
    {
      return -1;
    }

So it seems rather plain that the ATECC RNG is considered to provide
good random numbers.

[1] https://github.com/conorpp/u2f-zero/blob/master/firmware/src/custom.c
[2] https://github.com/conorpp/u2f-zero/blob/master/firmware/src/u2f_atecc.c

Cc: Andrej Shadura <andrew.shadura@collabora.co.uk>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 drivers/hid/hid-u2fzero.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/drivers/hid/hid-u2fzero.c b/drivers/hid/hid-u2fzero.c
index ad489caf53ad..744a91e6e78c 100644
--- a/drivers/hid/hid-u2fzero.c
+++ b/drivers/hid/hid-u2fzero.c
@@ -261,7 +261,6 @@ static int u2fzero_init_hwrng(struct u2fzero_device *dev,
 
 	dev->hwrng.name = dev->rng_name;
 	dev->hwrng.read = u2fzero_rng_read;
-	dev->hwrng.quality = 1;
 
 	return devm_hwrng_register(&dev->hdev->dev, &dev->hwrng);
 }
-- 
2.38.1