From:   Laurent Vivier <lvivier@redhat.com>
To:     linux-kernel@vger.kernel.org
Cc:     Herbert Xu <herbert@gondor.apana.org.au>,
        linux-crypto@vger.kernel.org, Matt Mackall <mpm@selenic.com>
Subject: [PATCH] hwrng: core - don't block in add_early_randomness()
Date:   Fri, 29 Mar 2019 17:30:11 +0100
Message-Id: <20190329163011.7370-1-lvivier@redhat.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Sender: linux-crypto-owner@vger.kernel.org
Precedence: bulk

If the device is not ready to provide data the kernel will
be stuck indefinitely in the init function.

This is not a problem if the device is driven using a module,
but if the driver is linked directly into the kernel then the
kernel boot sequence hangs.

This can happen with virtio-rng device with rng-egd backend
with no data provider, for instance with QEMU command line parameters:

...
    -chardev socket,id=charrng0,host=localhost,port=2345,server,nowait \
    -object rng-egd,id=objrng0,chardev=charrng0 \
    -device virtio-rng-pci,rng=objrng0,id=rng0

To avoid that, we can call rng_get_data() in non blocking mode because
the function already manages the case where byte_read is
0 (if the device is not already initialized).

See also commit d3cc7996473a
("hwrng: fetch randomness only after device init")

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 drivers/char/hw_random/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 95be7228f327..3866d6b8017c 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -67,7 +67,7 @@ static void add_early_randomness(struct hwrng *rng)
 	size_t size = min_t(size_t, 16, rng_buffer_size());
 
 	mutex_lock(&reading_mutex);
-	bytes_read = rng_get_data(rng, rng_buffer, size, 1);
+	bytes_read = rng_get_data(rng, rng_buffer, size, 0);
 	mutex_unlock(&reading_mutex);
 	if (bytes_read > 0)
 		add_device_randomness(rng_buffer, bytes_read);
-- 
2.20.1