Linux Virtualization - Jul 2014 - [PATCH v2 1/2] hwrng: fetch randomness only after device init

v2:
 - this now separates both the patches; the virtio-rng fix is self-contained
 - re-work hwrng core to fetch randomness at device init time if
   ->init() is registered by the device, instead of not calling it at all.
 - virtio-rng: introduce a probe_done bool to ensure we don't ask host
   for data before successful probe

Hi,

When booting a recent kernel under KVM with the virtio-rng device
enabled, the boot process was stalling.  Bisect pointed to a commit
made during the 3.15 window to fetch randomness from newly-registered
devices in the hwrng core.  The details are in the patches.

Turns out there were two bugs: the initial randomness was being
fetched w/o the device being initialized in cases where the init
callback was registered and the device wasn't the first device being
added to the hwrng core.  The second bug is virtio can't communicate
with the host without the device probe is successfully completed.

Please review and apply if appropriate,

Amit Shah (2):
  hwrng: fetch randomness only after device init
  virtio: rng: ensure reads happen after successful probe

 drivers/char/hw_random/core.c       | 37 +++++++++++++++++++++++++++++++------
 drivers/char/hw_random/virtio-rng.c | 10 ++++++++++
 2 files changed, 41 insertions(+), 6 deletions(-)

-- 
1.9.3

Commit d9e7972619334 "hwrng: add randomness to system from rng
sources"
added a call to rng_get_data() from the hwrng_register() function.
However, some rng devices need initialization before data can be read
from them.

This commit makes the call to rng_get_data() depend on no init fn
pointer being registered by the device.  If an init function is
registered, this call is made after device init.

CC: Kees Cook <keescook at chromium.org>
CC: Jason Cooper <jason at lakedaemon.net>
CC: Herbert Xu <herbert at gondor.apana.org.au>
CC: <stable at vger.kernel.org> # For v3.15+
Signed-off-by: Amit Shah <amit.shah at redhat.com>
---
 drivers/char/hw_random/core.c | 31 +++++++++++++++++++++++++------
 1 file changed, 25 insertions(+), 6 deletions(-)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 334601c..df95e2f 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -55,16 +55,37 @@ static DEFINE_MUTEX(rng_mutex);
 static int data_avail;
 static u8 *rng_buffer;
 
+static inline int rng_get_data(struct hwrng *rng, u8 *buffer, size_t size,
+			       int wait);
+
 static size_t rng_buffer_size(void)
 {
 	return SMP_CACHE_BYTES < 32 ? 32 : SMP_CACHE_BYTES;
 }
 
+static void add_early_randomness(struct hwrng *rng)
+{
+	unsigned char bytes[16];
+	int bytes_read;
+
+	bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
+	if (bytes_read > 0)
+		add_device_randomness(bytes, bytes_read);
+}
+
 static inline int hwrng_init(struct hwrng *rng)
 {
+	int ret;
+
 	if (!rng->init)
 		return 0;
-	return rng->init(rng);
+
+	ret =  rng->init(rng);
+	if (ret)
+		return ret;
+
+	add_early_randomness(rng);
+	return ret;
 }
 
 static inline void hwrng_cleanup(struct hwrng *rng)
@@ -304,8 +325,6 @@ int hwrng_register(struct hwrng *rng)
 {
 	int err = -EINVAL;
 	struct hwrng *old_rng, *tmp;
-	unsigned char bytes[16];
-	int bytes_read;
 
 	if (rng->name == NULL ||
 	    (rng->data_read == NULL && rng->read == NULL))
@@ -347,9 +366,9 @@ int hwrng_register(struct hwrng *rng)
 	INIT_LIST_HEAD(&rng->list);
 	list_add_tail(&rng->list, &rng_list);
 
-	bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
-	if (bytes_read > 0)
-		add_device_randomness(bytes, bytes_read);
+	if (!rng->init)
+		add_early_randomness(rng);
+
 out_unlock:
 	mutex_unlock(&rng_mutex);
 out:
-- 
1.9.3

The hwrng core asks for random data in the hwrng_register() call itself
from commit d9e7972619.  This doesn't play well with virtio -- the
DRIVER_OK bit is only set by virtio core on a successful probe, and
we're not yet out of our probe routine when this call is made.  This
causes the host to not acknowledge any requests we put in the virtqueue,
and the insmod or kernel boot process just waits for data to arrive from
the host, which never happens.

CC: Kees Cook <keescook at chromium.org>
CC: Jason Cooper <jason at lakedaemon.net>
CC: Herbert Xu <herbert at gondor.apana.org.au>
CC: <stable at vger.kernel.org> # For v3.15+
Signed-off-by: Amit Shah <amit.shah at redhat.com>
---
 drivers/char/hw_random/core.c       |  6 ++++++
 drivers/char/hw_random/virtio-rng.c | 10 ++++++++++
 2 files changed, 16 insertions(+)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index df95e2f..50f5b76 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -68,6 +68,12 @@ static void add_early_randomness(struct hwrng *rng)
 	unsigned char bytes[16];
 	int bytes_read;
 
+	/*
+	 * Currently only virtio-rng cannot return data during device
+	 * probe, and that's handled in virtio-rng.c itself.  If there
+	 * are more such devices, this call to rng_get_data can be
+	 * made conditional here instead of doing it per-device.
+	 */
 	bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
 	if (bytes_read > 0)
 		add_device_randomness(bytes, bytes_read);
diff --git a/drivers/char/hw_random/virtio-rng.c
b/drivers/char/hw_random/virtio-rng.c
index f3e7150..157f27c 100644
--- a/drivers/char/hw_random/virtio-rng.c
+++ b/drivers/char/hw_random/virtio-rng.c
@@ -38,6 +38,8 @@ struct virtrng_info {
 	int index;
 };
 
+bool probe_done;
+
 static void random_recv_done(struct virtqueue *vq)
 {
 	struct virtrng_info *vi = vq->vdev->priv;
@@ -67,6 +69,13 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t
size, bool wait)
 	int ret;
 	struct virtrng_info *vi = (struct virtrng_info *)rng->priv;
 
+	/*
+	 * Don't ask host for data till we're setup.  This call can
+	 * happen during hwrng_register(), after commit d9e7972619.
+	 */
+	if (unlikely(!probe_done))
+		return 0;
+
 	if (!vi->busy) {
 		vi->busy = true;
 		init_completion(&vi->have_data);
@@ -137,6 +146,7 @@ static int probe_common(struct virtio_device *vdev)
 		return err;
 	}
 
+	probe_done = true;
 	return 0;
 }
 
-- 
1.9.3

On Fri, Jul 4, 2014 at 10:34 PM, Amit Shah <amit.shah at redhat.com>
wrote:
> The hwrng core asks for random data in the hwrng_register() call itself
> from commit d9e7972619.  This doesn't play well with virtio -- the
> DRIVER_OK bit is only set by virtio core on a successful probe, and
> we're not yet out of our probe routine when this call is made.  This
> causes the host to not acknowledge any requests we put in the virtqueue,
> and the insmod or kernel boot process just waits for data to arrive from
> the host, which never happens.
Doesn't this mean that virtio-rng won't ever contribute entropy to the
system?

-Kees
>
> CC: Kees Cook <keescook at chromium.org>
> CC: Jason Cooper <jason at lakedaemon.net>
> CC: Herbert Xu <herbert at gondor.apana.org.au>
> CC: <stable at vger.kernel.org> # For v3.15+
> Signed-off-by: Amit Shah <amit.shah at redhat.com>
> ---
>  drivers/char/hw_random/core.c       |  6 ++++++
>  drivers/char/hw_random/virtio-rng.c | 10 ++++++++++
>  2 files changed, 16 insertions(+)
>
> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> index df95e2f..50f5b76 100644
> --- a/drivers/char/hw_random/core.c
> +++ b/drivers/char/hw_random/core.c
> @@ -68,6 +68,12 @@ static void add_early_randomness(struct hwrng *rng)
>         unsigned char bytes[16];
>         int bytes_read;
>
> +       /*
> +        * Currently only virtio-rng cannot return data during device
> +        * probe, and that's handled in virtio-rng.c itself.  If there
> +        * are more such devices, this call to rng_get_data can be
> +        * made conditional here instead of doing it per-device.
> +        */
>         bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
>         if (bytes_read > 0)
>                 add_device_randomness(bytes, bytes_read);
> diff --git a/drivers/char/hw_random/virtio-rng.c
b/drivers/char/hw_random/virtio-rng.c
> index f3e7150..157f27c 100644
> --- a/drivers/char/hw_random/virtio-rng.c
> +++ b/drivers/char/hw_random/virtio-rng.c
> @@ -38,6 +38,8 @@ struct virtrng_info {
>         int index;
>  };
>
> +bool probe_done;
> +
>  static void random_recv_done(struct virtqueue *vq)
>  {
>         struct virtrng_info *vi = vq->vdev->priv;
> @@ -67,6 +69,13 @@ static int virtio_read(struct hwrng *rng, void *buf,
size_t size, bool wait)
>         int ret;
>         struct virtrng_info *vi = (struct virtrng_info *)rng->priv;
>
> +       /*
> +        * Don't ask host for data till we're setup.  This call can
> +        * happen during hwrng_register(), after commit d9e7972619.
> +        */
> +       if (unlikely(!probe_done))
> +               return 0;
> +
>         if (!vi->busy) {
>                 vi->busy = true;
>                 init_completion(&vi->have_data);
> @@ -137,6 +146,7 @@ static int probe_common(struct virtio_device *vdev)
>                 return err;
>         }
>
> +       probe_done = true;
>         return 0;
>  }
>
> --
> 1.9.3
>


-- 
Kees Cook
Chrome OS Security

On Fri, Jul 4, 2014 at 10:34 PM, Amit Shah <amit.shah at redhat.com>
wrote:
> Commit d9e7972619334 "hwrng: add randomness to system from rng
sources"
> added a call to rng_get_data() from the hwrng_register() function.
> However, some rng devices need initialization before data can be read
> from them.
>
> This commit makes the call to rng_get_data() depend on no init fn
> pointer being registered by the device.  If an init function is
> registered, this call is made after device init.
Thanks, this seems pretty reasonable. One side-effect is that cycling
between hwrngs via sysfs (when they have init functions) will cause
them to add more entropy. I don't think this is a problem, but it is
kind of a weird side-effect.

Reviewed-by: Kees Cook <keescook at chromium.org>

-Kees
>
> CC: Kees Cook <keescook at chromium.org>
> CC: Jason Cooper <jason at lakedaemon.net>
> CC: Herbert Xu <herbert at gondor.apana.org.au>
> CC: <stable at vger.kernel.org> # For v3.15+
> Signed-off-by: Amit Shah <amit.shah at redhat.com>
> ---
>  drivers/char/hw_random/core.c | 31 +++++++++++++++++++++++++------
>  1 file changed, 25 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> index 334601c..df95e2f 100644
> --- a/drivers/char/hw_random/core.c
> +++ b/drivers/char/hw_random/core.c
> @@ -55,16 +55,37 @@ static DEFINE_MUTEX(rng_mutex);
>  static int data_avail;
>  static u8 *rng_buffer;
>
> +static inline int rng_get_data(struct hwrng *rng, u8 *buffer, size_t size,
> +                              int wait);
> +
>  static size_t rng_buffer_size(void)
>  {
>         return SMP_CACHE_BYTES < 32 ? 32 : SMP_CACHE_BYTES;
>  }
>
> +static void add_early_randomness(struct hwrng *rng)
> +{
> +       unsigned char bytes[16];
> +       int bytes_read;
> +
> +       bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
> +       if (bytes_read > 0)
> +               add_device_randomness(bytes, bytes_read);
> +}
> +
>  static inline int hwrng_init(struct hwrng *rng)
>  {
> +       int ret;
> +
>         if (!rng->init)
>                 return 0;
> -       return rng->init(rng);
> +
> +       ret =  rng->init(rng);
> +       if (ret)
> +               return ret;
> +
> +       add_early_randomness(rng);
> +       return ret;
>  }
>
>  static inline void hwrng_cleanup(struct hwrng *rng)
> @@ -304,8 +325,6 @@ int hwrng_register(struct hwrng *rng)
>  {
>         int err = -EINVAL;
>         struct hwrng *old_rng, *tmp;
> -       unsigned char bytes[16];
> -       int bytes_read;
>
>         if (rng->name == NULL ||
>             (rng->data_read == NULL && rng->read == NULL))
> @@ -347,9 +366,9 @@ int hwrng_register(struct hwrng *rng)
>         INIT_LIST_HEAD(&rng->list);
>         list_add_tail(&rng->list, &rng_list);
>
> -       bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
> -       if (bytes_read > 0)
> -               add_device_randomness(bytes, bytes_read);
> +       if (!rng->init)
> +               add_early_randomness(rng);
> +
>  out_unlock:
>         mutex_unlock(&rng_mutex);
>  out:
> --
> 1.9.3
>


-- 
Kees Cook
Chrome OS Security

On Sat, Jul 05, 2014 at 11:04:52AM +0530, Amit Shah
wrote:
> Commit d9e7972619334 "hwrng: add randomness to system from rng
sources"
> added a call to rng_get_data() from the hwrng_register() function.
> However, some rng devices need initialization before data can be read
> from them.
> 
> This commit makes the call to rng_get_data() depend on no init fn
> pointer being registered by the device.  If an init function is
> registered, this call is made after device init.
> 
> CC: Kees Cook <keescook at chromium.org>
> CC: Jason Cooper <jason at lakedaemon.net>
> CC: Herbert Xu <herbert at gondor.apana.org.au>
> CC: <stable at vger.kernel.org> # For v3.15+
> Signed-off-by: Amit Shah <amit.shah at redhat.com>
> ---
>  drivers/char/hw_random/core.c | 31 +++++++++++++++++++++++++------
>  1 file changed, 25 insertions(+), 6 deletions(-)
Thanks for cleaning this up!

Reviewed-by: Jason Cooper <jason at lakedaemon.net>

thx,

Jason.

On Sat, Jul 05, 2014 at 11:04:53AM +0530, Amit Shah
wrote:
> The hwrng core asks for random data in the hwrng_register() call itself
> from commit d9e7972619.  This doesn't play well with virtio -- the
> DRIVER_OK bit is only set by virtio core on a successful probe, and
> we're not yet out of our probe routine when this call is made.  This
> causes the host to not acknowledge any requests we put in the virtqueue,
> and the insmod or kernel boot process just waits for data to arrive from
> the host, which never happens.
> 
> CC: Kees Cook <keescook at chromium.org>
> CC: Jason Cooper <jason at lakedaemon.net>
> CC: Herbert Xu <herbert at gondor.apana.org.au>
> CC: <stable at vger.kernel.org> # For v3.15+
> Signed-off-by: Amit Shah <amit.shah at redhat.com>
> ---
>  drivers/char/hw_random/core.c       |  6 ++++++
>  drivers/char/hw_random/virtio-rng.c | 10 ++++++++++
>  2 files changed, 16 insertions(+)
Yeah, I don't think there's any viable way to get random data out of
virtio-rng at probe time...  :-(

Reviewed-by: Jason Cooper <jason at lakedaemon.net>
> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> index df95e2f..50f5b76 100644
> --- a/drivers/char/hw_random/core.c
> +++ b/drivers/char/hw_random/core.c
> @@ -68,6 +68,12 @@ static void add_early_randomness(struct hwrng *rng)
>  	unsigned char bytes[16];
>  	int bytes_read;
>  
> +	/*
> +	 * Currently only virtio-rng cannot return data during device
> +	 * probe, and that's handled in virtio-rng.c itself.  If there
> +	 * are more such devices, this call to rng_get_data can be
> +	 * made conditional here instead of doing it per-device.
> +	 */
>  	bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
>  	if (bytes_read > 0)
>  		add_device_randomness(bytes, bytes_read);
> diff --git a/drivers/char/hw_random/virtio-rng.c
b/drivers/char/hw_random/virtio-rng.c
> index f3e7150..157f27c 100644
> --- a/drivers/char/hw_random/virtio-rng.c
> +++ b/drivers/char/hw_random/virtio-rng.c
> @@ -38,6 +38,8 @@ struct virtrng_info {
>  	int index;
>  };
>  
> +bool probe_done;
> +
>  static void random_recv_done(struct virtqueue *vq)
>  {
>  	struct virtrng_info *vi = vq->vdev->priv;
> @@ -67,6 +69,13 @@ static int virtio_read(struct hwrng *rng, void *buf,
size_t size, bool wait)
>  	int ret;
>  	struct virtrng_info *vi = (struct virtrng_info *)rng->priv;
>  
> +	/*
> +	 * Don't ask host for data till we're setup.  This call can
> +	 * happen during hwrng_register(), after commit d9e7972619.
nit: There's no need to reference the commit in the comment, you already
mention it in the commit log.

thx,

Jason.
> +	 */
> +	if (unlikely(!probe_done))
> +		return 0;
> +
>  	if (!vi->busy) {
>  		vi->busy = true;
>  		init_completion(&vi->have_data);
> @@ -137,6 +146,7 @@ static int probe_common(struct virtio_device *vdev)
>  		return err;
>  	}
>  
> +	probe_done = true;
>  	return 0;
>  }
>  
> -- 
> 1.9.3
>