Re: [PATCH 1/2] virtio-rng: implement entropy leak feature

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On 20/1/23 13:32, Michael S. Tsirkin wrote:
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On Thu, Jan 19, 2023 at 07:43:47PM +0100, Babis Chalios wrote:
Implement the virtio-rng feature that allows a guest driver to request
from the device to perform certain operations in the event of an
"entropy leak", such as when taking a VM snapshot or restoring a VM from
a snapshot. The guest can request one of two operations: (i) fill a
buffer with random bytes, or (ii) perform a memory copy between two
bytes.

The feature is similar to Microsoft's Virtual Machine Generation ID and
it can be used to (1) avoid the race-condition that exists in our
current VMGENID implementation, between the time vcpus are resumed and
the ACPI notification is being handled and (2) provide mechanisms for
notifying user-space about snapshot-related events.

This commit implements the protocol between guest and device.
Additionally, it makes sure there is always a request for random bytes
in the event of entropy leak in-flight. Once such an event is observed,
the driver feeds these bytes to as entropy using
`add_device_randomness`.

Keep in mind that this commit does not solve the race-condition issue,
it adds fresh entropy whenever the driver handles the used buffer from
the fill-on-leak request. In order to close the race window, we need to
expose some API so that other kernel subsystems can request directly
notifications from the device.

Signed-off-by: Babis Chalios <bchalios@xxxxxxxxx>
---
  drivers/char/hw_random/virtio-rng.c | 200 +++++++++++++++++++++++++++-
  include/uapi/linux/virtio_rng.h     |   3 +
  2 files changed, 197 insertions(+), 6 deletions(-)

diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
index a6f3a8a2aca6..389a091a8801 100644
--- a/drivers/char/hw_random/virtio-rng.c
+++ b/drivers/char/hw_random/virtio-rng.c
@@ -4,12 +4,14 @@
   *  Copyright (C) 2007, 2008 Rusty Russell IBM Corporation
   */

+#include "asm-generic/errno.h"
Why are you pulling this in?
If you really need errno the thing to include
is linux/errno.h

checkpatch has a rule to catch such cases:

# warn if <asm/foo.h> is #included and <linux/foo.h> is available and includes
# itself <asm/foo.h> (uses RAW line)

did you run checkpatch?

You are right, this is wrong. I think my LSP client pulled this in and I missed it because, strangely enough, I did ran
checkpatch but it didn't complain about it.

  #include <linux/err.h>
  #include <linux/hw_random.h>
  #include <linux/scatterlist.h>
  #include <linux/spinlock.h>
  #include <linux/virtio.h>
  #include <linux/virtio_rng.h>
+#include <linux/random.h>
  #include <linux/module.h>
  #include <linux/slab.h>

@@ -18,6 +20,12 @@ static DEFINE_IDA(rng_index_ida);
  struct virtrng_info {
       struct hwrng hwrng;
       struct virtqueue *vq;
+     /* Leak queues */
+     bool has_leakqs;
+     struct virtqueue *leakq[2];
+     spinlock_t lock;
+     int active_leakq;
+
       char name[25];
       int index;
       bool hwrng_register_done;
@@ -29,27 +37,159 @@ struct virtrng_info {
       /* minimal size returned by rng_buffer_size() */
  #if SMP_CACHE_BYTES < 32
       u8 data[32];
+     u8 leak_data[32];
  #else
       u8 data[SMP_CACHE_BYTES];
+     u8 leak_data[SMP_CACHE_BYTES];
  #endif
  };

+/* Swaps the queues and returns the new active leak queue. */
+static struct virtqueue *swap_leakqs(struct virtrng_info *vi)
+{
+     vi->active_leakq = 1 - vi->active_leakq;
+     return vi->leakq[vi->active_leakq];
+}
+
+static struct virtqueue *get_active_leakq(struct virtrng_info *vi)
+{
+     return vi->leakq[vi->active_leakq];
+}
+
+int add_fill_on_leak_request(struct virtrng_info *vi, struct virtqueue *vq, void *data, size_t len)
+{
+     struct scatterlist sg;
+     int ret;
+
+     sg_init_one(&sg, data, len);
+     ret = virtqueue_add_inbuf(vq, &sg, 1, data, GFP_KERNEL);
+     if (ret)
+             goto err;
+
+err:
+     return ret;
+}
+
+int virtrng_fill_on_leak(struct virtrng_info *vi, void *data, size_t len)
+{
+     struct virtqueue *vq;
+     unsigned long flags;
+     int ret;
+
+     if (!vi->has_leakqs)
+             return -EOPNOTSUPP;
+
+     spin_lock_irqsave(&vi->lock, flags);
+
+     vq = get_active_leakq(vi);
+     ret = add_fill_on_leak_request(vi, vq, data, len);
+     if (ret)
+             virtqueue_kick(vq);
+
+     spin_unlock_irqrestore(&vi->lock, flags);
+
+     return ret;
+}
+
+int add_copy_on_leak_request(struct virtrng_info *vi, struct virtqueue *vq,
+             void *to, void *from, size_t len)
+{
+     int ret;
+     struct scatterlist out, in, *sgs[2];
+
+     sg_init_one(&out, from, len);
+     sgs[0] = &out;
+     sg_init_one(&in, to, len);
+     sgs[1] = &in;
+
+     ret = virtqueue_add_sgs(vq, sgs, 1, 1, to, GFP_KERNEL);
+     if (ret)
+             goto err;
+
+err:
+     return ret;
+}
+
+int virtrng_copy_on_leak(struct virtrng_info *vi, void *to, void *from, size_t len)
+{
+     struct virtqueue *vq;
+     unsigned long flags;
+     int ret;
+
+     if (!vi->has_leakqs)
+             return -EOPNOTSUPP;
+
+     spin_lock_irqsave(&vi->lock, flags);
+
+     vq = get_active_leakq(vi);
+     ret = add_copy_on_leak_request(vi, vq, to, from, len);
+     if (ret)
+             virtqueue_kick(vq);
+
+     spin_unlock_irqrestore(&vi->lock, flags);
+
+     return ret;
+}
+
+static void entropy_leak_detected(struct virtqueue *vq)
+{
+     struct virtrng_info *vi = vq->vdev->priv;
+     struct virtqueue *activeq;
+     unsigned int len;
+     unsigned long flags;
+     void *buffer;
+     bool kick_activeq = false;
+
+     spin_lock_irqsave(&vi->lock, flags);
+
+     activeq = get_active_leakq(vi);
+     /* Drain all the used buffers from the queue */
+     while ((buffer = virtqueue_get_buf(vq, &len)) != NULL) {
+             if (vq == activeq) {
+                     pr_debug("%s: entropy leak detected!", vi->name);
+                     activeq = swap_leakqs(vi);
+             }
+
+             if (buffer == vi->leak_data) {
+                     add_device_randomness(vi->leak_data, sizeof(vi->leak_data));
+
+                     /* Ensure we always have a pending request for random bytes on entropy
+                      * leak. Do it here, after we have swapped leak queues, so it gets handled
+                      * with the next entropy leak event.
+                      */
+                     add_fill_on_leak_request(vi, activeq, vi->leak_data, sizeof(vi->leak_data));
+                     kick_activeq = true;
+             }
+     }
+
+     if (kick_activeq)
+             virtqueue_kick(activeq);
+
+     spin_unlock_irqrestore(&vi->lock, flags);
+}
+
  static void random_recv_done(struct virtqueue *vq)
  {
       struct virtrng_info *vi = vq->vdev->priv;
+     unsigned long flags;

+     spin_lock_irqsave(&vi->lock, flags);
       /* We can get spurious callbacks, e.g. shared IRQs + virtio_pci. */
       if (!virtqueue_get_buf(vi->vq, &vi->data_avail))
-             return;
+             goto unlock;

       vi->data_idx = 0;

       complete(&vi->have_data);
+
+unlock:
+     spin_unlock_irqrestore(&vi->lock, flags);
  }

  static void request_entropy(struct virtrng_info *vi)
  {
       struct scatterlist sg;
+     unsigned long flags;

       reinit_completion(&vi->have_data);
       vi->data_avail = 0;
@@ -57,10 +197,12 @@ static void request_entropy(struct virtrng_info *vi)

       sg_init_one(&sg, vi->data, sizeof(vi->data));

+     spin_lock_irqsave(&vi->lock, flags);
       /* There should always be room for one buffer. */
       virtqueue_add_inbuf(vi->vq, &sg, 1, vi->data, GFP_KERNEL);

       virtqueue_kick(vi->vq);
+     spin_unlock_irqrestore(&vi->lock, flags);
  }

  static unsigned int copy_data(struct virtrng_info *vi, void *buf,
@@ -126,6 +268,40 @@ static void virtio_cleanup(struct hwrng *rng)
       complete(&vi->have_data);
  }

+static int init_virtqueues(struct virtrng_info *vi, struct virtio_device *vdev)
+{
+     int ret = -ENOMEM, total_vqs = 1;
+     struct virtqueue *vqs[3];
+     const char *names[3];
+     vq_callback_t *callbacks[3];
+
+     if (vi->has_leakqs)
+             total_vqs = 3;
+
+     callbacks[0] = random_recv_done;
+     names[0] = "input";
+     if (vi->has_leakqs) {
+             callbacks[1] = entropy_leak_detected;
+             names[1] = "leakq.1";
+             callbacks[2] = entropy_leak_detected;
+             names[2] = "leakq.2";
+     }
+
+     ret = virtio_find_vqs(vdev, total_vqs, vqs, callbacks, names, NULL);
+     if (ret)
+             goto err;
+
+     vi->vq = vqs[0];
+
+     if (vi->has_leakqs) {
+             vi->leakq[0] = vqs[1];
+             vi->leakq[1] = vqs[2];
+     }
+
+err:
+     return ret;
+}
+
  static int probe_common(struct virtio_device *vdev)
  {
       int err, index;
@@ -152,18 +328,24 @@ static int probe_common(struct virtio_device *vdev)
       };
       vdev->priv = vi;

-     /* We expect a single virtqueue. */
-     vi->vq = virtio_find_single_vq(vdev, random_recv_done, "input");
-     if (IS_ERR(vi->vq)) {
-             err = PTR_ERR(vi->vq);
-             goto err_find;
+     vi->has_leakqs = virtio_has_feature(vdev, VIRTIO_RNG_F_LEAK);
+     if (vi->has_leakqs) {
+             spin_lock_init(&vi->lock);
+             vi->active_leakq = 0;
       }

+     err = init_virtqueues(vi, vdev);
+     if (err)
+             goto err_find;
+
       virtio_device_ready(vdev);

       /* we always have a pending entropy request */
       request_entropy(vi);

+     /* we always have a fill_on_leak request pending */
+     virtrng_fill_on_leak(vi, vi->leak_data, sizeof(vi->leak_data));
+
       return 0;

  err_find:
@@ -246,7 +428,13 @@ static const struct virtio_device_id id_table[] = {
       { 0 },
  };

+static unsigned int features[] = {
+     VIRTIO_RNG_F_LEAK,
+};
+
  static struct virtio_driver virtio_rng_driver = {
+     .feature_table = features,
+     .feature_table_size = ARRAY_SIZE(features),
       .driver.name =  KBUILD_MODNAME,
       .driver.owner = THIS_MODULE,
       .id_table =     id_table,
diff --git a/include/uapi/linux/virtio_rng.h b/include/uapi/linux/virtio_rng.h
index c4d5de896f0c..d9774951547e 100644
--- a/include/uapi/linux/virtio_rng.h
+++ b/include/uapi/linux/virtio_rng.h
@@ -5,4 +5,7 @@
  #include <linux/virtio_ids.h>
  #include <linux/virtio_config.h>

+/* The feature bitmap for virtio entropy device */
+#define VIRTIO_RNG_F_LEAK 0
+
  #endif /* _LINUX_VIRTIO_RNG_H */
--
2.38.1

Amazon Spain Services sociedad limitada unipersonal, Calle Ramirez de Prado 5, 28045 Madrid. Registro Mercantil de Madrid . Tomo 22458 . Folio 102 . Hoja M-401234 . CIF B84570936

Amazon Spain Services sociedad limitada unipersonal, Calle Ramirez de Prado 5, 28045 Madrid. Registro Mercantil de Madrid . Tomo 22458 . Folio 102 . Hoja M-401234 . CIF B84570936




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