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Re: [Qemu-devel] [PATCH] virtio: destroy region cache during reset

From: Jason Wang
Subject: Re: [Qemu-devel] [PATCH] virtio: destroy region cache during reset
Date: Thu, 9 Mar 2017 10:19:47 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0

On 2017年03月08日 18:12, Cornelia Huck wrote:
On Wed, 8 Mar 2017 17:51:22 +0800
Jason Wang <address@hidden> wrote:

On 2017年03月08日 17:19, Cornelia Huck wrote:
On Wed, 8 Mar 2017 11:18:27 +0800
Jason Wang <address@hidden> wrote:

On 2017年03月07日 18:16, Cornelia Huck wrote:
On Tue,  7 Mar 2017 16:47:58 +0800
Jason Wang <address@hidden> wrote:

We don't destroy region cache during reset which can make the maps
of previous driver leaked to a buggy or malicious driver that don't
set vring address before starting to use the device. Fix this by
destroy the region cache during reset and validate it before trying to
use them. While at it, also validate address_space_cache_init() during
virtio_init_region_cache() to make sure we have a correct region

Signed-off-by: Jason Wang <address@hidden>
    hw/virtio/virtio.c | 88 
    1 file changed, 76 insertions(+), 12 deletions(-)
@@ -190,6 +211,10 @@ static inline uint16_t vring_avail_flags(VirtQueue *vq)
        VRingMemoryRegionCaches *caches = atomic_rcu_read(&vq->vring.caches);
        hwaddr pa = offsetof(VRingAvail, flags);
+    if (!caches) {
+        virtio_error(vq->vdev, "Cannot map avail flags");
I'm not sure that virtio_error is the right thing here; ending up in
this function with !caches indicates an error in our logic.
Probably not, this can be triggered by buggy guest.
I would think that even a buggy guest should not be able to trigger
accesses to vring structures that have not yet been set up. What am I
I think this may happen if a driver start the device without setting the
vring address. In this case, region caches cache the mapping of previous
driver. But maybe I was wrong.
So the sequence would be:

- Driver #1 sets up the device, then abandons it without cleaning up
via reset

If driver #1 reset the device in this case, the caches would be NULL.

- Driver #2 uses the device without doing a reset or proper setup

Without this patch, even if driver #2 do a reset, it can still use the old map if it don't set queue pfn.


I can't quite see why caches would be NULL in that case...

Having reset clean up the caches as well (IOW, the other part of your
patch) should make sure that we always have a consistent view of
descriptors and their caches,

And prevent it from being leaked to untrusted drivers.


I think. The checks for desc != NULL and
friends should catch other driver buggyness.

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