A smp_rmb() has been missed in vhost_enable_notify(), inspired by
Will Deacon <will@kernel.org>. Otherwise, it's not ensured the
available ring entries pushed by guest can be observed by vhost
in time, leading to stale available ring entries fetched by vhost
in vhost_get_vq_desc(), as reported by Yihuang Yu on NVidia's
grace-hopper (ARM64) platform.
/home/gavin/sandbox/qemu.main/build/qemu-system-aarch64 \
-accel kvm -machine virt,gic-version=host -cpu host \
-smp maxcpus=1,cpus=1,sockets=1,clusters=1,cores=1,threads=1 \
-m 4096M,slots=16,maxmem=64G \
-object memory-backend-ram,id=mem0,size=4096M \
: \
-netdev tap,id=vnet0,vhost=true \
-device virtio-net-pci,bus=pcie.8,netdev=vnet0,mac=52:54:00:f1:26:b0
:
guest# netperf -H 10.26.1.81 -l 60 -C -c -t UDP_STREAM
virtio_net virtio0: output.0:id 100 is not a head!
Add the missed smp_rmb() in vhost_enable_notify(). Note that it
should be safe until vq->avail_idx is changed by commit d3bb267bbdcb
("vhost: cache avail index in vhost_enable_notify()").
Fixes: d3bb267bbdcb ("vhost: cache avail index in vhost_enable_notify()")
Cc: <stable@kernel.org> # v5.18+
Reported-by: Yihuang Yu <yihyu@redhat.com>
Signed-off-by: Gavin Shan <gshan@redhat.com>
---
drivers/vhost/vhost.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 00445ab172b3..58f9d6a435f0 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -2847,9 +2847,18 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
&vq->avail->idx, r);
return false;
}
+
vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
+ if (vq->avail_idx != vq->last_avail_idx) {
+ /* Similar to what's done in vhost_get_vq_desc(), we need
+ * to ensure the available ring entries have been exposed
+ * by guest.
+ */
+ smp_rmb();
+ return true;
+ }
- return vq->avail_idx != vq->last_avail_idx;
+ return false;
}
EXPORT_SYMBOL_GPL(vhost_enable_notify);
--
2.44.0On Wed, Mar 27, 2024 at 7:39 AM Gavin Shan <gshan@redhat.com> wrote:
>
> A smp_rmb() has been missed in vhost_enable_notify(), inspired by
> Will Deacon <will@kernel.org>. Otherwise, it's not ensured the
> available ring entries pushed by guest can be observed by vhost
> in time, leading to stale available ring entries fetched by vhost
> in vhost_get_vq_desc(), as reported by Yihuang Yu on NVidia's
> grace-hopper (ARM64) platform.
>
> /home/gavin/sandbox/qemu.main/build/qemu-system-aarch64 \
> -accel kvm -machine virt,gic-version=host -cpu host \
> -smp maxcpus=1,cpus=1,sockets=1,clusters=1,cores=1,threads=1 \
> -m 4096M,slots=16,maxmem=64G \
> -object memory-backend-ram,id=mem0,size=4096M \
> : \
> -netdev tap,id=vnet0,vhost=true \
> -device virtio-net-pci,bus=pcie.8,netdev=vnet0,mac=52:54:00:f1:26:b0
> :
> guest# netperf -H 10.26.1.81 -l 60 -C -c -t UDP_STREAM
> virtio_net virtio0: output.0:id 100 is not a head!
>
> Add the missed smp_rmb() in vhost_enable_notify(). Note that it
> should be safe until vq->avail_idx is changed by commit d3bb267bbdcb
> ("vhost: cache avail index in vhost_enable_notify()").
>
> Fixes: d3bb267bbdcb ("vhost: cache avail index in vhost_enable_notify()")
> Cc: <stable@kernel.org> # v5.18+
> Reported-by: Yihuang Yu <yihyu@redhat.com>
> Signed-off-by: Gavin Shan <gshan@redhat.com>
> ---
> drivers/vhost/vhost.c | 11 ++++++++++-
> 1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 00445ab172b3..58f9d6a435f0 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2847,9 +2847,18 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
> &vq->avail->idx, r);
> return false;
> }
> +
> vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
> + if (vq->avail_idx != vq->last_avail_idx) {
> + /* Similar to what's done in vhost_get_vq_desc(), we need
> + * to ensure the available ring entries have been exposed
> + * by guest.
> + */
> + smp_rmb();
> + return true;
> + }
>
> - return vq->avail_idx != vq->last_avail_idx;
> + return false;
So we only care about the case when vhost_enable_notify() returns true.
In that case, I think you want to order with vhost_get_vq_desc():
last_avail_idx = vq->last_avail_idx;
if (vq->avail_idx == vq->last_avail_idx) { /* false */
}
vhost_get_avail_head(vq, &ring_head, last_avail_idx)
Assuming I understand the patch correctly.
Acked-by: Jason Wang <jasowang@redhat.com>
Thanks
> }
> EXPORT_SYMBOL_GPL(vhost_enable_notify);
>
> --
> 2.44.0
>
On 3/27/24 12:41, Jason Wang wrote:
> On Wed, Mar 27, 2024 at 7:39 AM Gavin Shan <gshan@redhat.com> wrote:
>>
>> A smp_rmb() has been missed in vhost_enable_notify(), inspired by
>> Will Deacon <will@kernel.org>. Otherwise, it's not ensured the
>> available ring entries pushed by guest can be observed by vhost
>> in time, leading to stale available ring entries fetched by vhost
>> in vhost_get_vq_desc(), as reported by Yihuang Yu on NVidia's
>> grace-hopper (ARM64) platform.
>>
>> /home/gavin/sandbox/qemu.main/build/qemu-system-aarch64 \
>> -accel kvm -machine virt,gic-version=host -cpu host \
>> -smp maxcpus=1,cpus=1,sockets=1,clusters=1,cores=1,threads=1 \
>> -m 4096M,slots=16,maxmem=64G \
>> -object memory-backend-ram,id=mem0,size=4096M \
>> : \
>> -netdev tap,id=vnet0,vhost=true \
>> -device virtio-net-pci,bus=pcie.8,netdev=vnet0,mac=52:54:00:f1:26:b0
>> :
>> guest# netperf -H 10.26.1.81 -l 60 -C -c -t UDP_STREAM
>> virtio_net virtio0: output.0:id 100 is not a head!
>>
>> Add the missed smp_rmb() in vhost_enable_notify(). Note that it
>> should be safe until vq->avail_idx is changed by commit d3bb267bbdcb
>> ("vhost: cache avail index in vhost_enable_notify()").
>>
>> Fixes: d3bb267bbdcb ("vhost: cache avail index in vhost_enable_notify()")
>> Cc: <stable@kernel.org> # v5.18+
>> Reported-by: Yihuang Yu <yihyu@redhat.com>
>> Signed-off-by: Gavin Shan <gshan@redhat.com>
>> ---
>> drivers/vhost/vhost.c | 11 ++++++++++-
>> 1 file changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index 00445ab172b3..58f9d6a435f0 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -2847,9 +2847,18 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>> &vq->avail->idx, r);
>> return false;
>> }
>> +
>> vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
>> + if (vq->avail_idx != vq->last_avail_idx) {
>> + /* Similar to what's done in vhost_get_vq_desc(), we need
>> + * to ensure the available ring entries have been exposed
>> + * by guest.
>> + */
>> + smp_rmb();
>> + return true;
>> + }
>>
>> - return vq->avail_idx != vq->last_avail_idx;
>> + return false;
>
> So we only care about the case when vhost_enable_notify() returns true.
>
> In that case, I think you want to order with vhost_get_vq_desc():
>
> last_avail_idx = vq->last_avail_idx;
>
> if (vq->avail_idx == vq->last_avail_idx) { /* false */
> }
>
> vhost_get_avail_head(vq, &ring_head, last_avail_idx)
>
> Assuming I understand the patch correctly.
>
> Acked-by: Jason Wang <jasowang@redhat.com>
>
Jason, thanks for your review and comments. Your understanding is exactly
what I understood.
>
>> }
>> EXPORT_SYMBOL_GPL(vhost_enable_notify);
>>
Thanks,
Gavin
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