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Re: [PATCH] vhost-user: Check for iotlb callback in iotlb_miss

From: Jason Wang
Subject: Re: [PATCH] vhost-user: Check for iotlb callback in iotlb_miss
Date: Fri, 29 Jan 2021 15:29:07 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 2021/1/29 下午3:22, Eugenio Perez Martin wrote:
On Fri, Jan 29, 2021 at 4:29 AM Jason Wang <jasowang@redhat.com> wrote:

On 2021/1/28 下午5:37, Eugenio Perez Martin wrote:
Hi Jason.

On Thu, Jan 28, 2021 at 3:32 AM Jason Wang <jasowang@redhat.com> wrote:
On 2021/1/28 上午4:44, Eugenio Pérez wrote:
Not registering this can lead to vhost_backend_handle_iotlb_msg and
vhost_device_iotlb_miss if backend issue a miss after qemu vhost device

This causes a try to access dev->vdev->dma_as with vdev == NULL.
Hi Eugenio:

What condition can we get this? Did you mean we receive IOTLB_MISS
before vhost_dev_start()?

In the VM reboot case, yes, between vhost_dev_stop() and
vhost_dev_start(). But I can reproduce the bug in VM shutdown too,
with no vhost_dev_start expected.

I didn't try to send IOTLB_MISS before starting vhost_dev, but this
patch should solve that problem too.

I think we can get this with whatever malicious/buggy vhost-user
backend sending unexpected iotlb misses, but I didn't test so deeply.

I see.

If yes, it looks to me a bug somewhere else.
Where should I look for it?

So I winder whether or not we can simply ignore IOTLB message if vhost
device is not started.

Do you mean like an early return in vhost_device_iotlb_miss?

Yes or probably a little bit earlier in vhost_backend_handle_iotlb_msg() which somehow a warn there.

Anyhow it's meaningless to process IOTLB message in this case and we don't need to introduce a dedicated variable for this.


That was
my first first option, but this seems cleaner to me. I'm ok with both

Or do you mean not to return -EFAULT but 0 if !u->iotlb_enabled ?





Reproduced rebooting a guest with testpmd in txonly forward mode.
    #0  0x0000559ffff94394 in vhost_device_iotlb_miss (
        dev=dev@entry=0x55a0012f6680, iova=10245279744, write=1)
        at ../hw/virtio/vhost.c:1013
    #1  0x0000559ffff9ac31 in vhost_backend_handle_iotlb_msg (
        imsg=0x7ffddcfd32c0, dev=0x55a0012f6680)
        at ../hw/virtio/vhost-backend.c:411
    #2  vhost_backend_handle_iotlb_msg (dev=dev@entry=0x55a0012f6680,
        at ../hw/virtio/vhost-backend.c:404
    #3  0x0000559fffeded7b in slave_read (opaque=0x55a0012f6680)
        at ../hw/virtio/vhost-user.c:1464
    #4  0x000055a0000c541b in aio_dispatch_handler (
        ctx=ctx@entry=0x55a0010a2120, node=0x55a0012d9e00)
        at ../util/aio-posix.c:329

Fixes: 6dcdd06e3b ("spec/vhost-user spec: Add IOMMU support")
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
    hw/virtio/vhost-user.c | 10 ++++++++--
    1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
index 2fdd5daf74..a49b2229fb 100644
--- a/hw/virtio/vhost-user.c
+++ b/hw/virtio/vhost-user.c
@@ -238,6 +238,7 @@ struct vhost_user {
        /* Shared between vhost devs of the same virtio device */
        VhostUserState *user;
        int slave_fd;
+    bool iotlb_enabled;
        NotifierWithReturn postcopy_notifier;
        struct PostCopyFD  postcopy_fd;
        uint64_t           postcopy_client_bases[VHOST_USER_MAX_RAM_SLOTS];
@@ -1461,7 +1462,11 @@ static void slave_read(void *opaque)

        switch (hdr.request) {
-        ret = vhost_backend_handle_iotlb_msg(dev, &payload.iotlb);
+        if (likely(u->iotlb_enabled)) {
+            ret = vhost_backend_handle_iotlb_msg(dev, &payload.iotlb);
+        } else {
+            ret = -EFAULT;
+        }
            ret = vhost_user_slave_handle_config_change(dev);
@@ -2044,7 +2049,8 @@ static int vhost_user_send_device_iotlb_msg(struct 
vhost_dev *dev,

    static void vhost_user_set_iotlb_callback(struct vhost_dev *dev, int 
-    /* No-op as the receive channel is not dedicated to IOTLB messages. */
+    struct vhost_user *u = dev->opaque;
+    u->iotlb_enabled = enabled;

    static int vhost_user_get_config(struct vhost_dev *dev, uint8_t *config,

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