Re: [PATCH V5 1/4] virtio_ring: validate used buffer length

[Michael Ellerman]

"Michael S. Tsirkin" <mst@xxxxxxxxxx> writes:
> On Tue, Nov 23, 2021 at 10:25:20AM +0800, Jason Wang wrote:
>> On Tue, Nov 23, 2021 at 4:24 AM Halil Pasic <pasic@xxxxxxxxxxxxx> wrote:
>> >
>> > On Mon, 22 Nov 2021 14:25:26 +0800
>> > Jason Wang <jasowang@xxxxxxxxxx> wrote:
>> >
>> > > I think the fixes are:
>> > >
>> > > 1) fixing the vhost vsock
>> > > 2) use suppress_used_validation=true to let vsock driver to validate
>> > > the in buffer length
>> > > 3) probably a new feature so the driver can only enable the validation
>> > > when the feature is enabled.
>> >
>> > I'm not sure, I would consider a F_DEV_Y_FIXED_BUG_X a perfectly good
>> > feature. Frankly the set of such bugs is device implementation
>> > specific and it makes little sense to specify a feature bit
>> > that says the device implementation claims to adhere to some
>> > aspect of the specification. Also what would be the semantic
>> > of not negotiating F_DEV_Y_FIXED_BUG_X?
>> 
>> Yes, I agree. Rethink of the feature bit, it seems unnecessary,
>> especially considering the driver should not care about the used
>> length for tx.
>> 
>> >
>> > On the other hand I see no other way to keep the validation
>> > permanently enabled for fixed implementations, and get around the problem
>> > with broken implementations. So we could have something like
>> > VHOST_USED_LEN_STRICT.
>> 
>> It's more about a choice of the driver's knowledge. For vsock TX it
>> should be fine. If we introduce a parameter and disable it by default,
>> it won't be very useful.
>> 
>> >
>> > Maybe, we can also think of 'warn and don't alter behavior' instead of
>> > 'warn' and alter behavior. Or maybe even not having such checks on in
>> > production, but only when testing.
>> 
>> I think there's an agreement that virtio drivers need more hardening,
>> that's why a lot of patches were merged. Especially considering the
>> new requirements came from confidential computing, smart NIC and
>> VDUSE. For virtio drivers, enabling the validation may help to
>> 
>> 1) protect the driver from the buggy and malicious device
>> 2) uncover the bugs of the devices (as vsock did, and probably rpmsg)
>> 3) force the have a smart driver that can do the validation itself
>> then we can finally remove the validation in the core
>> 
>> So I'd like to keep it enabled.
>> 
>> Thanks
>
> Let's see how far we can get. But yes, maybe we were too aggressive in
> breaking things by default, a warning might be a better choice for a
> couple of cycles.

This series appears to break the virtio_balloon driver as well.

The symptom is soft lockup warnings, eg:

  INFO: task kworker/1:1:109 blocked for more than 614 seconds.
        Not tainted 5.16.0-rc2-gcc-10.3.0 #21
  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  task:kworker/1:1     state:D stack:12496 pid:  109 ppid:     2 flags:0x00000800
  Workqueue: events_freezable update_balloon_size_func
  Call Trace:
  [c000000003cef7c0] [c000000003cef820] 0xc000000003cef820 (unreliable)
  [c000000003cef9b0] [c00000000001e238] __switch_to+0x1e8/0x2f0
  [c000000003cefa10] [c000000000f0a00c] __schedule+0x2cc/0xb50
  [c000000003cefae0] [c000000000f0a8fc] schedule+0x6c/0x140
  [c000000003cefb10] [c00000000095b6c4] tell_host+0xe4/0x130
  [c000000003cefba0] [c00000000095d234] update_balloon_size_func+0x394/0x3f0
  [c000000003cefc70] [c000000000178064] process_one_work+0x2c4/0x5b0
  [c000000003cefd10] [c0000000001783f8] worker_thread+0xa8/0x640
  [c000000003cefda0] [c000000000185444] kthread+0x1b4/0x1c0
  [c000000003cefe10] [c00000000000cee4] ret_from_kernel_thread+0x5c/0x64

Similar backtrace reported here by Luis:

  https://lore.kernel.org/lkml/YY2duTi0wAyAKUTJ@xxxxxxxxxxxxxxxxxxxxxx/

Bisect points to:

  # first bad commit: [939779f5152d161b34f612af29e7dc1ac4472fcf] virtio_ring: validate used buffer length

Adding suppress used validation to the virtio balloon driver "fixes" it, eg.

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index c22ff0117b46..a14b82ceebb2 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -1150,6 +1150,7 @@ static unsigned int features[] = {
 };
 
 static struct virtio_driver virtio_balloon_driver = {
+	.suppress_used_validation = true,
 	.feature_table = features,
 	.feature_table_size = ARRAY_SIZE(features),
 	.driver.name =	KBUILD_MODNAME,


cheers