Re: [virtio] [OASIS Issue Tracker] Created: (VIRTIO-35) race condition with multi-dword config accesses

[Rusty Russell <rusty@au1.ibm.com>]

OASIS Issues Tracker <workgroup_mailer@lists.oasis-open.org> writes:
> race condition with multi-dword config accesses
> -----------------------------------------------
>
>                  Key: VIRTIO-35
>                  URL: http://tools.oasis-open.org/issues/browse/VIRTIO-35
>              Project: OASIS Virtual I/O Device (VIRTIO) TC
>           Issue Type: Bug
>             Reporter: Michael Tsirkin
>
>
> on many architectures, accesses larger than 32 bit can not be atomic.
> Thus access to a device config field of >4 bytes is inherently racy
> in case field can change.
>
> For example, virtio-blk has
>                 u64 capacity;
> The following race can trigger:
>   driver reads low 32 bit
>   both low and high 32 bit change
>   driver reads high 32 bit
>
> as a result, capacity observed is composed of
> old low bits and new high bits which does not
> make sense.
>
> For legacy devices, spec allowed byte by byte access,
> making the race even more common.

Yes, in theory this is a problem with virtio_blk, which could change
capacity (the spec is silent on this, but the Linux driver will handle
it by re-reading capacity, exposing this race).

I don't think qemu supports changing a disk underneath a device anyway?
And in practice, I'm not sure it would ever change fast enough to
trigger this race...

So, should we try to fix it?
Rusty.