Re: [PATCH V4 mlx5-next 13/13] vfio/mlx5: Use its own PCI reset_done error handler

Alex Williamson
2021-10-27T16:49:02Z

On Wed, 27 Oct 2021 12:53:39 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Wed, Oct 27, 2021 at 09:29:43AM -0600, Alex Williamson wrote:
>
> > The reset_done handler sets deferred_reset = true and if it's possible
> > to get the state_mutex, will reset migration data and device_state as
> > part of releasing that mutex. If there's contention on state_mutex,
> > the deferred_reset field flags that this migration state is still stale.
> >
> > So, I assume that it's possible that a user resets the device via ioctl
> > or config space, there was contention and the migration state is still
> > stale, right?
>
> If this occurs it is a userspace bug and the goal here is to maintain
> kernel integrity.
>
> > The user then goes to read device_state, but the staleness of the
> > migration state is not resolved until *after* the stale device state is
> > copied to the user buffer.
>
> This is not preventable in the general case. Assume we have sane
> locking and it looks like this:
>
> CPU0 CPU1
> ioctl state change
> mutex_lock
> copy_to_user(state == !RUNNING)
> mutex_unlock
> ioctl reset
> mutex_lock
> state = RUNNING
> mutex_unlock
> return to userspace
> return to userspace
> Userspace sees state != RUNNING
>
> Same issue. Userspace cannot race state manipulating ioctls and expect
> things to make any sense.
>
> In all cases contention on the mutex during reset causes the reset to
> order after the mutex is released. This is true with this approach and
> it is true with a simple direct use of mutex.
>
> In either case userspace will see incoherent results, and it is
> userspace error to try and run the kernel ioctls this way.
>
> > What did the user do wrong to see stale data? Thanks,
>
> Userspace allowed two state effecting IOCTLs to run concurrently.
>
> Userspace must block reset while it is manipulating migration states.

Ok, I see. I didn't digest that contention on state_mutex can only
occur from a concurrent migration region access and the stale state is
resolved at the end of that concurrent access, not some subsequent
access. I agree we have no obligation to resolve anything about the
state that concurrent access would see. Thanks,

Alex