Subject: Re: [PATCH kernel v5 5/6] vfio/spapr: Reference mm in tce_container

From: David Gibson <[email-masked]>
Date: Wed, 23 Nov 2016 12:36:55 +1100

On Tue, Nov 22, 2016 at 06:34:25PM +1100, Alexey Kardashevskiy wrote:
> On 22/11/16 14:49, Alexey Kardashevskiy wrote:
> > On 22/11/16 13:38, David Gibson wrote:
> >> On Thu, Nov 17, 2016 at 06:39:41PM +1100, Alexey Kardashevskiy wrote:
> >>> On 11/11/16 23:32, Alexey Kardashevskiy wrote:
> >>>> In some situations the userspace memory context may live longer than
> >>>> the userspace process itself so if we need to do proper memory context
> >>>> cleanup, we better have tce_container take a reference to mm_struct and
> >>>> use it later when the process is gone (@current or @current->mm is NULL).
> >>>>
> >>>> This references mm and stores the pointer in the container; this is done
> >>>> in a new helper - tce_iommu_mm_set() - when one of the following happens:
> >>>> - a container is enabled (IOMMU v1);
> >>>> - a first attempt to pre-register memory is made (IOMMU v2);
> >>>> - a DMA window is created (IOMMU v2).
> >>>> The @mm stays referenced till the container is destroyed.
> >>>>
> >>>> This replaces current->mm with container->mm everywhere except debug
> >>>> prints.
> >>>>
> >>>> This adds a check that current->mm is the same as the one stored in
> >>>> the container to prevent userspace from making changes to a memory
> >>>> context of other processes.
> >>>>
> >>>> DMA map/unmap ioctls() do not check for @mm as they already check
> >>>> for @enabled which is set after tce_iommu_mm_set() is called.
> >>>>
> >>>> Signed-off-by: Alexey Kardashevskiy <[email-masked]>
> >>>> ---
> >>>> Changes:
> >>>> v5:
> >>>> * postpone referencing of mm
> >>>>
> >>>> v4:
> >>>> * added check for container->mm!=current->mm in tce_iommu_ioctl()
> >>>> for all ioctls and removed other redundand checks
> >>>> ---
> >>>>  drivers/vfio/vfio_iommu_spapr_tce.c | 159 ++++++++++++++++++++++--------------
> >>>>  1 file changed, 99 insertions(+), 60 deletions(-)
> >>>>
> >>>> diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
> >>>> index 1c02498..9a81a7e 100644
> >>>> --- a/drivers/vfio/vfio_iommu_spapr_tce.c
> >>>> +++ b/drivers/vfio/vfio_iommu_spapr_tce.c
> >>>> @@ -31,49 +31,49 @@
> >>>>  static void tce_iommu_detach_group(void *iommu_data,
> >>>>  		struct iommu_group *iommu_group);
> >>>>  
> >>>> -static long try_increment_locked_vm(long npages)
> >>>> +static long try_increment_locked_vm(struct mm_struct *mm, long npages)
> >>>>  {
> >>>>  	long ret = 0, locked, lock_limit;
> >>>>  
> >>>> -	if (!current || !current->mm)
> >>>> -		return -ESRCH; /* process exited */
> >>>> +	if (!mm)
> >>>> +		return -EPERM;
> >>>>  
> >>>>  	if (!npages)
> >>>>  		return 0;
> >>>>  
> >>>> -	down_write(&current->mm->mmap_sem);
> >>>> -	locked = current->mm->locked_vm + npages;
> >>>> +	down_write(&mm->mmap_sem);
> >>>> +	locked = mm->locked_vm + npages;
> >>>>  	lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> >>>>  	if (locked > lock_limit && !capable(CAP_IPC_LOCK))
> >>>
> >>>
> >>>
> >>> Oh boy. Now it seems I have to reference a task, not just mm (which I may
> >>> not have to reference at all after all as the task reference should keep mm
> >>> alive) as I missed the fact capable() and rlimit() are working with
> >>> @current.
> >>
> >> Sorry, what?  I'm not seeing how a task reference comes into this.
> > 
> > I reference @mm to make sure that just one mm uses a container. If mm
> > changes, I return an error, sanity check.
> > 
> > The code also increments locked_vm in mm. But it looks at the current task
> > if there is room for increments and for CAP_IPC_LOCK.
> > 
> > So, the options are:
> > 1. I do not reference the current task, and if mm changes, then the mm
> > sanity check won't let me proceed to the code which tries using current OR
> > 2. reference a task when I reference mm and do that sanity check not just
> > for mm but also for current task.
> > 
> > Makes sense?
> 
> I had a chat with Nick and now I think that having mm referenced and
> checked should be enough and I do not need to reference the task as
> multiple threads within the same mm are allowed to ioctl() to vfio and
> supposedly they will have same limits and capabilities and if they do not,
> we'll just fail and that's it, I cannot see any harm from this approach,
> can you?

Yeah, that sounds sane to me.  If the different threads in the mm
somehow have different caps /  limits, we could get some weird results
depending on which thread attempts to do the mapping, but it shouldn't
actually be harmful.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson