Re: [PATCH V5 2/7] vfio/type1: prevent locked_vm underflow

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Dec 15, 2022 at 01:56:59PM -0800, Steve Sistare wrote:
> When a vfio container is preserved across exec, the task does not change,
> but it gets a new mm with locked_vm=0.  If the user later unmaps a dma
> mapping, locked_vm underflows to a large unsigned value, and a subsequent
> dma map request fails with ENOMEM in __account_locked_vm.
> 
> To avoid underflow, grab and save the mm at the time a dma is mapped.
> Use that mm when adjusting locked_vm, rather than re-acquiring the saved
> task's mm, which may have changed.  If the saved mm is dead, do nothing.
> 
> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
> ---
>  drivers/vfio/vfio_iommu_type1.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)

Add fixes lines and a CC stable

The subject should be more like 'vfio/typ1: Prevent corruption of mm->locked_vm via exec()'

> @@ -1687,6 +1689,8 @@ static int vfio_dma_do_map(struct vfio_iommu *iommu,
>  	get_task_struct(current->group_leader);
>  	dma->task = current->group_leader;
>  	dma->lock_cap = capable(CAP_IPC_LOCK);
> +	dma->mm = dma->task->mm;

This should be current->mm, current->group_leader->mm is not quite the
same thing (and maybe another bug, I'm not sure)

Jason