Re: [PATCH v2] vfio/type1: Limit DMA mappings per container

Auger Eric
2019-04-02T19:14:45Z

Hi Alex,

On 4/2/19 6:15 PM, Alex Williamson wrote:
> Memory backed DMA mappings are accounted against a user's locked
> memory limit, including multiple mappings of the same memory. This
> accounting bounds the number of such mappings that a user can create.
> However, DMA mappings that are not backed by memory, such as DMA
> mappings of device MMIO via mmaps, do not make use of page pinning
> and therefore do not count against the user's locked memory limit.
> These mappings still consume memory, but the memory is not well
> associated to the process for the purpose of oom killing a task.
>
> To add bounding on this use case, we introduce a limit to the total
> number of concurrent DMA mappings that a user is allowed to create.
> This limit is exposed as a tunable module option where the default
> value of 64K is expected to be well in excess of any reasonable use
> case (a large virtual machine configuration would typically only make
> use of tens of concurrent mappings).
>
> This fixes CVE-2019-3882.
>
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>

Thanks

Eric

> ---
>
> v2: Remove unnecessary atomic, all runtime access occurs while
> holding vfio_iommu.lock. Change to unsigned int since we're
> no longer bound by the atomic_t.
>
> drivers/vfio/vfio_iommu_type1.c | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
>
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 73652e21efec..d0f731c9920a 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -58,12 +58,18 @@ module_param_named(disable_hugepages,
> MODULE_PARM_DESC(disable_hugepages,
> "Disable VFIO IOMMU support for IOMMU hugepages.");
>
> +static unsigned int dma_entry_limit __read_mostly = U16_MAX;
> +module_param_named(dma_entry_limit, dma_entry_limit, uint, 0644);
> +MODULE_PARM_DESC(dma_entry_limit,
> + "Maximum number of user DMA mappings per container (65535).");
> +
> struct vfio_iommu {
> struct list_head domain_list;
> struct vfio_domain *external_domain; /* domain for external user */
> struct mutex lock;
> struct rb_root dma_list;
> struct blocking_notifier_head notifier;
> + unsigned int dma_avail;
> bool v2;
> bool nesting;
> };
> @@ -836,6 +842,7 @@ static void vfio_remove_dma(struct vfio_iommu *iommu, struct vfio_dma *dma)
> vfio_unlink_dma(iommu, dma);
> put_task_struct(dma->task);
> kfree(dma);
> + iommu->dma_avail++;
> }
>
> static unsigned long vfio_pgsize_bitmap(struct vfio_iommu *iommu)
> @@ -1081,12 +1088,18 @@ static int vfio_dma_do_map(struct vfio_iommu *iommu,
> goto out_unlock;
> }
>
> + if (!iommu->dma_avail) {
> + ret = -ENOSPC;
> + goto out_unlock;
> + }
> +
> dma = kzalloc(sizeof(*dma), GFP_KERNEL);
> if (!dma) {
> ret = -ENOMEM;
> goto out_unlock;
> }
>
> + iommu->dma_avail--;
> dma->iova = iova;
> dma->vaddr = vaddr;
> dma->prot = prot;
> @@ -1583,6 +1596,7 @@ static void *vfio_iommu_type1_open(unsigned long arg)
>
> INIT_LIST_HEAD(&iommu->domain_list);
> iommu->dma_list = RB_ROOT;
> + iommu->dma_avail = dma_entry_limit;
> mutex_init(&iommu->lock);
> BLOCKING_INIT_NOTIFIER_HEAD(&iommu->notifier);
>
>