Re: [PATCH V1 5/5] vfio: block during VA suspend

Steven Sistare
2021-01-11T21:17:32Z

On 1/8/2021 4:32 PM, Alex Williamson wrote:
> On Tue, 5 Jan 2021 07:36:53 -0800
> Steve Sistare <steven.sistare@oracle.com> wrote:
>
>> Block translation of host virtual address while an iova range is suspended.
>>
>> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
>> ---
>> drivers/vfio/vfio_iommu_type1.c | 48 ++++++++++++++++++++++++++++++++++++++---
>> 1 file changed, 45 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>> index 2c164a6..8035b9a 100644
>> --- a/drivers/vfio/vfio_iommu_type1.c
>> +++ b/drivers/vfio/vfio_iommu_type1.c
>> @@ -31,6 +31,8 @@
>> #include <linux/rbtree.h>
>> #include <linux/sched/signal.h>
>> #include <linux/sched/mm.h>
>> +#include <linux/delay.h>
>> +#include <linux/kthread.h>
>> #include <linux/slab.h>
>> #include <linux/uaccess.h>
>> #include <linux/vfio.h>
>> @@ -484,6 +486,34 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
>> return ret;
>> }
>>
>> +static bool vfio_iommu_contained(struct vfio_iommu *iommu)
>> +{
>> + struct vfio_domain *domain = iommu->external_domain;
>> + struct vfio_group *group;
>> +
>> + if (!domain)
>> + domain = list_first_entry(&iommu->domain_list,
>> + struct vfio_domain, next);
>> +
>> + group = list_first_entry(&domain->group_list, struct vfio_group, next);
>> + return vfio_iommu_group_contained(group->iommu_group);
>> +}
>
> This seems really backwards for a vfio iommu backend to ask vfio-core
> whether its internal object is active.

Yes. I considered the architecturally purer approach of defining a new back end
interface and calling it from the core when the change to uncontained occurs,
but it seemed like overkill.

>> +
>> +
>> +bool vfio_vaddr_valid(struct vfio_iommu *iommu, struct vfio_dma *dma)
>> +{
>> + while (dma->suspended) {
>> + mutex_unlock(&iommu->lock);
>> + msleep_interruptible(10);
>> + mutex_lock(&iommu->lock);
>> + if (kthread_should_stop() || !vfio_iommu_contained(iommu) ||
>> + fatal_signal_pending(current)) {
>> + return false;
>> + }
>> + }
>> + return true;
>> +}
>> +
>> /*
>> * Attempt to pin pages. We really don't want to track all the pfns and
>> * the iommu can only map chunks of consecutive pfns anyway, so get the
>> @@ -690,6 +720,11 @@ static int vfio_iommu_type1_pin_pages(void *iommu_data,
>> continue;
>> }
>>
>> + if (!vfio_vaddr_valid(iommu, dma)) {
>> + ret = -EFAULT;
>> + goto pin_unwind;
>> + }
>
> This could have dropped iommu->lock, we have no basis to believe our
> vfio_dma object is still valid.

Ouch, I am embarrassed. To fix, I will pass iova to vfio_vaddr_valid() and call
vfio_find_dma after re-acquiring the lock.

> Also this is called with an elevated
> reference to the container, so we theoretically should not need to test
> whether the iommu is still contained.

We still have a reference to the container, but userland may have closed the container fd
and hence will never call the ioctl to replace the vaddr, so we must bail.

>> +
>> remote_vaddr = dma->vaddr + (iova - dma->iova);
>> ret = vfio_pin_page_external(dma, remote_vaddr, &phys_pfn[i],
>> do_accounting);
>> @@ -1514,12 +1549,16 @@ static int vfio_iommu_replay(struct vfio_iommu *iommu,
>> i += PAGE_SIZE;
>> }
>> } else {
>> - unsigned long pfn;
>> - unsigned long vaddr = dma->vaddr +
>> - (iova - dma->iova);
>> + unsigned long pfn, vaddr;
>> size_t n = dma->iova + dma->size - iova;
>> long npage;
>>
>> + if (!vfio_vaddr_valid(iommu, dma)) {
>> + ret = -EFAULT;
>> + goto unwind;
>> + }
>
> This one is even scarier, do we have any basis to assume either object
> is still valid after iommu->lock is released?

Another ouch. The iommu object still exists, but its dma_list may have changed.
We cannot even unwind and punt with EAGAIN.

I can fix this by adding a preamble loop that holds iommu->lock and verifies that
all dma_list entries are not suspended. If the loop hits a suspended entry, it
drops the lock and sleeps (like vfio_vaddr_valid), and retries from the beginning
of the list. On reaching the end of the list, the lock remains held and prevents
entries from being suspended.

Instead of retrying, I could return EAGAIN, but that would require unwinding of
earlier groups in __vfio_container_attach_groups.

> We can only get here if
> we're attaching a group to the iommu with suspended vfio_dmas. Do we
> need to allow that? It seems we could -EAGAIN and let the user figure
> out that they can't attach a group while mappings have invalid vaddrs.

I would like to allow it to minimize userland code changes.

>> + vaddr = dma->vaddr + (iova - dma->iova);
>> +
>> npage = vfio_pin_pages_remote(dma, vaddr,
>> n >> PAGE_SHIFT,
>> &pfn, limit);
>> @@ -2965,6 +3004,9 @@ static int vfio_iommu_type1_dma_rw_chunk(struct vfio_iommu *iommu,
>> if (count > dma->size - offset)
>> count = dma->size - offset;
>>
>> + if (!vfio_vaddr_valid(iommu, dma))
>
> Same as the pinning path, this should hold a container user reference
> but we cannot assume our vfio_dma object is valid if we've released the
> lock. Thanks,

Yup. Same fix needed.

- Steve

>> + goto out;
>> +
>> vaddr = dma->vaddr + offset;
>>
>> if (write) {
>