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SubjectRe: [PATCH v2] vfio/pci: Handle concurrent vma faults
On Mon, 28 Jun 2021 15:52:42 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Mon, Jun 28, 2021 at 12:36:21PM -0600, Alex Williamson wrote:
> > On Mon, 28 Jun 2021 14:30:28 -0300
> > Jason Gunthorpe <jgg@nvidia.com> wrote:
> >
> > > On Mon, Jun 28, 2021 at 10:46:53AM -0600, Alex Williamson wrote:
> > > > On Wed, 10 Mar 2021 11:58:07 -0700
> > > > Alex Williamson <alex.williamson@redhat.com> wrote:
> > > >
> > > > > vfio_pci_mmap_fault() incorrectly makes use of io_remap_pfn_range()
> > > > > from within a vm_ops fault handler. This function will trigger a
> > > > > BUG_ON if it encounters a populated pte within the remapped range,
> > > > > where any fault is meant to populate the entire vma. Concurrent
> > > > > inflight faults to the same vma will therefore hit this issue,
> > > > > triggering traces such as:
> > >
> > > If it is just about concurrancy can the vma_lock enclose
> > > io_remap_pfn_range() ?
> >
> > We could extend vma_lock around io_remap_pfn_range(), but that alone
> > would just block the concurrent faults to the same vma and once we
> > released them they'd still hit the BUG_ON in io_remap_pfn_range()
> > because the page is no longer pte_none(). We'd need to combine that
> > with something like __vfio_pci_add_vma() returning -EEXIST to skip the
> > io_remap_pfn_range(), but I've been advised that we shouldn't be
> > calling io_remap_pfn_range() from within the fault handler anyway, we
> > should be using something like vmf_insert_pfn() instead, which I
> > understand can be called safely in the same situation. That's rather
> > the testing I was hoping someone who reproduced the issue previously
> > could validate.
>
> Yes, using the vmf_ stuff is 'righter' for sure, but there isn't
> really a vmf for IO mappings..
>
> > > I assume there is a reason why vm_lock can't be used here, so I
> > > wouldn't object, though I don't especially like the loss of tracking
> > > either.
> >
> > There's no loss of tracking here, we were only expecting a single fault
> > per vma to add the vma to our list. This just skips adding duplicates
> > in these cases where we can have multiple faults in-flight. Thanks,
>
> I mean the arch tracking of IO maps that is hidden inside ioremap_pfn

Ok, so I take it you'd feel more comfortable with something like this,
right? Thanks,

Alex

diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
index 759dfb118712..74fc66cf9cf4 100644
--- a/drivers/vfio/pci/vfio_pci.c
+++ b/drivers/vfio/pci/vfio_pci.c
@@ -1584,6 +1584,7 @@ static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf)
{
struct vm_area_struct *vma = vmf->vma;
struct vfio_pci_device *vdev = vma->vm_private_data;
+ struct vfio_pci_mmap_vma *mmap_vma;
vm_fault_t ret = VM_FAULT_NOPAGE;

mutex_lock(&vdev->vma_lock);
@@ -1591,24 +1592,33 @@ static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf)

if (!__vfio_pci_memory_enabled(vdev)) {
ret = VM_FAULT_SIGBUS;
- mutex_unlock(&vdev->vma_lock);
goto up_out;
}

- if (__vfio_pci_add_vma(vdev, vma)) {
- ret = VM_FAULT_OOM;
- mutex_unlock(&vdev->vma_lock);
- goto up_out;
+ /*
+ * Skip existing vmas, assume concurrent in-flight faults to avoid
+ * BUG_ON from io_remap_pfn_range() hitting !pte_none() pages.
+ */
+ list_for_each_entry(mmap_vma, &vdev->vma_list, vma_next) {
+ if (mmap_vma->vma == vma)
+ goto up_out;
}

- mutex_unlock(&vdev->vma_lock);
-
if (io_remap_pfn_range(vma, vma->vm_start, vma->vm_pgoff,
- vma->vm_end - vma->vm_start, vma->vm_page_prot))
+ vma->vm_end - vma->vm_start,
+ vma->vm_page_prot)) {
ret = VM_FAULT_SIGBUS;
+ goto up_out;
+ }
+
+ if (__vfio_pci_add_vma(vdev, vma)) {
+ ret = VM_FAULT_OOM;
+ zap_vma_ptes(vma, vma->vm_start, vma->vm_end - vma->vm_start);
+ }

up_out:
up_read(&vdev->memory_lock);
+ mutex_unlock(&vdev->vma_lock);
return ret;
}

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