答复: [PATCH] vfio/pci: make the vfio_pci_mmap_fault reentrant

[Zengtao (B)]

Hi Alex:

> -----邮件原件-----
> 发件人: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> 发送时间: 2021年3月10日 14:09
> 收件人: Jason Gunthorpe <jgg@xxxxxxxxxx>
> 抄送: Peter Xu <peterx@xxxxxxxxxx>; Zengtao (B) <prime.zeng@xxxxxxxxxxxxx>;
> Cornelia Huck <cohuck@xxxxxxxxxx>; Kevin Tian <kevin.tian@xxxxxxxxx>;
> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>; Giovanni Cabiddu
> <giovanni.cabiddu@xxxxxxxxx>; Michel Lespinasse <walken@xxxxxxxxxx>; Jann
> Horn <jannh@xxxxxxxxxx>; Max Gurtovoy <mgurtovoy@xxxxxxxxxx>;
> kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Linuxarm
> <linuxarm@xxxxxxxxxx>
> 主题: Re: [PATCH] vfio/pci: make the vfio_pci_mmap_fault reentrant
> 
> On Tue, 9 Mar 2021 19:41:27 -0400
> Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
> 
> > On Tue, Mar 09, 2021 at 12:26:07PM -0700, Alex Williamson wrote:
> >
> > > In the new series, I think the fault handler becomes (untested):
> > >
> > > 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;
> > >         unsigned long base_pfn, pgoff;
> > >         vm_fault_t ret = VM_FAULT_SIGBUS;
> > >
> > >         if (vfio_pci_bar_vma_to_pfn(vma, &base_pfn))
> > >                 return ret;
> > >
> > >         pgoff = (vmf->address - vma->vm_start) >> PAGE_SHIFT;
> >
> > I don't think this math is completely safe, it needs to parse the
> > vm_pgoff..
> >
> > I'm worried userspace could split/punch/mangle a VMA using
> > munmap/mremap/etc/etc in a way that does update the pg_off but is
> > incompatible with the above.
> 
> parsing vm_pgoff is done in:
> 
> static int vfio_pci_bar_vma_to_pfn(struct vm_area_struct *vma,
>                                    unsigned long *pfn) {
>         struct vfio_pci_device *vdev = vma->vm_private_data;
>         struct pci_dev *pdev = vdev->pdev;
>         int index;
>         u64 pgoff;
> 
>         index = vma->vm_pgoff >> (VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT);
> 
>         if (index >= VFIO_PCI_ROM_REGION_INDEX ||
>             !vdev->bar_mmap_supported[index] || !vdev->barmap[index])
>                 return -EINVAL;
> 
>         pgoff = vma->vm_pgoff &
>                 ((1U << (VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT)) - 1);
> 
>         *pfn = (pci_resource_start(pdev, index) >> PAGE_SHIFT) + pgoff;
> 
>         return 0;
> }
> 
> But given Peter's concern about faulting individual pages, I think the fault handler
> becomes:
> 
> 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;
>         unsigned long vaddr, pfn;
>         vm_fault_t ret = VM_FAULT_SIGBUS;
> 
>         if (vfio_pci_bar_vma_to_pfn(vma, &pfn))
>                 return ret;
> 
>         down_read(&vdev->memory_lock);
> 
>         if (__vfio_pci_memory_enabled(vdev)) {
>                 for (vaddr = vma->vm_start;
>                      vaddr < vma->vm_end; vaddr += PAGE_SIZE, pfn++) {
One concern here is the performance, since you are doing the mapping for the
 whole vma, what about using block mapping if applicable?

>                         ret = vmf_insert_pfn_prot(vma, vaddr, pfn,
> 
> pgprot_decrypted(vma->vm_page_prot));
>                         if (ret != VM_FAULT_NOPAGE) {
>                                 zap_vma_ptes(vma, vma->vm_start,
>                                              vaddr - vma->vm_start);
>                                 break;
>                         }
>                 }
>         }
> 
>         up_read(&vdev->memory_lock);
> 
>         return ret;
> }
> 
> Thanks,
> Alex