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Re: [PATCH v14 Kernel 5/7] vfio iommu: Update UNMAP_DMA ioctl to get dir


From: Alex Williamson
Subject: Re: [PATCH v14 Kernel 5/7] vfio iommu: Update UNMAP_DMA ioctl to get dirty bitmap before unmap
Date: Fri, 20 Mar 2020 09:47:27 -0600

On Fri, 20 Mar 2020 09:40:39 -0600
Alex Williamson <address@hidden> wrote:

> On Fri, 20 Mar 2020 04:35:29 -0400
> Yan Zhao <address@hidden> wrote:
> 
> > On Thu, Mar 19, 2020 at 03:41:12AM +0800, Kirti Wankhede wrote:  
> > > DMA mapped pages, including those pinned by mdev vendor drivers, might
> > > get unpinned and unmapped while migration is active and device is still
> > > running. For example, in pre-copy phase while guest driver could access
> > > those pages, host device or vendor driver can dirty these mapped pages.
> > > Such pages should be marked dirty so as to maintain memory consistency
> > > for a user making use of dirty page tracking.
> > > 
> > > To get bitmap during unmap, user should set flag
> > > VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP, bitmap memory should be allocated 
> > > and
> > > zeroed by user space application. Bitmap size and page size should be set
> > > by user application.
> > > 
> > > Signed-off-by: Kirti Wankhede <address@hidden>
> > > Reviewed-by: Neo Jia <address@hidden>
> > > ---
> > >  drivers/vfio/vfio_iommu_type1.c | 55 
> > > ++++++++++++++++++++++++++++++++++++++---
> > >  include/uapi/linux/vfio.h       | 11 +++++++++
> > >  2 files changed, 62 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/drivers/vfio/vfio_iommu_type1.c 
> > > b/drivers/vfio/vfio_iommu_type1.c
> > > index d6417fb02174..aa1ac30f7854 100644
> > > --- a/drivers/vfio/vfio_iommu_type1.c
> > > +++ b/drivers/vfio/vfio_iommu_type1.c
> > > @@ -939,7 +939,8 @@ static int verify_bitmap_size(uint64_t npages, 
> > > uint64_t bitmap_size)
> > >  }
> > >  
> > >  static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
> > > -                      struct vfio_iommu_type1_dma_unmap *unmap)
> > > +                      struct vfio_iommu_type1_dma_unmap *unmap,
> > > +                      struct vfio_bitmap *bitmap)
> > >  {
> > >   uint64_t mask;
> > >   struct vfio_dma *dma, *dma_last = NULL;
> > > @@ -990,6 +991,10 @@ static int vfio_dma_do_unmap(struct vfio_iommu 
> > > *iommu,
> > >    * will be returned if these conditions are not met.  The v2 interface
> > >    * will only return success and a size of zero if there were no
> > >    * mappings within the range.
> > > +  *
> > > +  * When VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP flag is set, unmap request
> > > +  * must be for single mapping. Multiple mappings with this flag set is
> > > +  * not supported.
> > >    */
> > >   if (iommu->v2) {
> > >           dma = vfio_find_dma(iommu, unmap->iova, 1);
> > > @@ -997,6 +1002,13 @@ static int vfio_dma_do_unmap(struct vfio_iommu 
> > > *iommu,
> > >                   ret = -EINVAL;
> > >                   goto unlock;
> > >           }
> > > +
> > > +         if ((unmap->flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) &&
> > > +             (dma->iova != unmap->iova || dma->size != unmap->size)) {   
> > >  
> > dma is probably NULL here!  
> 
> Yep, I didn't look closely enough there.  This is situated right
> between the check to make sure we're not bisecting a mapping at the
> start of the unmap and the check to make sure we're not bisecting a
> mapping at the end of the unmap.  There's no guarantee that we have a
> valid pointer here.  The test should be in the while() loop below this
> code.

Actually the test could remain here, we can exit here if we can't find
a dma at the start of the unmap range with the GET_DIRTY_BITMAP flag,
but we absolutely cannot deref dma without testing it.

> > And this restriction on UNMAP would make some UNMAP operations of vIOMMU
> > fail.
> > 
> > e.g. below condition indeed happens in reality.
> > an UNMAP ioctl comes for IOVA range from 0xff800000, of size 0x200000
> > However, IOVAs in this range are mapped page by page.i.e., dma->size is 
> > 0x1000.
> > 
> > Previous, this UNMAP ioctl could unmap successfully as a whole.  
> 
> What triggers this in the guest?  Note that it's only when using the
> GET_DIRTY_BITMAP flag that this is restricted.  Does the event you're
> referring to potentially occur under normal circumstances in that mode?
> Thanks,
> 
> Alex
> 
> 
> > > +                 ret = -EINVAL;
> > > +                 goto unlock;
> > > +         }
> > > +
> > >           dma = vfio_find_dma(iommu, unmap->iova + unmap->size - 1, 0);
> > >           if (dma && dma->iova + dma->size != unmap->iova + unmap->size) {
> > >                   ret = -EINVAL;
> > > @@ -1014,6 +1026,12 @@ static int vfio_dma_do_unmap(struct vfio_iommu 
> > > *iommu,
> > >           if (dma->task->mm != current->mm)
> > >                   break;
> > >  
> > > +         if ((unmap->flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) &&
> > > +              iommu->dirty_page_tracking)
> > > +                 vfio_iova_dirty_bitmap(iommu, dma->iova, dma->size,
> > > +                                 bitmap->pgsize,
> > > +                                 (unsigned char __user *) bitmap->data);
> > > +
> > >           if (!RB_EMPTY_ROOT(&dma->pfn_list)) {
> > >                   struct vfio_iommu_type1_dma_unmap nb_unmap;
> > >  
> > > @@ -2369,17 +2387,46 @@ static long vfio_iommu_type1_ioctl(void 
> > > *iommu_data,
> > >  
> > >   } else if (cmd == VFIO_IOMMU_UNMAP_DMA) {
> > >           struct vfio_iommu_type1_dma_unmap unmap;
> > > -         long ret;
> > > +         struct vfio_bitmap bitmap = { 0 };
> > > +         int ret;
> > >  
> > >           minsz = offsetofend(struct vfio_iommu_type1_dma_unmap, size);
> > >  
> > >           if (copy_from_user(&unmap, (void __user *)arg, minsz))
> > >                   return -EFAULT;
> > >  
> > > -         if (unmap.argsz < minsz || unmap.flags)
> > > +         if (unmap.argsz < minsz ||
> > > +             unmap.flags & ~VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP)
> > >                   return -EINVAL;
> > >  
> > > -         ret = vfio_dma_do_unmap(iommu, &unmap);
> > > +         if (unmap.flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) {
> > > +                 unsigned long pgshift;
> > > +                 uint64_t iommu_pgsize =
> > > +                                  1 << __ffs(vfio_pgsize_bitmap(iommu));
> > > +
> > > +                 if (unmap.argsz < (minsz + sizeof(bitmap)))
> > > +                         return -EINVAL;
> > > +
> > > +                 if (copy_from_user(&bitmap,
> > > +                                    (void __user *)(arg + minsz),
> > > +                                    sizeof(bitmap)))
> > > +                         return -EFAULT;
> > > +
> > > +                 /* allow only min supported pgsize */
> > > +                 if (bitmap.pgsize != iommu_pgsize)
> > > +                         return -EINVAL;
> > > +                 if (!access_ok((void __user *)bitmap.data, bitmap.size))
> > > +                         return -EINVAL;
> > > +
> > > +                 pgshift = __ffs(bitmap.pgsize);
> > > +                 ret = verify_bitmap_size(unmap.size >> pgshift,
> > > +                                          bitmap.size);
> > > +                 if (ret)
> > > +                         return ret;
> > > +
> > > +         }
> > > +
> > > +         ret = vfio_dma_do_unmap(iommu, &unmap, &bitmap);
> > >           if (ret)
> > >                   return ret;
> > >  
> > > diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> > > index 043e9eafb255..a704e5380f04 100644
> > > --- a/include/uapi/linux/vfio.h
> > > +++ b/include/uapi/linux/vfio.h
> > > @@ -1010,12 +1010,23 @@ struct vfio_bitmap {
> > >   * field.  No guarantee is made to the user that arbitrary unmaps of iova
> > >   * or size different from those used in the original mapping call will
> > >   * succeed.
> > > + * VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP should be set to get dirty bitmap
> > > + * before unmapping IO virtual addresses. When this flag is set, user 
> > > must
> > > + * provide data[] as structure vfio_bitmap. User must allocate memory to 
> > > get
> > > + * bitmap, clear the bitmap memory by setting zero and must set size of
> > > + * allocated memory in vfio_bitmap.size field. One bit in bitmap
> > > + * represents per page, page of user provided page size in 'pgsize',
> > > + * consecutively starting from iova offset. Bit set indicates page at 
> > > that
> > > + * offset from iova is dirty. Bitmap of pages in the range of unmapped 
> > > size is
> > > + * returned in vfio_bitmap.data
> > >   */
> > >  struct vfio_iommu_type1_dma_unmap {
> > >   __u32   argsz;
> > >   __u32   flags;
> > > +#define VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP (1 << 0)
> > >   __u64   iova;                           /* IO virtual address */
> > >   __u64   size;                           /* Size of mapping (bytes) */
> > > + __u8    data[];
> > >  };
> > >  
> > >  #define VFIO_IOMMU_UNMAP_DMA _IO(VFIO_TYPE, VFIO_BASE + 14)
> > > -- 
> > > 2.7.0
> > >     
> >   
> 




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