Re: [PATCH v14 Kernel 4/7] vfio iommu: Implementation of ioctl for dirty pages tracking.

[Kirti Wankhede <kwankhede@xxxxxxxxxx>]

On 3/19/2020 9:15 AM, Alex Williamson wrote:
> On Thu, 19 Mar 2020 01:11:11 +0530
> Kirti Wankhede <kwankhede@xxxxxxxxxx> wrote:
>
> > VFIO_IOMMU_DIRTY_PAGES ioctl performs three operations:
> > - Start dirty pages tracking while migration is active
> > - Stop dirty pages tracking.
> > - Get dirty pages bitmap. Its user space application's responsibility to
> >    copy content of dirty pages from source to destination during migration.
> >
> > To prevent DoS attack, memory for bitmap is allocated per vfio_dma
> > structure. Bitmap size is calculated considering smallest supported page
> > size. Bitmap is allocated for all vfio_dmas when dirty logging is enabled
> >
> > Bitmap is populated for already pinned pages when bitmap is allocated for
> > a vfio_dma with the smallest supported page size. Update bitmap from
> > pinning functions when tracking is enabled. When user application queries
> > bitmap, check if requested page size is same as page size used to
> > populated bitmap. If it is equal, copy bitmap, but if not equal, return
> > error.
> >
> > Signed-off-by: Kirti Wankhede <kwankhede@xxxxxxxxxx>
> > Reviewed-by: Neo Jia <cjia@xxxxxxxxxx>
> > ---
> >   drivers/vfio/vfio_iommu_type1.c | 205 +++++++++++++++++++++++++++++++++++++++-
> >   1 file changed, 203 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > index 70aeab921d0f..d6417fb02174 100644
> > --- a/drivers/vfio/vfio_iommu_type1.c
> > +++ b/drivers/vfio/vfio_iommu_type1.c
> > @@ -71,6 +71,7 @@ struct vfio_iommu {
> >   	unsigned int		dma_avail;
> >   	bool			v2;
> >   	bool			nesting;
> > +	bool			dirty_page_tracking;
> >   };
> >   
> >   struct vfio_domain {
> > @@ -91,6 +92,7 @@ struct vfio_dma {
> >   	bool			lock_cap;	/* capable(CAP_IPC_LOCK) */
> >   	struct task_struct	*task;
> >   	struct rb_root		pfn_list;	/* Ex-user pinned pfn list */
> > +	unsigned long		*bitmap;
>
> We've made the bitmap a width invariant u64 else, should be here as
> well.
>
> >   };
> >   
> >   struct vfio_group {
> > @@ -125,7 +127,10 @@ struct vfio_regions {
> >   #define IS_IOMMU_CAP_DOMAIN_IN_CONTAINER(iommu)	\
> >   					(!list_empty(&iommu->domain_list))
> >   
> > +#define DIRTY_BITMAP_BYTES(n)	(ALIGN(n, BITS_PER_TYPE(u64)) / BITS_PER_BYTE)
> > +
> >   static int put_pfn(unsigned long pfn, int prot);
> > +static unsigned long vfio_pgsize_bitmap(struct vfio_iommu *iommu);
> >   
> >   /*
> >    * This code handles mapping and unmapping of user data buffers
> > @@ -175,6 +180,55 @@ static void vfio_unlink_dma(struct vfio_iommu *iommu, struct vfio_dma *old)
> >   	rb_erase(&old->node, &iommu->dma_list);
> >   }
> >   
> > +static int vfio_dma_bitmap_alloc(struct vfio_iommu *iommu, uint64_t pgsize)
> > +{
> > +	struct rb_node *n = rb_first(&iommu->dma_list);
> > +
> > +	for (; n; n = rb_next(n)) {
> > +		struct vfio_dma *dma = rb_entry(n, struct vfio_dma, node);
> > +		struct rb_node *p;
> > +		unsigned long npages = dma->size / pgsize;
> > +
> > +		dma->bitmap = kvzalloc(DIRTY_BITMAP_BYTES(npages), GFP_KERNEL);
> > +		if (!dma->bitmap) {
> > +			struct rb_node *p = rb_prev(n);
> > +
> > +			for (; p; p = rb_prev(p)) {
> > +				struct vfio_dma *dma = rb_entry(n,
> > +							struct vfio_dma, node);
> > +
> > +				kfree(dma->bitmap);
> > +				dma->bitmap = NULL;
> > +			}
> > +			return -ENOMEM;
> > +		}
> > +
> > +		if (RB_EMPTY_ROOT(&dma->pfn_list))
> > +			continue;
> > +
> > +		for (p = rb_first(&dma->pfn_list); p; p = rb_next(p)) {
> > +			struct vfio_pfn *vpfn = rb_entry(p, struct vfio_pfn,
> > +							 node);
> > +
> > +			bitmap_set(dma->bitmap,
> > +					(vpfn->iova - dma->iova) / pgsize, 1);
> > +		}
> > +	}
> > +	return 0;
> > +}
> > +
> > +static void vfio_dma_bitmap_free(struct vfio_iommu *iommu)
> > +{
> > +	struct rb_node *n = rb_first(&iommu->dma_list);
> > +
> > +	for (; n; n = rb_next(n)) {
> > +		struct vfio_dma *dma = rb_entry(n, struct vfio_dma, node);
> > +
> > +		kfree(dma->bitmap);
> > +		dma->bitmap = NULL;
> > +	}
> > +}
> > +
> >   /*
> >    * Helper Functions for host iova-pfn list
> >    */
> > @@ -567,6 +621,14 @@ static int vfio_iommu_type1_pin_pages(void *iommu_data,
> >   			vfio_unpin_page_external(dma, iova, do_accounting);
> >   			goto pin_unwind;
> >   		}
> > +
> > +		if (iommu->dirty_page_tracking) {
> > +			unsigned long pgshift =
> > +					 __ffs(vfio_pgsize_bitmap(iommu));
> > +
> > +			bitmap_set(dma->bitmap,
> > +				   (vpfn->iova - dma->iova) >> pgshift, 1);
> > +		}
> >   	}
> >   
> >   	ret = i;
> > @@ -801,6 +863,7 @@ static void vfio_remove_dma(struct vfio_iommu *iommu, struct vfio_dma *dma)
> >   	vfio_unmap_unpin(iommu, dma, true);
> >   	vfio_unlink_dma(iommu, dma);
> >   	put_task_struct(dma->task);
> > +	kfree(dma->bitmap);
> >   	kfree(dma);
> >   	iommu->dma_avail++;
> >   }
> > @@ -831,6 +894,50 @@ static unsigned long vfio_pgsize_bitmap(struct vfio_iommu *iommu)
> >   	return bitmap;
> >   }
> >   
> > +static int vfio_iova_dirty_bitmap(struct vfio_iommu *iommu, dma_addr_t iova,
> > +				  size_t size, uint64_t pgsize,
> > +				  unsigned char __user *bitmap)
>
> And here, why do callers cast to an unsigned char pointer when we're
> going to cast to a void pointer anyway?  Should be a u64 __user pointer.
>
> > +{
> > +	struct vfio_dma *dma;
> > +	unsigned long pgshift = __ffs(pgsize);
> > +	unsigned int npages, bitmap_size;
> > +
> > +	dma = vfio_find_dma(iommu, iova, 1);
> > +
> > +	if (!dma)
> > +		return -EINVAL;
> > +
> > +	if (dma->iova != iova || dma->size != size)
> > +		return -EINVAL;
> > +
> > +	npages = dma->size >> pgshift;
> > +	bitmap_size = DIRTY_BITMAP_BYTES(npages);
> > +
> > +	/* mark all pages dirty if all pages are pinned and mapped. */
> > +	if (dma->iommu_mapped)
> > +		bitmap_set(dma->bitmap, 0, npages);
> > +
> > +	if (copy_to_user((void __user *)bitmap, dma->bitmap, bitmap_size))
> > +		return -EFAULT;
> > +
> > +	return 0;
> > +}
> > +
> > +static int verify_bitmap_size(uint64_t npages, uint64_t bitmap_size)
> > +{
> > +	uint64_t bsize;
> > +
> > +	if (!npages || !bitmap_size || bitmap_size > UINT_MAX)
>
> As commented previously, how do we derive this UINT_MAX limitation?

Sorry, I missed that earlier

> UINT_MAX seems arbitrary, is this specified in our API?  The size of a
> vfio_dma is limited to what the user is able to pin, and therefore
> their locked memory limit, but do we have an explicit limit elsewhere
> that results in this limit here.  I think a 4GB bitmap would track
> something like 2^47 bytes of memory, that's pretty excessive, but still
> an arbitrary limit.

There has to be some upper limit check. In core KVM, in
virt/kvm/kvm_main.c there is max number of pages check:

if (new.npages > KVM_MEM_MAX_NR_PAGES)

Where
/*
 * Some of the bitops functions do not support too long bitmaps.
 * This number must be determined not to exceed such limits.
 */
#define KVM_MEM_MAX_NR_PAGES ((1UL << 31) - 1)

Though I don't know which bitops functions do not support long bitmaps.

Something similar as above can be done or same as you also mentioned of 
4GB bitmap limit? that is U32_MAX instead of UINT_MAX?

> > +		return -EINVAL;
> > +
> > +	bsize = DIRTY_BITMAP_BYTES(npages);
> > +
> > +	if (bitmap_size < bsize)
> > +		return -EINVAL;
> > +
> > +	return 0;
> > +}
> > +
> >   static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
> >   			     struct vfio_iommu_type1_dma_unmap *unmap)
> >   {
>
> We didn't address that vfio_dma_do_map() needs to kvzalloc a bitmap for
> any new vfio_dma created while iommu->dirty_page_tracking = true.

Good point. Adding it.

Thanks,
Kirti

> > @@ -2278,6 +2385,93 @@ static long vfio_iommu_type1_ioctl(void *iommu_data,
> >   
> >   		return copy_to_user((void __user *)arg, &unmap, minsz) ?
> >   			-EFAULT : 0;
> > +	} else if (cmd == VFIO_IOMMU_DIRTY_PAGES) {
> > +		struct vfio_iommu_type1_dirty_bitmap dirty;
> > +		uint32_t mask = VFIO_IOMMU_DIRTY_PAGES_FLAG_START |
> > +				VFIO_IOMMU_DIRTY_PAGES_FLAG_STOP |
> > +				VFIO_IOMMU_DIRTY_PAGES_FLAG_GET_BITMAP;
> > +		int ret = 0;
> > +
> > +		if (!iommu->v2)
> > +			return -EACCES;
> > +
> > +		minsz = offsetofend(struct vfio_iommu_type1_dirty_bitmap,
> > +				    flags);
> > +
> > +		if (copy_from_user(&dirty, (void __user *)arg, minsz))
> > +			return -EFAULT;
> > +
> > +		if (dirty.argsz < minsz || dirty.flags & ~mask)
> > +			return -EINVAL;
> > +
> > +		/* only one flag should be set at a time */
> > +		if (__ffs(dirty.flags) != __fls(dirty.flags))
> > +			return -EINVAL;
> > +
> > +		if (dirty.flags & VFIO_IOMMU_DIRTY_PAGES_FLAG_START) {
> > +			uint64_t pgsize = 1 << __ffs(vfio_pgsize_bitmap(iommu));
> > +
> > +			mutex_lock(&iommu->lock);
> > +			if (!iommu->dirty_page_tracking) {
> > +				ret = vfio_dma_bitmap_alloc(iommu, pgsize);
> > +				if (!ret)
> > +					iommu->dirty_page_tracking = true;
> > +			}
> > +			mutex_unlock(&iommu->lock);
> > +			return ret;
> > +		} else if (dirty.flags & VFIO_IOMMU_DIRTY_PAGES_FLAG_STOP) {
> > +			mutex_lock(&iommu->lock);
> > +			if (iommu->dirty_page_tracking) {
> > +				iommu->dirty_page_tracking = false;
> > +				vfio_dma_bitmap_free(iommu);
> > +			}
> > +			mutex_unlock(&iommu->lock);
> > +			return 0;
> > +		} else if (dirty.flags &
> > +				 VFIO_IOMMU_DIRTY_PAGES_FLAG_GET_BITMAP) {
> > +			struct vfio_iommu_type1_dirty_bitmap_get range;
> > +			unsigned long pgshift;
> > +			size_t data_size = dirty.argsz - minsz;
> > +			uint64_t iommu_pgsize =
> > +					 1 << __ffs(vfio_pgsize_bitmap(iommu));
> > +
> > +			if (!data_size || data_size < sizeof(range))
> > +				return -EINVAL;
> > +
> > +			if (copy_from_user(&range, (void __user *)(arg + minsz),
> > +					   sizeof(range)))
> > +				return -EFAULT;
> > +
> > +			/* allow only min supported pgsize */
> > +			if (range.bitmap.pgsize != iommu_pgsize)
> > +				return -EINVAL;
> > +			if (range.iova & (iommu_pgsize - 1))
> > +				return -EINVAL;
> > +			if (!range.size || range.size & (iommu_pgsize - 1))
> > +				return -EINVAL;
> > +			if (range.iova + range.size < range.iova)
> > +				return -EINVAL;
> > +			if (!access_ok((void __user *)range.bitmap.data,
> > +				       range.bitmap.size))
> > +				return -EINVAL;
> > +
> > +			pgshift = __ffs(range.bitmap.pgsize);
> > +			ret = verify_bitmap_size(range.size >> pgshift,
> > +						 range.bitmap.size);
> > +			if (ret)
> > +				return ret;
> > +
> > +			mutex_lock(&iommu->lock);
> > +			if (iommu->dirty_page_tracking)
> > +				ret = vfio_iova_dirty_bitmap(iommu, range.iova,
> > +					 range.size, range.bitmap.pgsize,
> > +				    (unsigned char __user *)range.bitmap.data);
> > +			else
> > +				ret = -EINVAL;
> > +			mutex_unlock(&iommu->lock);
> > +
> > +			return ret;
> > +		}
> >   	}
> >   
> >   	return -ENOTTY;
> > @@ -2345,10 +2539,17 @@ static int vfio_iommu_type1_dma_rw_chunk(struct vfio_iommu *iommu,
> >   
> >   	vaddr = dma->vaddr + offset;
> >   
> > -	if (write)
> > +	if (write) {
> >   		*copied = __copy_to_user((void __user *)vaddr, data,
> >   					 count) ? 0 : count;
> > -	else
> > +		if (*copied && iommu->dirty_page_tracking) {
> > +			unsigned long pgshift =
> > +				__ffs(vfio_pgsize_bitmap(iommu));
> > +
> > +			bitmap_set(dma->bitmap, offset >> pgshift,
> > +				   *copied >> pgshift);
> > +		}
> > +	} else
>
> Great, thanks for adding this!
>
> >   		*copied = __copy_from_user(data, (void __user *)vaddr,
> >   					   count) ? 0 : count;
> >   	if (kthread)