Re: [PATCH v12 Kernel 4/7] vfio iommu: Implementation of ioctl to for dirty pages tracking.

Alex Williamson
2020-02-10T17:25:31Z

On Sat, 8 Feb 2020 01:12:31 +0530
Kirti Wankhede <kwankhede@nvidia.com> wrote:

> VFIO_IOMMU_DIRTY_PAGES ioctl performs three operations:
> - Start pinned and unpinned pages tracking while migration is active
> - Stop pinned and unpinned dirty pages tracking. This is also used to
> stop dirty pages tracking if migration failed or cancelled.
> - Get dirty pages bitmap. This ioctl returns bitmap of dirty pages, its
> user space application 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 when dirty logging is enabled for those
> vfio_dmas whose vpfn list is not empty or whole range is mapped, in
> case of pass-through device.
>
> There could be multiple option as to when bitmap should be populated:
> * Polulate bitmap for already pinned pages when bitmap is allocated for
> a vfio_dma with the smallest supported page size. Updates bitmap from
> page pinning and unpinning functions. 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,
> re-populated bitmap according to requested page size and then copy to
> user.
> Pros: Bitmap gets populated on the fly after dirty tracking has
> started.
> Cons: If requested page size is different than smallest supported
> page size, then bitmap has to be re-populated again, with
> additional overhead of allocating bitmap memory again for
> re-population of bitmap.

No memory needs to be allocated to re-populate the bitmap. The bitmap
is clear-on-read and by tracking the bitmap in the smallest supported
page size we can guarantee that we can fit the user requested bitmap
size within the space occupied by that minimal page size range of the
bitmap. Therefore we'd destructively translate the requested region of
the bitmap to a different page size, write it out to the user, and
clear it. Also we expect userspace to use the minimum page size almost
exclusively, which is optimized by this approach as dirty bit tracking
is spread out over each page pinning operation.

>
> * Populate bitmap when bitmap is queried by user application.
> Pros: Bitmap is populated with requested page size. This eliminates
> the need to re-populate bitmap if requested page size is
> different than smallest supported pages size.
> Cons: There is one time processing time, when bitmap is queried.

Another significant Con is that the vpfn list needs to track and manage
unpinned pages, which makes it more complex and intrusive. The
previous option seems to have both time and complexity advantages,
especially in the case we expect to be most common of the user
accessing the bitmap with the minimum page size, ie. PAGE_SIZE. It's
also not clear why we pre-allocate the bitmap at all with this approach.

> I prefer later option with simple logic and to eliminate over-head of
> bitmap repopulation in case of differnt page sizes. Later option is
> implemented in this patch.

Hmm, we'll see below, but I not convinced based on the above rationale.

> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
> Reviewed-by: Neo Jia <cjia@nvidia.com>
> ---
> drivers/vfio/vfio_iommu_type1.c | 299 ++++++++++++++++++++++++++++++++++++++--
> 1 file changed, 287 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index d386461e5d11..df358dc1c85b 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -70,6 +70,7 @@ struct vfio_iommu {
> unsigned int dma_avail;
> bool v2;
> bool nesting;
> + bool dirty_page_tracking;
> };
>
> struct vfio_domain {
> @@ -90,6 +91,7 @@ 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;
> };
>
> struct vfio_group {
> @@ -125,6 +127,7 @@ struct vfio_regions {
> (!list_empty(&iommu->domain_list))
>
> 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
> @@ -174,6 +177,57 @@ static void vfio_unlink_dma(struct vfio_iommu *iommu, struct vfio_dma *old)
> rb_erase(&old->node, &iommu->dma_list);
> }
>
> +static inline unsigned long dirty_bitmap_bytes(unsigned int npages)
> +{
> + if (!npages)
> + return 0;
> +
> + return ALIGN(npages, BITS_PER_LONG) / sizeof(unsigned long);
> +}
> +
> +static int vfio_dma_bitmap_alloc(struct vfio_iommu *iommu,
> + struct vfio_dma *dma, unsigned long pgsizes)
> +{
> + unsigned long pgshift = __ffs(pgsizes);
> +
> + if (!RB_EMPTY_ROOT(&dma->pfn_list) || dma->iommu_mapped) {
> + unsigned long npages = dma->size >> pgshift;
> + unsigned long bsize = dirty_bitmap_bytes(npages);
> +
> + dma->bitmap = kvzalloc(bsize, GFP_KERNEL);

nit, we don't need to store bsize in a local variable.

> + if (!dma->bitmap)
> + return -ENOMEM;
> + }
> + return 0;
> +}
> +
> +static int vfio_dma_all_bitmap_alloc(struct vfio_iommu *iommu,
> + unsigned long pgsizes)
> +{
> + struct rb_node *n = rb_first(&iommu->dma_list);
> + int ret;
> +
> + for (; n; n = rb_next(n)) {
> + struct vfio_dma *dma = rb_entry(n, struct vfio_dma, node);
> +
> + ret = vfio_dma_bitmap_alloc(iommu, dma, pgsizes);
> + if (ret)
> + return ret;

This doesn't unwind on failure, so we're left with partially allocated
bitmap cruft.

> + }
> + return 0;
> +}
> +
> +static void vfio_dma_all_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);

We don't set dma->bitmap = NULL and we don't even prevent the case of a
user making multiple STOP calls, so we have a user triggerable double
free :(

> + }
> +}
> +
> /*
> * Helper Functions for host iova-pfn list
> */
> @@ -244,6 +298,29 @@ static void vfio_remove_from_pfn_list(struct vfio_dma *dma,
> kfree(vpfn);
> }
>
> +static void vfio_remove_unpinned_from_pfn_list(struct vfio_dma *dma)
> +{
> + struct rb_node *n = rb_first(&dma->pfn_list);
> +
> + for (; n; n = rb_next(n)) {
> + struct vfio_pfn *vpfn = rb_entry(n, struct vfio_pfn, node);
> +
> + if (!vpfn->ref_count)
> + vfio_remove_from_pfn_list(dma, vpfn);
> + }
> +}
> +
> +static void vfio_remove_unpinned_from_dma_list(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);
> +
> + vfio_remove_unpinned_from_pfn_list(dma);
> + }
> +}
> +
> static struct vfio_pfn *vfio_iova_get_vfio_pfn(struct vfio_dma *dma,
> unsigned long iova)
> {
> @@ -261,7 +338,8 @@ static int vfio_iova_put_vfio_pfn(struct vfio_dma *dma, struct vfio_pfn *vpfn)
> vpfn->ref_count--;
> if (!vpfn->ref_count) {
> ret = put_pfn(vpfn->pfn, dma->prot);
> - vfio_remove_from_pfn_list(dma, vpfn);
> + if (!dma->bitmap)
> + vfio_remove_from_pfn_list(dma, vpfn);
> }
> return ret;
> }
> @@ -483,13 +561,14 @@ static int vfio_pin_page_external(struct vfio_dma *dma, unsigned long vaddr,
> return ret;
> }
>
> -static int vfio_unpin_page_external(struct vfio_dma *dma, dma_addr_t iova,
> +static int vfio_unpin_page_external(struct vfio_iommu *iommu,

We added a parameter but didn't use it in this patch.

> + struct vfio_dma *dma, dma_addr_t iova,
> bool do_accounting)
> {
> int unlocked;
> struct vfio_pfn *vpfn = vfio_find_vpfn(dma, iova);
>
> - if (!vpfn)
> + if (!vpfn || !vpfn->ref_count)
> return 0;
>
> unlocked = vfio_iova_put_vfio_pfn(dma, vpfn);
> @@ -510,6 +589,7 @@ static int vfio_iommu_type1_pin_pages(void *iommu_data,
> unsigned long remote_vaddr;
> struct vfio_dma *dma;
> bool do_accounting;
> + unsigned long iommu_pgsizes = vfio_pgsize_bitmap(iommu);
>
> if (!iommu || !user_pfn || !phys_pfn)
> return -EINVAL;
> @@ -551,8 +631,10 @@ static int vfio_iommu_type1_pin_pages(void *iommu_data,
>
> vpfn = vfio_iova_get_vfio_pfn(dma, iova);
> if (vpfn) {
> - phys_pfn[i] = vpfn->pfn;
> - continue;
> + if (vpfn->ref_count > 1) {
> + phys_pfn[i] = vpfn->pfn;
> + continue;
> + }
> }
>
> remote_vaddr = dma->vaddr + iova - dma->iova;
> @@ -560,11 +642,23 @@ static int vfio_iommu_type1_pin_pages(void *iommu_data,
> do_accounting);
> if (ret)
> goto pin_unwind;
> -
> - ret = vfio_add_to_pfn_list(dma, iova, phys_pfn[i]);
> - if (ret) {
> - vfio_unpin_page_external(dma, iova, do_accounting);
> - goto pin_unwind;
> + if (!vpfn) {
> + ret = vfio_add_to_pfn_list(dma, iova, phys_pfn[i]);
> + if (ret) {
> + vfio_unpin_page_external(iommu, dma, iova,
> + do_accounting);
> + goto pin_unwind;
> + }
> + } else
> + vpfn->pfn = phys_pfn[i];
> +
> + if (iommu->dirty_page_tracking && !dma->bitmap) {
> + ret = vfio_dma_bitmap_alloc(iommu, dma, iommu_pgsizes);
> + if (ret) {
> + vfio_unpin_page_external(iommu, dma, iova,
> + do_accounting);
> + goto pin_unwind;
> + }
> }
> }
>
> @@ -578,7 +672,7 @@ static int vfio_iommu_type1_pin_pages(void *iommu_data,
>
> iova = user_pfn[j] << PAGE_SHIFT;
> dma = vfio_find_dma(iommu, iova, PAGE_SIZE);
> - vfio_unpin_page_external(dma, iova, do_accounting);
> + vfio_unpin_page_external(iommu, dma, iova, do_accounting);
> phys_pfn[j] = 0;
> }
> pin_done:
> @@ -612,7 +706,7 @@ static int vfio_iommu_type1_unpin_pages(void *iommu_data,
> dma = vfio_find_dma(iommu, iova, PAGE_SIZE);
> if (!dma)
> goto unpin_exit;
> - vfio_unpin_page_external(dma, iova, do_accounting);
> + vfio_unpin_page_external(iommu, dma, iova, do_accounting);
> }
>
> unpin_exit:
> @@ -830,6 +924,113 @@ 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)
> +{
> + struct vfio_dma *dma;
> + dma_addr_t i = iova, iova_limit;
> + unsigned int bsize, nbits = 0, l = 0;
> + unsigned long pgshift = __ffs(pgsize);
> +
> + while ((dma = vfio_find_dma(iommu, i, pgsize))) {
> + int ret, j;
> + unsigned int npages = 0, shift = 0;
> + unsigned char temp = 0;
> +
> + /* mark all pages dirty if all pages are pinned and mapped. */
> + if (dma->iommu_mapped) {
> + iova_limit = min(dma->iova + dma->size, iova + size);
> + npages = iova_limit/pgsize;
> + bitmap_set(dma->bitmap, 0, npages);

npages is derived from iova_limit, which is the number of bits to set
dirty relative to the first requested iova, not iova zero, ie. the set
of dirty bits is offset from those requested unless iova == dma->iova.

Also I hope dma->bitmap was actually allocated. Not only does the
START error path potentially leave dirty tracking enabled without all
the bitmap allocated, when does the bitmap get allocated for a new
vfio_dma when dirty tracking is enabled? Seems it only occurs if a
vpfn gets marked dirty.

> + } else if (dma->bitmap) {
> + struct rb_node *n = rb_first(&dma->pfn_list);
> + bool found = false;
> +
> + for (; n; n = rb_next(n)) {
> + struct vfio_pfn *vpfn = rb_entry(n,
> + struct vfio_pfn, node);
> + if (vpfn->iova >= i) {
> + found = true;
> + break;
> + }
> + }
> +
> + if (!found) {
> + i += dma->size;
> + continue;
> + }
> +
> + for (; n; n = rb_next(n)) {
> + unsigned int s;
> + struct vfio_pfn *vpfn = rb_entry(n,
> + struct vfio_pfn, node);
> +
> + if (vpfn->iova >= iova + size)
> + break;
> +
> + s = (vpfn->iova - dma->iova) >> pgshift;
> + bitmap_set(dma->bitmap, s, 1);
> +
> + iova_limit = vpfn->iova + pgsize;
> + }
> + npages = iova_limit/pgsize;

Isn't iova_limit potentially uninitialized here? For example, if our
vfio_dma covers {0,8192} and we ask for the bitmap of {0,4096} and
there's a vpfn at {4096,8192}. I think that means vpfn->iova >= i
(4096 >= 0), so we break with found = true, then we test 4096 >= 0 +
4096 and break, and npages = ????/pgsize.

> + }
> +
> + bsize = dirty_bitmap_bytes(npages);
> + shift = nbits % BITS_PER_BYTE;
> +
> + if (npages && shift) {
> + l--;
> + if (!access_ok((void __user *)bitmap + l,
> + sizeof(unsigned char)))
> + return -EINVAL;
> +
> + ret = __get_user(temp, bitmap + l);

I don't understand why we care to get the user's bitmap, are we trying
to leave whatever garbage they might have set in it and only also set
the dirty bits? That seems unnecessary.

Also why do we need these access_ok() checks when we already checked
the range at the start of the ioctl?

> + if (ret)
> + return ret;
> + }
> +
> + for (j = 0; j < bsize; j++, l++) {
> + temp = temp |
> + (*((unsigned char *)dma->bitmap + j) << shift);

|=

> + if (!access_ok((void __user *)bitmap + l,
> + sizeof(unsigned char)))
> + return -EINVAL;
> +
> + ret = __put_user(temp, bitmap + l);
> + if (ret)
> + return ret;
> + if (shift) {
> + temp = *((unsigned char *)dma->bitmap + j) >>
> + (BITS_PER_BYTE - shift);
> + }

When shift == 0, temp just seems to accumulate bits that never get
cleared.

> + }
> +
> + nbits += npages;
> +
> + i = min(dma->iova + dma->size, iova + size);
> + if (i >= iova + size)
> + break;

So whether we error or succeed, we leave cruft in dma->bitmap for the
next pass. It doesn't seem to make any sense why we pre-allocated the
bitmap, we might as well just allocate it on demand here. Actually, if
we're not going to do a copy_to_user() for some range of the bitmap,
I'm not sure what it's purpose is at all. I think the big advantages
of the bitmap are that we can't amortize the cost across every pinned
page or DMA mapping, we don't need the overhead of tracking unmapped
vpfns, and we can use copy_to_user() to push the bitmap out. We're not
getting any of those advantages here.

> + }
> + return 0;
> +}
> +
> +static long verify_bitmap_size(unsigned long npages, unsigned long bitmap_size)
> +{
> + long bsize;
> +
> + if (!bitmap_size || bitmap_size > SIZE_MAX)
> + return -EINVAL;
> +
> + bsize = dirty_bitmap_bytes(npages);
> +
> + if (bitmap_size < bsize)
> + return -EINVAL;
> +
> + return bsize;
> +}

Seems like this could simply return int, -errno or zero for success.
The returned bsize is not used for anything else.

> +
> static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
> struct vfio_iommu_type1_dma_unmap *unmap)
> {
> @@ -2277,6 +2478,80 @@ 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 range;
> + uint32_t mask = VFIO_IOMMU_DIRTY_PAGES_FLAG_START |
> + VFIO_IOMMU_DIRTY_PAGES_FLAG_STOP |
> + VFIO_IOMMU_DIRTY_PAGES_FLAG_GET_BITMAP;
> + int ret;
> +
> + if (!iommu->v2)
> + return -EACCES;
> +
> + minsz = offsetofend(struct vfio_iommu_type1_dirty_bitmap,
> + bitmap);

We require the user to provide iova, size, pgsize, bitmap_size, and
bitmap fields to START/STOP? Why?

> +
> + if (copy_from_user(&range, (void __user *)arg, minsz))
> + return -EFAULT;
> +
> + if (range.argsz < minsz || range.flags & ~mask)
> + return -EINVAL;
> +
> + /* only one flag should be set at a time */
> + if (__ffs(range.flags) != __fls(range.flags))
> + return -EINVAL;
> +
> + if (range.flags & VFIO_IOMMU_DIRTY_PAGES_FLAG_START) {
> + unsigned long iommu_pgsizes = vfio_pgsize_bitmap(iommu);
> +
> + mutex_lock(&iommu->lock);
> + iommu->dirty_page_tracking = true;
> + ret = vfio_dma_all_bitmap_alloc(iommu, iommu_pgsizes);

So dirty page tracking is enabled even if we fail to allocate all the
bitmaps? Shouldn't this return an error if dirty tracking is already
enabled?

> + mutex_unlock(&iommu->lock);
> + return ret;
> + } else if (range.flags & VFIO_IOMMU_DIRTY_PAGES_FLAG_STOP) {
> + mutex_lock(&iommu->lock);
> + iommu->dirty_page_tracking = false;

Shouldn't we only allow STOP if tracking is enabled?

> + vfio_dma_all_bitmap_free(iommu);

Here's where that user induced double free enters the picture.

> + vfio_remove_unpinned_from_dma_list(iommu);
> + mutex_unlock(&iommu->lock);
> + return 0;
> + } else if (range.flags &
> + VFIO_IOMMU_DIRTY_PAGES_FLAG_GET_BITMAP) {
> + long bsize;
> + unsigned long pgshift = __ffs(range.pgsize);
> + uint64_t iommu_pgsizes = vfio_pgsize_bitmap(iommu);
> + uint64_t iommu_pgmask =
> + ((uint64_t)1 << __ffs(iommu_pgsizes)) - 1;
> +
> + if ((range.pgsize & iommu_pgsizes) != range.pgsize)
> + return -EINVAL;
> + if (range.iova & iommu_pgmask)
> + return -EINVAL;
> + if (!range.size || range.size & iommu_pgmask)
> + return -EINVAL;
> + if (range.iova + range.size < range.iova)
> + return -EINVAL;
> + if (!access_ok((void __user *)range.bitmap,
> + range.bitmap_size))
> + return -EINVAL;
> +
> + bsize = verify_bitmap_size(range.size >> pgshift,
> + range.bitmap_size);
> + if (bsize < 0)
> + return bsize;
> +
> + mutex_lock(&iommu->lock);
> + if (iommu->dirty_page_tracking)
> + ret = vfio_iova_dirty_bitmap(iommu, range.iova,
> + range.size, range.pgsize,
> + (unsigned char __user *)range.bitmap);
> + else
> + ret = -EINVAL;
> + mutex_unlock(&iommu->lock);
> +
> + return ret;
> + }
> }
>
> return -ENOTTY;

Thanks,
Alex