Subject: Re: [PATCH v3 2/2] vhost-vdpa: fix page pinning leakage in error path

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 14 Oct 2020 02:52:13 -0400

On Tue, Oct 13, 2020 at 04:42:59PM -0700, si-wei liu wrote:
> 
> On 10/9/2020 7:27 PM, Jason Wang wrote:
> > 
> > On 2020/10/3 下午1:02, Si-Wei Liu wrote:
> > > Pinned pages are not properly accounted particularly when
> > > mapping error occurs on IOTLB update. Clean up dangling
> > > pinned pages for the error path. As the inflight pinned
> > > pages, specifically for memory region that strides across
> > > multiple chunks, would need more than one free page for
> > > book keeping and accounting. For simplicity, pin pages
> > > for all memory in the IOVA range in one go rather than
> > > have multiple pin_user_pages calls to make up the entire
> > > region. This way it's easier to track and account the
> > > pages already mapped, particularly for clean-up in the
> > > error path.
> > > 
> > > Fixes: 4c8cf31885f6 ("vhost: introduce vDPA-based backend")
> > > Signed-off-by: Si-Wei Liu <[email-masked]>
> > > ---
> > > Changes in v3:
> > > - Factor out vhost_vdpa_map() change to a separate patch
> > > 
> > > Changes in v2:
> > > - Fix incorrect target SHA1 referenced
> > > 
> > >   drivers/vhost/vdpa.c | 119
> > > ++++++++++++++++++++++++++++++---------------------
> > >   1 file changed, 71 insertions(+), 48 deletions(-)
> > > 
> > > diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> > > index 0f27919..dad41dae 100644
> > > --- a/drivers/vhost/vdpa.c
> > > +++ b/drivers/vhost/vdpa.c
> > > @@ -595,21 +595,19 @@ static int
> > > vhost_vdpa_process_iotlb_update(struct vhost_vdpa *v,
> > >       struct vhost_dev *dev = &v->vdev;
> > >       struct vhost_iotlb *iotlb = dev->iotlb;
> > >       struct page **page_list;
> > > -    unsigned long list_size = PAGE_SIZE / sizeof(struct page *);
> > > +    struct vm_area_struct **vmas;
> > >       unsigned int gup_flags = FOLL_LONGTERM;
> > > -    unsigned long npages, cur_base, map_pfn, last_pfn = 0;
> > > -    unsigned long locked, lock_limit, pinned, i;
> > > +    unsigned long map_pfn, last_pfn = 0;
> > > +    unsigned long npages, lock_limit;
> > > +    unsigned long i, nmap = 0;
> > >       u64 iova = msg->iova;
> > > +    long pinned;
> > >       int ret = 0;
> > >         if (vhost_iotlb_itree_first(iotlb, msg->iova,
> > >                       msg->iova + msg->size - 1))
> > >           return -EEXIST;
> > >   -    page_list = (struct page **) __get_free_page(GFP_KERNEL);
> > > -    if (!page_list)
> > > -        return -ENOMEM;
> > > -
> > >       if (msg->perm & VHOST_ACCESS_WO)
> > >           gup_flags |= FOLL_WRITE;
> > >   @@ -617,61 +615,86 @@ static int
> > > vhost_vdpa_process_iotlb_update(struct vhost_vdpa *v,
> > >       if (!npages)
> > >           return -EINVAL;
> > >   +    page_list = kvmalloc_array(npages, sizeof(struct page *),
> > > GFP_KERNEL);
> > > +    vmas = kvmalloc_array(npages, sizeof(struct vm_area_struct *),
> > > +                  GFP_KERNEL);
> > 
> > 
> > This will result high order memory allocation which was what the code
> > tried to avoid originally.
> > 
> > Using an unlimited size will cause a lot of side effects consider VM or
> > userspace may try to pin several TB of memory.
> Hmmm, that's a good point. Indeed, if the guest memory demand is huge or the
> host system is running short of free pages, kvmalloc will be problematic and
> less efficient than the __get_free_page implementation.

OK so ... Jason, what's the plan?

How about you send a patchset with
1. revert this change
2. fix error handling leak

> > 
> > 
> > > +    if (!page_list || !vmas) {
> > > +        ret = -ENOMEM;
> > > +        goto free;
> > > +    }
> > 
> > 
> > Any reason that you want to use vmas?
> Without providing custom vmas, it's subject to high order allocation
> failure. While page_list and vmas can now fallback to virtual memory
> allocation if need be.
> 
> > 
> > 
> > > +
> > >       mmap_read_lock(dev->mm);
> > >   -    locked = atomic64_add_return(npages, &dev->mm->pinned_vm);
> > >       lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> > > -
> > > -    if (locked > lock_limit) {
> > > +    if (npages + atomic64_read(&dev->mm->pinned_vm) > lock_limit) {
> > >           ret = -ENOMEM;
> > > -        goto out;
> > > +        goto unlock;
> > >       }
> > >   -    cur_base = msg->uaddr & PAGE_MASK;
> > > -    iova &= PAGE_MASK;
> > > +    pinned = pin_user_pages(msg->uaddr & PAGE_MASK, npages, gup_flags,
> > > +                page_list, vmas);
> > > +    if (npages != pinned) {
> > > +        if (pinned < 0) {
> > > +            ret = pinned;
> > > +        } else {
> > > +            unpin_user_pages(page_list, pinned);
> > > +            ret = -ENOMEM;
> > > +        }
> > > +        goto unlock;
> > > +    }
> > >   -    while (npages) {
> > > -        pinned = min_t(unsigned long, npages, list_size);
> > > -        ret = pin_user_pages(cur_base, pinned,
> > > -                     gup_flags, page_list, NULL);
> > > -        if (ret != pinned)
> > > -            goto out;
> > > -
> > > -        if (!last_pfn)
> > > -            map_pfn = page_to_pfn(page_list[0]);
> > > -
> > > -        for (i = 0; i < ret; i++) {
> > > -            unsigned long this_pfn = page_to_pfn(page_list[i]);
> > > -            u64 csize;
> > > -
> > > -            if (last_pfn && (this_pfn != last_pfn + 1)) {
> > > -                /* Pin a contiguous chunk of memory */
> > > -                csize = (last_pfn - map_pfn + 1) << PAGE_SHIFT;
> > > -                if (vhost_vdpa_map(v, iova, csize,
> > > -                           map_pfn << PAGE_SHIFT,
> > > -                           msg->perm))
> > > -                    goto out;
> > > -                map_pfn = this_pfn;
> > > -                iova += csize;
> > > +    iova &= PAGE_MASK;
> > > +    map_pfn = page_to_pfn(page_list[0]);
> > > +
> > > +    /* One more iteration to avoid extra vdpa_map() call out of
> > > loop. */
> > > +    for (i = 0; i <= npages; i++) {
> > > +        unsigned long this_pfn;
> > > +        u64 csize;
> > > +
> > > +        /* The last chunk may have no valid PFN next to it */
> > > +        this_pfn = i < npages ? page_to_pfn(page_list[i]) : -1UL;
> > > +
> > > +        if (last_pfn && (this_pfn == -1UL ||
> > > +                 this_pfn != last_pfn + 1)) {
> > > +            /* Pin a contiguous chunk of memory */
> > > +            csize = last_pfn - map_pfn + 1;
> > > +            ret = vhost_vdpa_map(v, iova, csize << PAGE_SHIFT,
> > > +                         map_pfn << PAGE_SHIFT,
> > > +                         msg->perm);
> > > +            if (ret) {
> > > +                /*
> > > +                 * Unpin the rest chunks of memory on the
> > > +                 * flight with no corresponding vdpa_map()
> > > +                 * calls having been made yet. On the other
> > > +                 * hand, vdpa_unmap() in the failure path
> > > +                 * is in charge of accounting the number of
> > > +                 * pinned pages for its own.
> > > +                 * This asymmetrical pattern of accounting
> > > +                 * is for efficiency to pin all pages at
> > > +                 * once, while there is no other callsite
> > > +                 * of vdpa_map() than here above.
> > > +                 */
> > > +                unpin_user_pages(&page_list[nmap],
> > > +                         npages - nmap);
> > > +                goto out;
> > >               }
> > > -
> > > -            last_pfn = this_pfn;
> > > +            atomic64_add(csize, &dev->mm->pinned_vm);
> > > +            nmap += csize;
> > > +            iova += csize << PAGE_SHIFT;
> > > +            map_pfn = this_pfn;
> > >           }
> > > -
> > > -        cur_base += ret << PAGE_SHIFT;
> > > -        npages -= ret;
> > > +        last_pfn = this_pfn;
> > >       }
> > 
> > 
> > So what I suggest is to fix the pinning leakage first and do the
> > possible optimization on top (which is still questionable to me).
> OK. Unfortunately, this was picked and got merged in upstream. So I will
> post a follow up patch set to 1) revert the commit to the original
> __get_free_page() implementation, and 2) fix the accounting and leakage on
> top. Will it be fine?
> 
> 
> -Siwei
> > 
> > Thanks
> > 
> > 
> > >   -    /* Pin the rest chunk */
> > > -    ret = vhost_vdpa_map(v, iova, (last_pfn - map_pfn + 1) <<
> > > PAGE_SHIFT,
> > > -                 map_pfn << PAGE_SHIFT, msg->perm);
> > > +    WARN_ON(nmap != npages);
> > >   out:
> > > -    if (ret) {
> > > +    if (ret)
> > >           vhost_vdpa_unmap(v, msg->iova, msg->size);
> > > -        atomic64_sub(npages, &dev->mm->pinned_vm);
> > > -    }
> > > +unlock:
> > >       mmap_read_unlock(dev->mm);
> > > -    free_page((unsigned long)page_list);
> > > +free:
> > > +    kvfree(vmas);
> > > +    kvfree(page_list);
> > >       return ret;
> > >   }
> >