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

From: Jason Wang
Date: Fri Oct 09 2020 - 22:32:19 EST


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 <si-wei.liu@xxxxxxxxxx>
---
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.


+ if (!page_list || !vmas) {
+ ret = -ENOMEM;
+ goto free;
+ }


Any reason that you want to use vmas?


+
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).

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;
}