Messages in this thread Patch in this message | | | Subject | [PATCH] vfio/type1: Unpin zero pages | From | Alex Williamson <> | Date | Mon, 29 Aug 2022 21:05:40 -0600 |
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There's currently a reference count leak on the zero page. We increment the reference via pin_user_pages_remote(), but the page is later handled as an invalid/reserved page, therefore it's not accounted against the user and not unpinned by our put_pfn().
Introducing special zero page handling in put_pfn() would resolve the leak, but without accounting of the zero page, a single user could still create enough mappings to generate a reference count overflow.
The zero page is always resident, so for our purposes there's no reason to keep it pinned. Therefore, add a loop to walk pages returned from pin_user_pages_remote() and unpin any zero pages.
Cc: David Hildenbrand <david@redhat.com> Cc: stable@vger.kernel.org Reported-by: Luboslav Pivarc <lpivarc@redhat.com> Signed-off-by: Alex Williamson <alex.williamson@redhat.com> --- drivers/vfio/vfio_iommu_type1.c | 12 ++++++++++++ 1 file changed, 12 insertions(+)
diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c index db516c90a977..8706482665d1 100644 --- a/drivers/vfio/vfio_iommu_type1.c +++ b/drivers/vfio/vfio_iommu_type1.c @@ -558,6 +558,18 @@ static int vaddr_get_pfns(struct mm_struct *mm, unsigned long vaddr, ret = pin_user_pages_remote(mm, vaddr, npages, flags | FOLL_LONGTERM, pages, NULL, NULL); if (ret > 0) { + int i; + + /* + * The zero page is always resident, we don't need to pin it + * and it falls into our invalid/reserved test so we don't + * unpin in put_pfn(). Unpin all zero pages in the batch here. + */ + for (i = 0 ; i < ret; i++) { + if (unlikely(is_zero_pfn(page_to_pfn(pages[i])))) + unpin_user_page(pages[i]); + } + *pfn = page_to_pfn(pages[0]); goto done; }
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