From 873aefb376bbc0ed1dd2381ea1d6ec88106fdbd4 Mon Sep 17 00:00:00 2001
From: Alex Williamson <alex.williamson@redhat.com>
Date: Mon, 29 Aug 2022 21:05:40 -0600
Subject: [PATCH] vfio/type1: Unpin zero pages
Git-commit: 873aefb376bbc0ed1dd2381ea1d6ec88106fdbd4
Patch-mainline: v6.0-rc5
References: git-fixes
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: stable@vger.kernel.org
Reported-by: Luboslav Pivarc <lpivarc@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/166182871735.3518559.8884121293045337358.stgit@omen
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Takashi Iwai <tiwai@suse.de>
---
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;
}
--
2.35.3