Using x86_64 as an example, for a 32KB struct page[] area describing a
2MB hugeTLB, HVO reduces the area to 4KB by the following steps:
1. Split the (r/w vmemmap) PMD mapping the area into 512 (r/w) PTEs;
2. For the 8 PTEs mapping the area, remap PTE 1-7 to the page mapped
by PTE 0, and at the same time change the permission from r/w to
r/o;
3. Free the pages PTE 1-7 used to map, hence the reduction from 32KB
to 4KB.
However, the following race can happen due to improperly memory loads
ordering:
CPU 1 (HVO) CPU 2 (speculative PFN walker)
page_ref_freeze()
synchronize_rcu()
rcu_read_lock()
page_is_fake_head() is false
vmemmap_remap_pte()
XXX: struct page[] becomes r/o
page_ref_unfreeze()
page_ref_count() is not zero
atomic_add_unless(&page->_refcount)
XXX: try to modify r/o struct page[]
Specifically, page_is_fake_head() must be ordered after
page_ref_count() on CPU 2 so that it can only return true for this
case, to avoid the later attempt to modify r/o struct page[].
This patch adds the missing memory barrier and makes the tests on
page_is_fake_head() and page_ref_count() done in the proper order.
Fixes: bd225530a4c7 ("mm/hugetlb_vmemmap: fix race with speculative PFN walkers")
Reported-by: Will Deacon <will@xxxxxxxxxx>
Closes: https://lore.kernel.org/20241128142028.GA3506@willie-the-truck/
Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
---
include/linux/page-flags.h | 2 +-
include/linux/page_ref.h | 8 ++++++--
2 files changed, 7 insertions(+), 3 deletions(-)
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 691506bdf2c5..6b8ecf86f1b6 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -212,7 +212,7 @@ static __always_inline const struct page *page_fixed_fake_head(const struct page
* cold cacheline in some cases.
*/
if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) &&
- test_bit(PG_head, &page->flags)) {
+ test_bit_acquire(PG_head, &page->flags)) {
/*
* We can safely access the field of the @page[1] with PG_head
* because the @page is a compound page composed with at least
diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
index 8c236c651d1d..5becea98bd79 100644
--- a/include/linux/page_ref.h
+++ b/include/linux/page_ref.h
@@ -233,8 +233,12 @@ static inline bool page_ref_add_unless(struct page *page, int nr, int u)
bool ret = false;
rcu_read_lock();
- /* avoid writing to the vmemmap area being remapped */
- if (!page_is_fake_head(page) && page_ref_count(page) != u)
+ /*
+ * To avoid writing to the vmemmap area remapped into r/o in parallel,
+ * the page_ref_count() test must precede the page_is_fake_head() test
+ * so that test_bit_acquire() in the latter is ordered after the former.
+ */
+ if (page_ref_count(page) != u && !page_is_fake_head(page))
ret = atomic_add_unless(&page->_refcount, nr, u);
rcu_read_unlock();
--
2.47.1.613.gc27f4b7a9f-goog
