[PATCH v4] mm/userfaultfd: fix hugetlb fault mutex hash calculation

[Jianhui Zhou]

In mfill_atomic_hugetlb(), linear_page_index() is used to calculate the
page index for hugetlb_fault_mutex_hash(). However, linear_page_index()
returns the index in PAGE_SIZE units, while hugetlb_fault_mutex_hash()
expects the index in huge page units. This mismatch means that different
addresses within the same huge page can produce different hash values,
leading to the use of different mutexes for the same huge page. This can
cause races between faulting threads, which can corrupt the reservation
map and trigger the BUG_ON in resv_map_release().

Fix this by introducing hugetlb_linear_page_index(), which returns the
page index in huge page granularity, and using it in place of
linear_page_index().

Fixes: a08c7193e4f1 ("mm/filemap: remove hugetlb special casing in filemap.c")
Reported-by: syzbot+f525fd79634858f478e7@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=f525fd79634858f478e7
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Jianhui Zhou <jianhuizzzzz@xxxxxxxxx>
---
v4:
- Introduce hugetlb_linear_page_index() instead of exposing
  vma_hugecache_offset(); call hstate_vma() internally to simplify
  the API (David Hildenbrand)

v3:
- Fix Fixes tag to a08c7193e4f1 (Hugh Dickins)

v2:
- Remove unnecessary !CONFIG_HUGETLB_PAGE stub for vma_hugecache_offset()
  (Peter Xu, SeongJae Park)

 include/linux/hugetlb.h | 17 +++++++++++++++++
 mm/userfaultfd.c        |  2 +-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 65910437be1c..67d4f0924646 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -796,6 +796,23 @@ static inline unsigned huge_page_shift(struct hstate *h)
 	return h->order + PAGE_SHIFT;
 }
 
+/**
+ * hugetlb_linear_page_index() - linear_page_index() but in hugetlb
+ *				 page size granularity.
+ * @vma: the hugetlb VMA
+ * @address: the virtual address within the VMA
+ *
+ * Return: the page offset within the mapping in huge page units.
+ */
+static inline pgoff_t hugetlb_linear_page_index(struct vm_area_struct *vma,
+		unsigned long address)
+{
+	struct hstate *h = hstate_vma(vma);
+
+	return ((address - vma->vm_start) >> huge_page_shift(h)) +
+		(vma->vm_pgoff >> huge_page_order(h));
+}
+
 static inline bool order_is_gigantic(unsigned int order)
 {
 	return order > MAX_PAGE_ORDER;
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 927086bb4a3c..5590989e18c7 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -573,7 +573,7 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
 		 * in the case of shared pmds.  fault mutex prevents
 		 * races with other faulting threads.
 		 */
-		idx = linear_page_index(dst_vma, dst_addr);
+		idx = hugetlb_linear_page_index(dst_vma, dst_addr);
 		mapping = dst_vma->vm_file->f_mapping;
 		hash = hugetlb_fault_mutex_hash(mapping, idx);
 		mutex_lock(&hugetlb_fault_mutex_table[hash]);
-- 
2.43.0