[PATCH v3] 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 (as calculated by
vma_hugecache_offset()). 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 replacing linear_page_index() with vma_hugecache_offset()
and applying huge_page_mask() to align the address properly. To make
vma_hugecache_offset() available outside of mm/hugetlb.c, move it to
include/linux/hugetlb.h as a static inline function.

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>
---
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 | 11 +++++++++++
 mm/hugetlb.c            | 11 -----------
 mm/userfaultfd.c        |  5 ++++-
 3 files changed, 15 insertions(+), 12 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 65910437be1c..f003afe0cc91 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -796,6 +796,17 @@ static inline unsigned huge_page_shift(struct hstate *h)
 	return h->order + PAGE_SHIFT;
 }
 
+/*
+ * Convert the address within this vma to the page offset within
+ * the mapping, huge page units here.
+ */
+static inline pgoff_t vma_hugecache_offset(struct hstate *h,
+		struct vm_area_struct *vma, unsigned long address)
+{
+	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/hugetlb.c b/mm/hugetlb.c
index 0beb6e22bc26..b87ed652c748 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1006,17 +1006,6 @@ static long region_count(struct resv_map *resv, long f, long t)
 	return chg;
 }
 
-/*
- * Convert the address within this vma to the page offset within
- * the mapping, huge page units here.
- */
-static pgoff_t vma_hugecache_offset(struct hstate *h,
-			struct vm_area_struct *vma, unsigned long address)
-{
-	return ((address - vma->vm_start) >> huge_page_shift(h)) +
-			(vma->vm_pgoff >> huge_page_order(h));
-}
-
 /**
  * vma_kernel_pagesize - Page size granularity for this VMA.
  * @vma: The user mapping.
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 927086bb4a3c..8efebc47a410 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -507,6 +507,7 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
 	pgoff_t idx;
 	u32 hash;
 	struct address_space *mapping;
+	struct hstate *h;
 
 	/*
 	 * There is no default zero huge page for all huge page sizes as
@@ -564,6 +565,8 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
 			goto out_unlock;
 	}
 
+	h = hstate_vma(dst_vma);
+
 	while (src_addr < src_start + len) {
 		VM_WARN_ON_ONCE(dst_addr >= dst_start + len);
 
@@ -573,7 +576,7 @@ 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 = vma_hugecache_offset(h, dst_vma, dst_addr & huge_page_mask(h));
 		mapping = dst_vma->vm_file->f_mapping;
 		hash = hugetlb_fault_mutex_hash(mapping, idx);
 		mutex_lock(&hugetlb_fault_mutex_table[hash]);
-- 
2.43.0