[PATCH] mm/memory: use pmdp_get() for the lockless pmd read in do_fault_around()

[Hui Zhu <hui.zhu@linux.dev>]

From: Hui Zhu <zhuhui@kylinos.cn>

KCSAN reports a data-race in do_read_fault / pmd_install; the same code
is in mainline:

  write (marked) to 0xffff8d52a75fb810 of 8 bytes by task 27487 on cpu 5:
   native_set_pmd arch/x86/include/asm/pgtable_64.h:83 [inline]
   pmd_populate arch/x86/include/asm/pgalloc.h:84 [inline]
   pmd_install+0xe0/0x120 mm/memory.c:444
   finish_fault+0x5bc/0xb00 mm/memory.c:5204
   do_cow_fault mm/memory.c:5432 [inline]
   do_fault+0x5d0/0x6e0 mm/memory.c:5526
   handle_mm_fault+0x211/0x4c0 mm/memory.c:6218
   __mm_populate+0x144/0x270 mm/gup.c:2207
   __do_sys_mlockall+0x2cd/0x380 mm/mlock.c:764

  read to 0xffff8d52a75fb810 of 8 bytes by task 27527 on cpu 9:
   do_fault_around mm/memory.c:5341 [inline]
   do_read_fault+0x162/0x670 mm/memory.c:5381
   do_fault+0x31a/0x6e0 mm/memory.c:5524
   handle_mm_fault+0x211/0x4c0 mm/memory.c:6218
   faultin_page_range+0x227/0x530 mm/gup.c:2156
   __x64_sys_madvise+0x67/0x80 mm/madvise.c:1479

  value changed: 0x0000000000000000 -> 0x000000001386c067

do_fault_around() reads vmf->pmd without the pmd lock to decide whether
->map_pages() needs a preallocated PTE page, while pmd_install() publishes
the entry with WRITE_ONCE() under the pmd lock.  Both paths above only hold
mmap_lock for read, so they can fault into the same VMA concurrently.

The race is benign.  The pmd is a naturally aligned 8 byte object, the
store is a single WRITE_ONCE() and pmd_none() only tests the low word, so
no torn value is possible.  A stale none pmd only allocates an extra
prealloc_pte, which pmd_install() leaves unused because it rechecks
pmd_none() under the pmd lock, and do_fault() frees it on the error path.
A pmd already seen as populated makes ->map_pages() find no PTE, and
do_read_fault() then falls back to __do_fault() and finish_fault(), which
allocate the page table properly.

Read the entry with pmdp_get() so that the load becomes a READ_ONCE()
pairing with the WRITE_ONCE() in pmd_install().

No functional change intended.

Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
---
 mm/memory.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 0fb676dd2bd63..90afbd124d503 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6017,7 +6017,11 @@ static vm_fault_t do_fault_around(struct vm_fault *vmf)
 	to_pte = min3(from_pte + nr_pages, (pgoff_t)PTRS_PER_PTE,
 		      pte_off + vma_pages(vmf->vma) - vma_off) - 1;
 
-	if (pmd_none(*vmf->pmd)) {
+	/*
+	 * Lockless hint: pmd_install() may populate this pmd concurrently and
+	 * rechecks it under the pmd lock, so a stale value is harmless.
+	 */
+	if (pmd_none(pmdp_get(vmf->pmd))) {
 		vmf->prealloc_pte = pte_alloc_one(vmf->vma->vm_mm);
 		if (!vmf->prealloc_pte)
 			return VM_FAULT_OOM;
-- 
2.43.0