[PATCH v3] mm/memory: use pmdp_get_lockless() for the 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

Both paths only hold mmap_lock for read, so do_fault_around() tests
vmf->pmd while pmd_install() populates it under the pmd lock.  The race is
benign: a stale pmd_none() only wastes the prealloc_pte, which do_fault()
frees again, and a pmd seen as populated only makes ->map_pages() find no
PTE and do_read_fault() fall back to __do_fault().

The load still has to be lockless-safe.  pmdp_get() is a plain READ_ONCE()
and can tear where pmd_t is wider than a word, as on i386-PAE where
pmd_install() publishes two words with a single cmpxchg8b.  Use
pmdp_get_lockless(), the accessor for pmd reads without the pmd lock and
what __pte_offset_map() already uses on this very pmd a few instructions
later.  It is pmdp_get() everywhere else, so the report is silenced the
same way.

No functional change on architectures that can load a pmd in one go.

Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
---
Changelog:
v3:
Shorten the comment to one line (David Hildenbrand).
v2:
Use pmdp_get_lockless() instead of pmdp_get() (David Hildenbrand).

 mm/memory.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 0fb676dd2bd63..f25c2ce0ef153 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6017,7 +6017,8 @@ 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)) {
+	/* pmd_install() rechecks pmd_none() under the pmd lock. */
+	if (pmd_none(pmdp_get_lockless(vmf->pmd))) {
 		vmf->prealloc_pte = pte_alloc_one(vmf->vma->vm_mm);
 		if (!vmf->prealloc_pte)
 			return VM_FAULT_OOM;
-- 
2.43.0