[PATCH] mm: fix a data race in put_page()

[Qian Cai <cai@xxxxxx>]

page->flags could be accessed concurrently as noticied by KCSAN,

 BUG: KCSAN: data-race in page_cpupid_xchg_last / put_page

 write (marked) to 0xfffffc0d48ec1a00 of 8 bytes by task 91442 on cpu 3:
  page_cpupid_xchg_last+0x51/0x80
  page_cpupid_xchg_last at mm/mmzone.c:109 (discriminator 11)
  wp_page_reuse+0x3e/0xc0
  wp_page_reuse at mm/memory.c:2453
  do_wp_page+0x472/0x7b0
  do_wp_page at mm/memory.c:2798
  __handle_mm_fault+0xcb0/0xd00
  handle_pte_fault at mm/memory.c:4049
  (inlined by) __handle_mm_fault at mm/memory.c:4163
  handle_mm_fault+0xfc/0x2f0
  handle_mm_fault at mm/memory.c:4200
  do_page_fault+0x263/0x6f9
  do_user_addr_fault at arch/x86/mm/fault.c:1465
  (inlined by) do_page_fault at arch/x86/mm/fault.c:1539
  page_fault+0x34/0x40

 read to 0xfffffc0d48ec1a00 of 8 bytes by task 94817 on cpu 69:
  put_page+0x15a/0x1f0
  page_zonenum at include/linux/mm.h:923
  (inlined by) is_zone_device_page at include/linux/mm.h:929
  (inlined by) page_is_devmap_managed at include/linux/mm.h:948
  (inlined by) put_page at include/linux/mm.h:1023
  wp_page_copy+0x571/0x930
  wp_page_copy at mm/memory.c:2615
  do_wp_page+0x107/0x7b0
  __handle_mm_fault+0xcb0/0xd00
  handle_mm_fault+0xfc/0x2f0
  do_page_fault+0x263/0x6f9
  page_fault+0x34/0x40

 Reported by Kernel Concurrency Sanitizer on:
 CPU: 69 PID: 94817 Comm: systemd-udevd Tainted: G        W  O L 5.5.0-next-20200204+ #6
 Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 07/10/2019

Both the read and write are done only with the non-exclusive mmap_sem
held. Since the read will check for specific bits (up to three bits for
now) in the flag, load tearing could in theory trigger a logic bug.

To fix it, it could introduce put_page_lockless() in those places but
that could be an overkill, and difficult to use. Thus, just add
READ_ONCE() for the read in page_zonenum() for now where it should not
affect the performance and correctness with a small trade-off that
compilers might generate less efficient optimization in some places.

Signed-off-by: Qian Cai <cai@xxxxxx>
---
 include/linux/mm.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 52269e56c514..f8529aa971c0 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -920,7 +920,7 @@ vm_fault_t alloc_set_pte(struct vm_fault *vmf, struct mem_cgroup *memcg,
 
 static inline enum zone_type page_zonenum(const struct page *page)
 {
-	return (page->flags >> ZONES_PGSHIFT) & ZONES_MASK;
+	return (READ_ONCE(page->flags) >> ZONES_PGSHIFT) & ZONES_MASK;
 }
 
 #ifdef CONFIG_ZONE_DEVICE
-- 
1.8.3.1