[PATCH v3] mm/ksm: Fix NULL pointer dereference when KSM zero page is enabled

[Muchun Song]

The find_mergeable_vma can return NULL. In this case, it leads
to crash when we access vma->vm_mm(its offset is 0x40) later in
write_protect_page. And this case did happen on our server. The
following calltrace is captured in kernel 4.19 with the following
patch applied and KSM zero page enabled on our server.

  commit e86c59b1b12d ("mm/ksm: improve deduplication of zero pages with colouring")

So add a vma check to fix it.

--------------------------------------------------------------------------
  BUG: unable to handle kernel NULL pointer dereference at 0000000000000040
  Oops: 0000 [#1] SMP NOPTI
  CPU: 9 PID: 510 Comm: ksmd Kdump: loaded Tainted: G OE 4.19.36.bsk.9-amd64 #4.19.36.bsk.9
  Hardware name: FOXCONN R-5111/GROOT, BIOS IC1B111F 08/17/2019
  RIP: 0010:try_to_merge_one_page+0xc7/0x760
  Code: 24 58 65 48 33 34 25 28 00 00 00 89 e8 0f 85 a3 06 00 00 48 83 c4
        60 5b 5d 41 5c 41 5d 41 5e 41 5f c3 48 8b 46 08 a8 01 75 b8 <49>
        8b 44 24 40 4c 8d 7c 24 20 b9 07 00 00 00 4c 89 e6 4c 89 ff 48
  RSP: 0018:ffffadbdd9fffdb0 EFLAGS: 00010246
  RAX: ffffda83ffd4be08 RBX: ffffda83ffd4be40 RCX: 0000002c6e800000
  RDX: 0000000000000000 RSI: ffffda83ffd4be40 RDI: 0000000000000000
  RBP: ffffa11939f02ec0 R08: 0000000094e1a447 R09: 00000000abe76577
  R10: 0000000000000962 R11: 0000000000004e6a R12: 0000000000000000
  R13: ffffda83b1e06380 R14: ffffa18f31f072c0 R15: ffffda83ffd4be40
  FS: 0000000000000000(0000) GS:ffffa0da43b80000(0000) knlGS:0000000000000000
  CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000000000040 CR3: 0000002c77c0a003 CR4: 00000000007626e0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  PKRU: 55555554
  Call Trace:
    ? follow_page_pte+0x36d/0x5e0
    ksm_scan_thread+0x115e/0x1960
    ? remove_wait_queue+0x60/0x60
    kthread+0xf5/0x130
    ? try_to_merge_with_ksm_page+0x90/0x90
    ? kthread_create_worker_on_cpu+0x70/0x70
    ret_from_fork+0x1f/0x30
--------------------------------------------------------------------------

Fixes: e86c59b1b12d ("mm/ksm: improve deduplication of zero pages with colouring")
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Co-developed-by: Xiongchun Duan <duanxiongchun@xxxxxxxxxxxxx>
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
---

Change in v3:
    1. Update "Signed-off-by" to "Co-developed-by"
    2. Update commit message

Change in v2:
    1. Update commit message.
    2. Update patch subject from:
         "mm/ksm: Fix kernel NULL pointer dereference at 0000000000000040"
       to:
         "mm/ksm: Fix NULL pointer dereference when KSM zero page is enabled"

 mm/ksm.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index a558da9e71770..69b2f85e22d5b 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2112,8 +2112,11 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
 
 		down_read(&mm->mmap_sem);
 		vma = find_mergeable_vma(mm, rmap_item->address);
-		err = try_to_merge_one_page(vma, page,
-					    ZERO_PAGE(rmap_item->address));
+		if (vma)
+			err = try_to_merge_one_page(vma, page,
+					ZERO_PAGE(rmap_item->address));
+		else
+			err = -EFAULT;
 		up_read(&mm->mmap_sem);
 		/*
 		 * In case of failure, the page was not really empty, so we
-- 
2.11.0