+ mm-ksm-fix-null-pointer-dereference-when-ksm-zero-page-is-enabled.patch added to -mm tree

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/ksm: fix NULL pointer dereference when KSM zero page is enabled
has been added to the -mm tree.  Its filename is
     mm-ksm-fix-null-pointer-dereference-when-ksm-zero-page-is-enabled.patch

This patch should soon appear at
    http://ozlabs.org/~akpm/mmots/broken-out/mm-ksm-fix-null-pointer-dereference-when-ksm-zero-page-is-enabled.patch
and later at
    http://ozlabs.org/~akpm/mmotm/broken-out/mm-ksm-fix-null-pointer-dereference-when-ksm-zero-page-is-enabled.patch

Before you just go and hit "reply", please:
   a) Consider who else should be cc'ed
   b) Prefer to cc a suitable mailing list as well
   c) Ideally: find the original patch on the mailing list and do a
      reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next and is updated
there every 3-4 working days

------------------------------------------------------
From: Muchun Song <songmuchun@bytedance.com>
Subject: mm/ksm: fix NULL pointer dereference when KSM zero page is enabled

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
--------------------------------------------------------------------------

Link: http://lkml.kernel.org/r/20200414132905.83819-1-songmuchun@bytedance.com
Fixes: e86c59b1b12d ("mm/ksm: improve deduplication of zero pages with colouring")
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Co-developed-by: Xiongchun Duan <duanxiongchun@bytedance.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Cc: Markus Elfring <Markus.Elfring@web.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

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

--- a/mm/ksm.c~mm-ksm-fix-null-pointer-dereference-when-ksm-zero-page-is-enabled
+++ a/mm/ksm.c
@@ -2112,8 +2112,11 @@ static void cmp_and_merge_page(struct pa
 
 		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
_

Patches currently in -mm which might be from songmuchun@bytedance.com are

mm-ksm-fix-null-pointer-dereference-when-ksm-zero-page-is-enabled.patch