It is possible to hit a zero entry while traversing the vmas in
unuse_mm(), called from the swapoff path. Not checking the zero entry
can result into operating on it as vma which leads into oops.

The issue is manifested from the below race between the fork() on a
process and swapoff:
fork(dup_mmap())			swapoff(unuse_mm)
---------------                         -----------------
1) Identical mtree is built using
   __mt_dup().

2) copy_pte_range()-->
	copy_nonpresent_pte():
       The dst mm is added into the
    mmlist to be visible to the
    swapoff operation.

3) Fatal signal is sent to the parent
process(which is the current during the
fork) thus skip the duplication of the
vmas and mark the vma range with
XA_ZERO_ENTRY as a marker for this process
that helps during exit_mmap().

				     4) swapoff is tried on the
					'mm' added to the 'mmlist' as
					part of the 2.

				     5) unuse_mm(), that iterates
					through the vma's of this 'mm'
					will hit the non-NULL zero entry
					and operating on this zero entry
					as a vma is resulting into the
					oops.

Crash reported:
--------------
Unable to handle kernel NULL pointer dereference at virtual address
0000000000000446--> Loading the memory from offset 0x40 on the
XA_ZERO_ENTRY as address.
Mem abort info:
  ESR = 0x0000000096000005
  EC = 0x25: DABT (current EL), IL = 32 bits
  SET = 0, FnV = 0
  EA = 0, S1PTW = 0
  FSC = 0x05: level 1 translation fault

 pc : try_to_unuse+0x188/0x79c
 lr : try_to_unuse+0x440/0x79c
 Call trace:
  try_to_unuse+0x188/0x79c
  __arm64_sys_swapoff+0x248/0x4d0
  invoke_syscall+0x58/0x10c
  el0_svc_common+0xa8/0xdc
  do_el0_svc+0x1c/0x28
  el0_svc+0x38/0x88
  el0t_64_sync_handler+0x70/0xbc
  el0t_64_sync+0x1a8/0x1ac

Fix this by checking if vma is zero entry before operating on it.

Signed-off-by: Charan Teja Kalla <quic_charante@quicinc.com>
---
 mm/swapfile.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 68ce283e84be..91513830ef9c 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -2245,6 +2245,12 @@ static int unuse_mm(struct mm_struct *mm, unsigned int type)
 
 	mmap_read_lock(mm);
 	for_each_vma(vmi, vma) {
+		/*
+		 * zero entries in mm_struct->mm_mt is a marker to stop
+		 * looking for vma's. see comment in exit_mmap().
+		 */
+		if (xa_is_zero(vma))
+			break;
 		if (vma->anon_vma && !is_vm_hugetlb_page(vma)) {
 			ret = unuse_vma(vma, type);
 			if (ret)
-- 
2.34.1