To properly clear the slab on free with slab_want_init_on_free, we walk
the list of free objects using get_freepointer/set_freepointer.

The value we get from get_freepointer may not be valid.  This isn't an
issue since an actual value will get written later but this means
there's a chance of triggering a bug if we use this value with
set_freepointer:

  kernel BUG at mm/slub.c:306!
  invalid opcode: 0000 [#1] PREEMPT PTI
  CPU: 0 PID: 0 Comm: swapper Not tainted 5.2.0-05754-g6471384a #4
  RIP: 0010:kfree+0x58a/0x5c0
  Code: 48 83 05 78 37 51 02 01 0f 0b 48 83 05 7e 37 51 02 01 48 83 05 7e 37 51 02 01 48 83 05 7e 37 51 02 01 48 83 05 d6 37 51 02 01 <0f> 0b 48 83 05 d4 37 51 02 01 48 83 05 d4 37 51 02 01 48 83 05 d4
  RSP: 0000:ffffffff82603d90 EFLAGS: 00010002
  RAX: ffff8c3976c04320 RBX: ffff8c3976c04300 RCX: 0000000000000000
  RDX: ffff8c3976c04300 RSI: 0000000000000000 RDI: ffff8c3976c04320
  RBP: ffffffff82603db8 R08: 0000000000000000 R09: 0000000000000000
  R10: ffff8c3976c04320 R11: ffffffff8289e1e0 R12: ffffd52cc8db0100
  R13: ffff8c3976c01a00 R14: ffffffff810f10d4 R15: ffff8c3976c04300
  FS:  0000000000000000(0000) GS:ffffffff8266b000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: ffff8c397ffff000 CR3: 0000000125020000 CR4: 00000000000406b0
  Call Trace:
   apply_wqattrs_prepare+0x154/0x280
   apply_workqueue_attrs_locked+0x4e/0xe0
   apply_workqueue_attrs+0x36/0x60
   alloc_workqueue+0x25a/0x6d0
   workqueue_init_early+0x246/0x348
   start_kernel+0x3c7/0x7ec
   x86_64_start_reservations+0x40/0x49
   x86_64_start_kernel+0xda/0xe4
   secondary_startup_64+0xb6/0xc0
  Modules linked in:
  ---[ end trace f67eb9af4d8d492b ]---

Fix this by ensuring the value we set with set_freepointer is either NULL
or another value in the chain.
Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Laura Abbott <labbott@redhat.com>
Fixes: 6471384a ("mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options")
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>