Re: NULL pointer dereference in the kernel 3.10

[Hillf Danton]

On April 10, 2017 5:54 PM Xishi Qiu wrote: 
> On 2017/4/10 17:37, Hillf Danton wrote:
> 
> > On April 10, 2017 4:57 PM Xishi Qiu wrote:
> >> On 2017/4/10 14:42, Hillf Danton wrote:
> >>
> >>> On April 08, 2017 9:40 PM zhong Jiang wrote:
> >>>>
> >>>> when runing the stabile docker cases in the vm.   The following issue will come up.
> >>>>
> >>>> #40 [ffff8801b57ffb30] async_page_fault at ffffffff8165c9f8
> >>>>     [exception RIP: down_read_trylock+5]
> >>>>     RIP: ffffffff810aca65  RSP: ffff8801b57ffbe8  RFLAGS: 00010202
> >>>>     RAX: 0000000000000000  RBX: ffff88018ae858c1  RCX: 0000000000000000
> >>>>     RDX: 0000000000000000  RSI: 0000000000000000  RDI: 0000000000000008
> >>>>     RBP: ffff8801b57ffc10   R8: ffffea0006903de0   R9: ffff8800b3c61810
> >>>>     R10: 00000000000022cb  R11: 0000000000000000  R12: ffff88018ae858c0
> >>>>     R13: ffffea0006903dc0  R14: 0000000000000008  R15: ffffea0006903dc0
> >>>>     ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0000
> >>>> #41 [ffff8801b57ffbe8] page_lock_anon_vma_read at ffffffff811b241c
> >>>> #42 [ffff8801b57ffc18] page_referenced at ffffffff811b26a7
> >>>> #43 [ffff8801b57ffc90] shrink_active_list at ffffffff8118d634
> >>>> #44 [ffff8801b57ffd48] balance_pgdat at ffffffff8118f088
> >>>> #45 [ffff8801b57ffe20] kswapd at ffffffff8118f633
> >>>> #46 [ffff8801b57ffec8] kthread at ffffffff810a795f
> >>>> #47 [ffff8801b57fff50] ret_from_fork at ffffffff81665398
> >>>> crash> struct page.mapping ffffea0006903dc0
> >>>>   mapping = 0xffff88018ae858c1
> >>>> crash> struct anon_vma 0xffff88018ae858c0
> >>>> struct anon_vma {
> >>>>   root = 0x0,
> >>>>   rwsem = {
> >>>>     count = 0,
> >>>>     wait_lock = {
> >>>>       raw_lock = {
> >>>>         {
> >>>>           head_tail = 1,
> >>>>           tickets = {
> >>>>             head = 1,
> >>>>             tail = 0
> >>>>           }
> >>>>         }
> >>>>       }
> >>>>     },
> >>>>     wait_list = {
> >>>>       next = 0x0,
> >>>>       prev = 0x0
> >>>>     }
> >>>>   },
> >>>>   refcount = {
> >>>>     counter = 0
> >>>>   },
> >>>>   rb_root = {
> >>>>     rb_node = 0x0
> >>>>   }
> >>>> }
> >>>>
> >>>> This maks me wonder,  the anon_vma do not come from slab structure.
> >>>> and the content is abnormal. IMO,  At least anon_vma->root will not NULL.
> >>>> The issue can be reproduced every other week.
> >>>>
> >>> Check please if commit
> >>> 624483f3ea8 ("mm: rmap: fix use-after-free in __put_anon_vma")
> >>> is included in the 3.10 you are running.
> >>>
> >> We missed this patch in RHEL 7.2
> >> Could you please give more details for how it triggered?
> >
> > Sorry, I could not.
> > I guess it is UAF as described in the log of that commit.
> > And if it works for you, we know how.
> >
> > Hillf
> >
> 
> __put_anon_vma            |   page_lock_anon_vma_read
>   anon_vma_free(root)     |
>                           |     root_anon_vma = ACCESS_ONCE(anon_vma->root)
>                           |     down_read_trylock(&root_anon_vma->rwsem)
>   anon_vma_free(anon_vma) |
> 
> I find anon_vma was created by SLAB_DESTROY_BY_RCU, so it will not merge
> by other slabs, and free_slab() will not free it during page_lock_anon_vma_read(),
> because it holds rcu_read_lock(), right?
> 
Dunno frankly, Sir, you know, I am not an rmap expert like you.
And pretty much probable I made a wrong guess, and sorry again.

> If root_anon_vma was reuse by someone, why "crash> struct anon_vma"
> shows almost zero?
> 
thank you very much
Hillf