Re: A small window for a race condition in mm/rmap.c:page_lock_anon_vma_read

[Michal Hocko]

anon_vma locking is clever^Wsubtle as hell. CC Peter...

On Tue 20-12-16 09:32:27, Dashi DS1 Cao wrote:
> I've collected four crash dumps with similar backtrace. 
> 
> PID: 247    TASK: ffff881fcfad8000  CPU: 14  COMMAND: "kswapd1"
>  #0 [ffff881fcfad7978] machine_kexec at ffffffff81051e9b
>  #1 [ffff881fcfad79d8] crash_kexec at ffffffff810f27e2
>  #2 [ffff881fcfad7aa8] oops_end at ffffffff8163f448
>  #3 [ffff881fcfad7ad0] die at ffffffff8101859b
>  #4 [ffff881fcfad7b00] do_general_protection at ffffffff8163ed3e
>  #5 [ffff881fcfad7b30] general_protection at ffffffff8163e5e8
>     [exception RIP: down_read_trylock+9]
>     RIP: ffffffff810aa9f9  RSP: ffff881fcfad7be0  RFLAGS: 00010286
>     RAX: 0000000000000000  RBX: ffff882b47ddadc0  RCX: 0000000000000000
>     RDX: 0000000000000000  RSI: 0000000000000000  RDI: 91550b2b32f5a3e8

rdi is obviously a mess - smells like a string. So either sombody has
overwritten root_anon_vma or this is really a use after free...

>     RBP: ffff881fcfad7be0   R8: ffffea00ecc28860   R9: ffff883fcffeae28
>     R10: ffffffff81a691a0  R11: 0000000000000001  R12: ffff882b47ddadc1
>     R13: ffffea00ecc28840  R14: 91550b2b32f5a3e8  R15: ffffea00ecc28840
>     ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0000
>  #6 [ffff881fcfad7be8] page_lock_anon_vma_read at ffffffff811a3365
>  #7 [ffff881fcfad7c18] page_referenced at ffffffff811a35e7
>  #8 [ffff881fcfad7c90] shrink_active_list at ffffffff8117e8cc
>  #9 [ffff881fcfad7d48] balance_pgdat at ffffffff81180288
> #10 [ffff881fcfad7e20] kswapd at ffffffff81180813
> #11 [ffff881fcfad7ec8] kthread at ffffffff810a5b8f
> #12 [ffff881fcfad7f50] ret_from_fork at ffffffff81646a98
> 
> I suspect my customer hits into a small window of a race condition in mm/rmap.c: page_lock_anon_vma_read.
> struct anon_vma *page_lock_anon_vma_read(struct page *page)
> {
>         struct anon_vma *anon_vma = NULL;
>         struct anon_vma *root_anon_vma;
>         unsigned long anon_mapping;
> 
>         rcu_read_lock();
>         anon_mapping = (unsigned long)READ_ONCE(page->mapping);
>         if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
>                 goto out;
>         if (!page_mapped(page))
>                 goto out;
> 
>         anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
>         root_anon_vma = READ_ONCE(anon_vma->root);

Could you dump the anon_vma and struct page as well?

>         if (down_read_trylock(&root_anon_vma->rwsem)) {
>                 /*
>                  * If the page is still mapped, then this anon_vma is still
>                  * its anon_vma, and holding the mutex ensures that it will
>                  * not go away, see anon_vma_free().
>                  */
>                 if (!page_mapped(page)) {
>                         up_read(&root_anon_vma->rwsem);
>                         anon_vma = NULL;
>                 }
>                 goto out;
>         }
> ...
> }
> 
> Between the time the two "page_mapped(page)" are checked, the address
> (anon_mapping - PAGE_MAPPING_ANON) is unmapped! However it seems
> that anon_vma->root could still be read in but the value is wild. So
> the kernel crashes in down_read_trylock. But it's weird that all the
> "struct page" has its member "_mapcount" still with value 0, not -1,
> in the four crashes.

-- 
Michal Hocko
SUSE Labs