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
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>