Re: [PATCH v3] zsmalloc: fix migrate_zspage-zs_free race condition

[Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>]

Cc Vlastimil,


Hello,

On (01/18/16 10:15), Junil Lee wrote:
> To prevent unlock at the not correct situation, tagging the new obj to
> assure lock in migrate_zspage() before right unlock path.
> 
> Two functions are in race condition by tag which set 1 on last bit of
> obj, however unlock succrently when update new obj to handle before call
> unpin_tag() which is right unlock path.
> 
> summarize this problem by call flow as below:
> 
> 		CPU0								CPU1
> migrate_zspage
> find_alloced_obj()
> 	trypin_tag() -- obj |= HANDLE_PIN_BIT
> obj_malloc() -- new obj is not set			zs_free
> record_obj() -- unlock and break sync		pin_tag() -- get lock
> unpin_tag()

Junil, can something like this be a bit simpler problem description?

---

record_obj() in migrate_zspage() does not preserve handle's
HANDLE_PIN_BIT, set by find_alloced_obj()->trypin_tag(), and
implicitly (accidentally) un-pins the handle, while migrate_zspage()
still performs an explicit unpin_tag() on the that handle.
This additional explicit unpin_tag() introduces a race condition
with zs_free(), which can pin that handle by this time, so the handle
becomes un-pinned. Schematically, it goes like this:

CPU0							CPU1
migrate_zspage
  find_alloced_obj
    trypin_tag
      set HANDLE_PIN_BIT				zs_free()
							  pin_tag()
  obj_malloc() -- new object, no tag
  record_obj() -- remove HANDLE_PIN_BIT			   set HANDLE_PIN_BIT
  unpin_tag()  -- remove zs_free's HANDLE_PIN_BIT

The race condition may result in a NULL pointer dereference:
	Unable to handle kernel NULL pointer dereference at virtual address 00000000
	CPU: 0 PID: 19001 Comm: CookieMonsterCl Tainted:
	PC is at get_zspage_mapping+0x0/0x24
	LR is at obj_free.isra.22+0x64/0x128
	Call trace:
		[<ffffffc0001a3aa8>] get_zspage_mapping+0x0/0x24
		[<ffffffc0001a4918>] zs_free+0x88/0x114
		[<ffffffc00053ae54>] zram_free_page+0x64/0xcc
		[<ffffffc00053af4c>] zram_slot_free_notify+0x90/0x108
		[<ffffffc000196638>] swap_entry_free+0x278/0x294
		[<ffffffc000199008>] free_swap_and_cache+0x38/0x11c
		[<ffffffc0001837ac>] unmap_single_vma+0x480/0x5c8
		[<ffffffc000184350>] unmap_vmas+0x44/0x60
		[<ffffffc00018a53c>] exit_mmap+0x50/0x110
		[<ffffffc00009e408>] mmput+0x58/0xe0
		[<ffffffc0000a2854>] do_exit+0x320/0x8dc
		[<ffffffc0000a3cb4>] do_group_exit+0x44/0xa8
		[<ffffffc0000ae1bc>] get_signal+0x538/0x580
		[<ffffffc000087e44>] do_signal+0x98/0x4b8
		[<ffffffc00008843c>] do_notify_resume+0x14/0x5c

Fix the race by removing explicit unpin_tag() from migrate_zspage().

---


> and for test, print obj value after pin_tag() in zs_free().
> Sometimes obj is even number means break synchronization.
> 
> After patched, crash is not occurred and obj is only odd number in same
> situation.
> 
> Signed-off-by: Junil Lee <junil0814.lee@lge.com>

I believe Vlastimil deserves a credit here (at least Suggested-by)
Suggested-by: Vlastimil Babka <vbabka@suse.cz>


now, can the compiler re-order

	record_obj(handle, free_obj);
	obj_free(pool, class, used_obj);

?

	-ss

> ---
>  mm/zsmalloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index e7414ce..0acfa20 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1635,8 +1635,8 @@ static int migrate_zspage(struct zs_pool *pool, struct size_class *class,
>  		free_obj = obj_malloc(d_page, class, handle);
>  		zs_object_copy(free_obj, used_obj, class);
>  		index++;
> +		/* This also effectively unpins the handle */
>  		record_obj(handle, free_obj);
> -		unpin_tag(handle);
>  		obj_free(pool, class, used_obj);
>  	}
>  
> -- 
> 2.6.2
> 

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