Hello Junil, On Mon, Jan 18, 2016 at 02:39:56PM +0900, Junil Lee wrote: > record_obj() in migrate_zspage() does not preserve handle's > HANDLE_PIN_BIT, set by find_aloced_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(). > > Signed-off-by: Junil Lee <junil0814.lee@xxxxxxx> > --- > 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 */ As reply of Vlastimil, I relied that I guess it doesn't work. We shouldn't omit unpin_tag and we should add WRITE_ONCE in record_obj. As well, it's worth to dobule check with locking guys. I will send updated version. Thanks. > record_obj(handle, free_obj); > - unpin_tag(handle); > obj_free(pool, class, used_obj); > } > > -- > 2.6.2 > -- 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>