• Junil Lee's avatar
    zsmalloc: fix migrate_zspage-zs_free race condition · c102f07c
    Junil Lee authored
    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:
         get_zspage_mapping+0x0/0x24
         zs_free+0x88/0x114
         zram_free_page+0x64/0xcc
         zram_slot_free_notify+0x90/0x108
         swap_entry_free+0x278/0x294
         free_swap_and_cache+0x38/0x11c
         unmap_single_vma+0x480/0x5c8
         unmap_vmas+0x44/0x60
         exit_mmap+0x50/0x110
         mmput+0x58/0xe0
         do_exit+0x320/0x8dc
         do_group_exit+0x44/0xa8
         get_signal+0x538/0x580
         do_signal+0x98/0x4b8
         do_notify_resume+0x14/0x5c
    
    This patch keeps the lock bit in migration path and update value
    atomically.
    Signed-off-by: default avatarJunil Lee <junil0814.lee@lge.com>
    Signed-off-by: default avatarMinchan Kim <minchan@kernel.org>
    Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
    Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
    Cc: <stable@vger.kernel.org> [4.1+]
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    c102f07c
zsmalloc.c 48.2 KB