Commit c102f07c authored Jan 20, 2016 by Junil Lee Committed by Linus Torvalds Jan 20, 2016

zsmalloc: fix migrate_zspage-zs_free race condition



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: Junil Lee <junil0814.lee@lge.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: <stable@vger.kernel.org> [4.1+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent dcd6c87c

mm/zsmalloc.c

+13 −1

Original line number	Original line	Diff line number	Diff line
	@@ -309,7 +309,12 @@ static void free_handle(struct zs_pool *pool, unsigned long handle)

	static void record_obj(unsigned long handle, unsigned long obj)		static void record_obj(unsigned long handle, unsigned long obj)
	{		{
	(unsigned long )handle = obj;		/*
			* lsb of @obj represents handle lock while other bits
			* represent object value the handle is pointing so
			* updating shouldn't do store tearing.
			*/
			WRITE_ONCE((unsigned long )handle, obj);
	}		}

	/* zpool driver */		/* zpool driver */
	@@ -1635,6 +1640,13 @@ static int migrate_zspage(struct zs_pool pool, struct size_class class,
	free_obj = obj_malloc(d_page, class, handle);		free_obj = obj_malloc(d_page, class, handle);
	zs_object_copy(free_obj, used_obj, class);		zs_object_copy(free_obj, used_obj, class);
	index++;		index++;
			/*
			* record_obj updates handle's value to free_obj and it will
			* invalidate lock bit(ie, HANDLE_PIN_BIT) of handle, which
			* breaks synchronization using pin_tag(e,g, zs_free) so
			* let's keep the lock bit.
			*/
			free_obj \|= BIT(HANDLE_PIN_BIT);
	record_obj(handle, free_obj);		record_obj(handle, free_obj);
	unpin_tag(handle);		unpin_tag(handle);
	obj_free(pool, class, used_obj);		obj_free(pool, class, used_obj);