Patch "zsmalloc: fix migrate_zspage-zs_free race condition" has been added to the 4.3-stable tree

This is a note to let you know that I've just added the patch titled

zsmalloc: fix migrate_zspage-zs_free race condition

to the 4.3-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
zsmalloc-fix-migrate_zspage-zs_free-race-condition.patch
and it can be found in the queue-4.3 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Junil Lee <[email protected]>
Date: Wed, 20 Jan 2016 14:58:18 -0800
Subject: zsmalloc: fix migrate_zspage-zs_free race condition

From: Junil Lee <[email protected]>

commit c102f07ca0b04f2cb49cfc161c83f6239d17f491 upstream.

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.

---
 mm/zsmalloc.c |   14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

@@ -304,7 +304,12 @@ static void free_handle(struct zs_pool *
 
 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 */

@@ -1629,6 +1634,13 @@ static int migrate_zspage(struct zs_pool
 		free_obj = obj_malloc(d_page, class, handle);
 		zs_object_copy(free_obj, used_obj, class);
 		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);
 		unpin_tag(handle);
 		obj_free(pool, class, used_obj);

Patches currently in stable-queue which might be from [email protected] are

queue-4.3/zsmalloc-fix-migrate_zspage-zs_free-race-condition.patch