The patch titled
Subject: zsmalloc: fix migrate_zspage-zs_free race condition
has been added to the -mm tree. Its filename is
zsmalloc-fix-migrate_zspage-zs_free-race-condition.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/zsmalloc-fix-migrate_zspage-zs_free-race-condition.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/zsmalloc-fix-migrate_zspage-zs_free-race-condition.patch
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------------------------------------------------------
From: Junil Lee <junil0814.lee@xxxxxxx>
Subject: 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:
[<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
This patch keeps the lock bit in migration path and update
value atomically.
Signed-off-by: Junil Lee <junil0814.lee@xxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [4.1+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/zsmalloc.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)
diff -puN mm/zsmalloc.c~zsmalloc-fix-migrate_zspage-zs_free-race-condition mm/zsmalloc.c
--- a/mm/zsmalloc.c~zsmalloc-fix-migrate_zspage-zs_free-race-condition
+++ a/mm/zsmalloc.c
@@ -309,7 +309,12 @@ 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 */
@@ -1635,6 +1640,13 @@ 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 -mm which might be from junil0814.lee@xxxxxxx are
zsmalloc-fix-migrate_zspage-zs_free-race-condition.patch
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