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Message-Id: <20160214222144.942363390@linuxfoundation.org>
Date: Sun, 14 Feb 2016 14:22:34 -0800
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-kernel@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
stable@...r.kernel.org, Junil Lee <junil0814.lee@....com>,
Minchan Kim <minchan@...nel.org>,
Vlastimil Babka <vbabka@...e.cz>,
Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 4.4 117/117] zsmalloc: fix migrate_zspage-zs_free race condition
4.4-stable review patch. If anyone has any objections, please let me know.
------------------
From: Junil Lee <junil0814.lee@....com>
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.
Signed-off-by: Junil Lee <junil0814.lee@....com>
Signed-off-by: Minchan Kim <minchan@...nel.org>
Acked-by: Vlastimil Babka <vbabka@...e.cz>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
mm/zsmalloc.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)
--- a/mm/zsmalloc.c
+++ b/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);
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