swap_readpage: avoid blk_wake_io_task() if !synchronous

Commit Message

Oleg Nesterov July 4, 2019, 4:03 p.m. UTC

swap_readpage() sets waiter = bio->bi_private even if synchronous = F,
this means that the caller can get the spurious wakeup after return. This
can be fatal if blk_wake_io_task() does set_current_state(TASK_RUNNING)
after the caller does set_special_state(), in the worst case the kernel
can crash in do_task_dead().

Reported-by: Qian Cai <[e-mail]>
Acked-by: Hugh Dickins <[e-mail]>
Reviewed-by: Jens Axboe <[e-mail]>
Signed-off-by: Oleg Nesterov <[e-mail]>
---
 mm/page_io.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

Comments

Andrew Morton July 4, 2019, 7:32 p.m. UTC | #1

On Thu, 4 Jul 2019 18:03:01 +0200 Oleg Nesterov <[e-mail]> wrote:

> swap_readpage() sets waiter = bio->bi_private even if synchronous = F,
> this means that the caller can get the spurious wakeup after return. This
> can be fatal if blk_wake_io_task() does set_current_state(TASK_RUNNING)
> after the caller does set_special_state(), in the worst case the kernel
> can crash in do_task_dead().

I think we need a Fixes: and a cc:stable here?

IIRC, we're fixing 0619317ff8baa2 ("block: add polled wakeup task helper").

Hugh Dickins July 4, 2019, 9:15 p.m. UTC | #2

On Thu, 4 Jul 2019, Andrew Morton wrote:
> On Thu, 4 Jul 2019 18:03:01 +0200 Oleg Nesterov <[e-mail]> wrote:
> 
> > swap_readpage() sets waiter = bio->bi_private even if synchronous = F,
> > this means that the caller can get the spurious wakeup after return. This
> > can be fatal if blk_wake_io_task() does set_current_state(TASK_RUNNING)
> > after the caller does set_special_state(), in the worst case the kernel
> > can crash in do_task_dead().
> 
> I think we need a Fixes: and a cc:stable here?
> 
> IIRC, we're fixing 0619317ff8baa2 ("block: add polled wakeup task helper").

Yes, you are right.

But catch me by surprise: I had been thinking this was a 5.2 regression.
I guess something in 5.2 (doesn't matter what) has made it significantly
easier to hit: but now I look at old records, see that I hit it once on
5.0-rc1, then never again until 5.2.

Thanks, and to Oleg,
Hugh

Patch

diff --git a/mm/page_io.c b/mm/page_io.c
index 2e8019d..3098895 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -140,8 +140,10 @@  static void end_swap_bio_read(struct bio *bio)
 	unlock_page(page);
 	WRITE_ONCE(bio->bi_private, NULL);
 	bio_put(bio);
-	blk_wake_io_task(waiter);
-	put_task_struct(waiter);
+	if (waiter) {
+		blk_wake_io_task(waiter);
+		put_task_struct(waiter);
+	}
 }
 
 int generic_swapfile_activate(struct swap_info_struct *sis,
@@ -398,11 +400,12 @@  int swap_readpage(struct page *page, bool synchronous)
 	 * Keep this task valid during swap readpage because the oom killer may
 	 * attempt to access it in the page fault retry time check.
 	 */
-	get_task_struct(current);
-	bio->bi_private = current;
 	bio_set_op_attrs(bio, REQ_OP_READ, 0);
-	if (synchronous)
+	if (synchronous) {
 		bio->bi_opf |= REQ_HIPRI;
+		get_task_struct(current);
+		bio->bi_private = current;
+	}
 	count_vm_event(PSWPIN);
 	bio_get(bio);
 	qc = submit_bio(bio);