Re: [PATCH] bdi: Fix another oops in wb_workfn()

Jan Kara
2018-06-13T14:46:06Z

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On Wed 13-06-18 19:43:47, Tetsuo Handa wrote:
> Can't we utilize RCU grace period (like shown below) ?

Honestly, the variant 1 looks too ugly to me. However variant 2 looks
mostly OK. We can also avoid the schedule_timeout_uninterruptible(HZ / 10)
from your patch by careful handling of the bit waitqueues. Also I'd avoid
the addition argument to wb_writeback() and split the function instead. The
patch resulting from your and mine ideas is attached. Thoughts?

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR

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>From d0fbfad964ae5b0a331f0d814d527150bd0c305c Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Date: Wed, 13 Jun 2018 16:39:00 +0200
Subject: [PATCH] bdi: Fix another oops in wb_workfn()

syzbot is reporting NULL pointer dereference at wb_workfn() [1] due to
wb->bdi->dev being NULL. And Dmitry confirmed that wb->state was
WB_shutting_down after wb->bdi->dev became NULL. This indicates that
unregister_bdi() failed to call wb_shutdown() on one of wb objects.

The problem is in cgwb_bdi_unregister() which does cgwb_kill() and thus
drops bdi's reference to wb structures before going through the list of
wbs again and calling wb_shutdown() on each of them. This way the loop
iterating through all wbs can easily miss a wb if that wb has already
passed through cgwb_remove_from_bdi_list() called from wb_shutdown()
from cgwb_release_workfn() and as a result fully shutdown bdi although
wb_workfn() for this wb structure is still running. In fact there are
also other ways cgwb_bdi_unregister() can race with
cgwb_release_workfn() leading e.g. to use-after-free issues:

CPU1 CPU2
cgwb_bdi_unregister()
cgwb_kill(*slot);

cgwb_release()
queue_work(cgwb_release_wq, &wb->release_work);
cgwb_release_workfn()
wb = list_first_entry(&bdi->wb_list, ...)
spin_unlock_irq(&cgwb_lock);
wb_shutdown(wb);
...
kfree_rcu(wb, rcu);
wb_shutdown(wb); -> oops use-after-free

We solve these issues by making wb_shutdown() remove the writeback
structure from the bdi_list only after all the shutdown is done and by
making sure cgwb_bdi_unregister() properly synchronizes with concurrent
shutdowns using WB_shutting_down bit.

Signed-off-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Jan Kara <jack@suse.cz>
---
mm/backing-dev.c | 69 ++++++++++++++++++++++++++++++++++++++++++++------------
1 file changed, 55 insertions(+), 14 deletions(-)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 347cc834c04a..433807ae364e 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -350,27 +350,21 @@ static int wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi,

static void cgwb_remove_from_bdi_list(struct bdi_writeback *wb);

-/*
- * Remove bdi from the global list and shutdown any threads we have running
- */
-static void wb_shutdown(struct bdi_writeback *wb)
+static bool wb_start_shutdown(struct bdi_writeback *wb)
{
/* Make sure nobody queues further work */
spin_lock_bh(&wb->work_lock);
if (!test_and_clear_bit(WB_registered, &wb->state)) {
spin_unlock_bh(&wb->work_lock);
- /*
- * Wait for wb shutdown to finish if someone else is just
- * running wb_shutdown(). Otherwise we could proceed to wb /
- * bdi destruction before wb_shutdown() is finished.
- */
- wait_on_bit(&wb->state, WB_shutting_down, TASK_UNINTERRUPTIBLE);
- return;
+ return false;
}
set_bit(WB_shutting_down, &wb->state);
spin_unlock_bh(&wb->work_lock);
+ return true;
+}

- cgwb_remove_from_bdi_list(wb);
+static void __wb_shutdown(struct bdi_writeback *wb)
+{
/*
* Drain work list and shutdown the delayed_work. !WB_registered
* tells wb_workfn() that @wb is dying and its work_list needs to
@@ -379,6 +373,7 @@ static void wb_shutdown(struct bdi_writeback *wb)
mod_delayed_work(bdi_wq, &wb->dwork, 0);
flush_delayed_work(&wb->dwork);
WARN_ON(!list_empty(&wb->work_list));
+ cgwb_remove_from_bdi_list(wb);
/*
* Make sure bit gets cleared after shutdown is finished. Matches with
* the barrier provided by test_and_clear_bit() above.
@@ -387,6 +382,23 @@ static void wb_shutdown(struct bdi_writeback *wb)
clear_and_wake_up_bit(WB_shutting_down, &wb->state);
}

+/*
+ * Remove bdi from the global list and shutdown any threads we have running
+ */
+static void wb_shutdown(struct bdi_writeback *wb)
+{
+ if (!wb_start_shutdown(wb)) {
+ /*
+ * Wait for wb shutdown to finish if someone else is just
+ * running wb_shutdown(). Otherwise we could proceed to wb /
+ * bdi destruction before wb_shutdown() is finished.
+ */
+ wait_on_bit(&wb->state, WB_shutting_down, TASK_UNINTERRUPTIBLE);
+ return;
+ }
+ __wb_shutdown(wb);
+}
+
static void wb_exit(struct bdi_writeback *wb)
{
int i;
@@ -706,6 +718,35 @@ static int cgwb_bdi_init(struct backing_dev_info *bdi)
return ret;
}

+/*
+ * Mark writeback structure as shutting down. The function returns true if
+ * we successfully marked the structure, false if shutdown was already in
+ * progress (in which case we also wait for it to finish).
+ *
+ * The function requires cgwb_lock to be held and releases it. Note that the
+ * caller need not hold reference to the writeback structure and thus once
+ * cgwb_lock is released, the writeback structure can be freed.
+ */
+static bool cgwb_start_shutdown(struct bdi_writeback *wb)
+ __releases(cgwb_lock)
+{
+ if (!wb_start_shutdown(wb)) {
+ DEFINE_WAIT(wait);
+ wait_queue_head_t *wqh = bit_waitqueue(&wb->state,
+ WB_shutting_down);
+ bool sleep;
+
+ prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
+ sleep = test_bit(WB_shutting_down, &wb->state);
+ spin_unlock_irq(&cgwb_lock);
+ if (sleep)
+ schedule();
+ return false;
+ }
+ spin_unlock_irq(&cgwb_lock);
+ return true;
+}
+
static void cgwb_bdi_unregister(struct backing_dev_info *bdi)
{
struct radix_tree_iter iter;
@@ -721,8 +762,8 @@ static void cgwb_bdi_unregister(struct backing_dev_info *bdi)
while (!list_empty(&bdi->wb_list)) {
wb = list_first_entry(&bdi->wb_list, struct bdi_writeback,
bdi_node);
- spin_unlock_irq(&cgwb_lock);
- wb_shutdown(wb);
+ if (cgwb_start_shutdown(wb))
+ __wb_shutdown(wb);
spin_lock_irq(&cgwb_lock);
}
spin_unlock_irq(&cgwb_lock);
--
2.13.7