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

Jan Kara
2018-06-15T12:06:20Z

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On Wed 13-06-18 07:33:15, Tejun Heo wrote:
> Hello, Jan.
>
> On Tue, Jun 12, 2018 at 05:57:54PM +0200, Jan Kara wrote:
> > > Yeah, right, so the root cause is that we're walking the wb_list while
> > > holding lock and expecting the object to stay there even after lock is
> > > released. Hmm... we can use a mutex to synchronize the two
> > > destruction paths. It's not like they're hot paths anyway.
> >
> > Hmm, do you mean like having a per-bdi or even a global mutex that would
> > protect whole wb_shutdown()? Yes, that should work and we could get rid of
> > WB_shutting_down bit as well with that. Just it seems a bit strange to
>
> Yeap.
>
> > introduce a mutex only to synchronize these two shutdown paths - usually
> > locks protect data structures and in this case we have cgwb_lock for
> > that so it looks like a duplication from a first look.
>
> Yeah, I feel a bit reluctant too but I think that's the right thing to
> do here. This is an inherently weird case where there are two ways
> that an object can go away with the immediate drain requirement from
> one side. It's not a hot path and the dumber the synchronization the
> better, right?

Yeah, fair enough. Something like attached patch? It is indeed considerably
simpler than fixing synchronization using WB_shutting_down. This one even
got some testing using scsi_debug, I want to do more testing next week with
more cgroup writeback included.

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

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>From 95fa7fe8ebc7dcf4d09a2e097c06abdf24ba81b5 Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@suse.cz>
Date: Thu, 14 Jun 2018 14:30:03 +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 synchronizing writeback structure shutdown from
cgwb_bdi_unregister() with cgwb_release_workfn() using a new mutex.

Signed-off-by: Jan Kara <jack@suse.cz>
---
include/linux/backing-dev-defs.h | 1 +
mm/backing-dev.c | 7 +++++++
2 files changed, 8 insertions(+)

diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index 0bd432a4d7bd..661e3121e580 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -189,6 +189,7 @@ struct backing_dev_info {
#ifdef CONFIG_CGROUP_WRITEBACK
struct radix_tree_root cgwb_tree; /* radix tree of active cgroup wbs */
struct rb_root cgwb_congested_tree; /* their congested states */
+ struct mutex cgwb_release_mutex; /* protect shutdown of wb structs */
#else
struct bdi_writeback_congested *wb_congested;
#endif
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 347cc834c04a..078cb4e0fb62 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -508,10 +508,12 @@ static void cgwb_release_workfn(struct work_struct *work)
struct bdi_writeback *wb = container_of(work, struct bdi_writeback,
release_work);

+ mutex_lock(&wb->bdi->cgwb_release_mutex);
wb_shutdown(wb);

css_put(wb->memcg_css);
css_put(wb->blkcg_css);
+ mutex_unlock(&wb->bdi->cgwb_release_mutex);

fprop_local_destroy_percpu(&wb->memcg_completions);
percpu_ref_exit(&wb->refcnt);
@@ -697,6 +699,7 @@ static int cgwb_bdi_init(struct backing_dev_info *bdi)

INIT_RADIX_TREE(&bdi->cgwb_tree, GFP_ATOMIC);
bdi->cgwb_congested_tree = RB_ROOT;
+ mutex_init(&bdi->cgwb_release_mutex);

ret = wb_init(&bdi->wb, bdi, 1, GFP_KERNEL);
if (!ret) {
@@ -717,7 +720,10 @@ static void cgwb_bdi_unregister(struct backing_dev_info *bdi)
spin_lock_irq(&cgwb_lock);
radix_tree_for_each_slot(slot, &bdi->cgwb_tree, &iter, 0)
cgwb_kill(*slot);
+ spin_unlock_irq(&cgwb_lock);

+ mutex_lock(&bdi->cgwb_release_mutex);
+ spin_lock_irq(&cgwb_lock);
while (!list_empty(&bdi->wb_list)) {
wb = list_first_entry(&bdi->wb_list, struct bdi_writeback,
bdi_node);
@@ -726,6 +732,7 @@ static void cgwb_bdi_unregister(struct backing_dev_info *bdi)
spin_lock_irq(&cgwb_lock);
}
spin_unlock_irq(&cgwb_lock);
+ mutex_unlock(&bdi->cgwb_release_mutex);
}

/**
--
2.16.4