Syzbot found a corrupted list bug scenario that can be triggered from cgroup_subtree_control_write(cgrp). The reproduces writes to cgroup.subtree_control file, which invokes: cgroup_apply_control_enable()->css_create()->css_populate_dir(), which then fails with a fault injected -ENOMEM. In such scenario the css_killed_work_fn will be en-queued via cgroup_apply_control_disable(cgrp)->kill_css(css), and bail out to cgroup_kn_unlock(). Then cgroup_kn_unlock() will call: cgroup_put(cgrp)->css_put(&cgrp->self), which will try to enqueue css_release_work_fn for the same css instance, causing a list_add corruption bug, as can be seen in the syzkaller report [1].
Fix this by synchronizing the css ref_kill and css_release jobs. css_release() function will check if the css_killed_work_fn() has been scheduled for the css and only en-queue the css_release_work_fn() if css_killed_work_fn wasn't already en-queued. Otherwise css_release() will set the CSS_REL_LATER flag for that css. This will cause the css_release_work_fn() work to be executed after css_killed_work_fn() is finished.
Two scc flags have been introduced to implement this serialization mechanizm:
* CSS_KILL_ENQED, which will be set when css_killed_work_fn() is en-queued, and * CSS_REL_LATER, which, if set, will cause the css_release_work_fn() to be scheduled after the css_killed_work_fn is finished.
There is also a new lock, which will protect the integrity of the css flags.
[1] https://syzkaller.appspot.com/bug?id=e26e54d6eac9d9fb50b221ec3e4627b327465db...
Cc: Tejun Heo tj@kernel.org Cc: Michal Koutny mkoutny@suse.com Cc: Zefan Li lizefan.x@bytedance.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Christian Brauner brauner@kernel.org Cc: Alexei Starovoitov ast@kernel.org Cc: Daniel Borkmann daniel@iogearbox.net Cc: Andrii Nakryiko andrii@kernel.org Cc: Martin KaFai Lau kafai@fb.com Cc: Song Liu songliubraving@fb.com Cc: Yonghong Song yhs@fb.com Cc: John Fastabend john.fastabend@gmail.com Cc: KP Singh kpsingh@kernel.org Cc: cgroups@vger.kernel.org Cc: netdev@vger.kernel.org Cc: bpf@vger.kernel.org Cc: stable@vger.kernel.org Cc: linux-kernel@vger.kernel.org
Reported-and-tested-by: syzbot+e42ae441c3b10acf9e9d@syzkaller.appspotmail.com Fixes: 8f36aaec9c92 ("cgroup: Use rcu_work instead of explicit rcu and work item") Signed-off-by: Tadeusz Struk tadeusz.struk@linaro.org --- include/linux/cgroup-defs.h | 4 ++++ kernel/cgroup/cgroup.c | 35 ++++++++++++++++++++++++++++++++--- 2 files changed, 36 insertions(+), 3 deletions(-)
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h index 1bfcfb1af352..8dc8b4edb242 100644 --- a/include/linux/cgroup-defs.h +++ b/include/linux/cgroup-defs.h @@ -53,6 +53,8 @@ enum { CSS_RELEASED = (1 << 2), /* refcnt reached zero, released */ CSS_VISIBLE = (1 << 3), /* css is visible to userland */ CSS_DYING = (1 << 4), /* css is dying */ + CSS_KILL_ENQED = (1 << 5), /* kill work enqueued for the css */ + CSS_REL_LATER = (1 << 6), /* release needs to be done after kill */ };
/* bits in struct cgroup flags field */ @@ -162,6 +164,8 @@ struct cgroup_subsys_state { */ int id;
+ /* lock to protect flags */ + spinlock_t lock; unsigned int flags;
/* diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c index 1779ccddb734..a0ceead4b390 100644 --- a/kernel/cgroup/cgroup.c +++ b/kernel/cgroup/cgroup.c @@ -5210,8 +5210,23 @@ static void css_release(struct percpu_ref *ref) struct cgroup_subsys_state *css = container_of(ref, struct cgroup_subsys_state, refcnt);
- INIT_WORK(&css->destroy_work, css_release_work_fn); - queue_work(cgroup_destroy_wq, &css->destroy_work); + spin_lock_bh(&css->lock); + + /* + * Check if the css_killed_work_fn work has been scheduled for this + * css and enqueue css_release_work_fn only if it wasn't. + * Otherwise set the CSS_REL_LATER flag, which will cause + * release to be enqueued after css_killed_work_fn is finished. + * This is to prevent list corruption by en-queuing two instance + * of the same work struct on the same WQ, namely cgroup_destroy_wq. + */ + if (!(css->flags & CSS_KILL_ENQED)) { + INIT_WORK(&css->destroy_work, css_release_work_fn); + queue_work(cgroup_destroy_wq, &css->destroy_work); + } else { + css->flags |= CSS_REL_LATER; + } + spin_unlock_bh(&css->lock); }
static void init_and_link_css(struct cgroup_subsys_state *css, @@ -5230,6 +5245,7 @@ static void init_and_link_css(struct cgroup_subsys_state *css, INIT_LIST_HEAD(&css->rstat_css_node); css->serial_nr = css_serial_nr_next++; atomic_set(&css->online_cnt, 0); + spin_lock_init(&css->lock);
if (cgroup_parent(cgrp)) { css->parent = cgroup_css(cgroup_parent(cgrp), ss); @@ -5545,10 +5561,12 @@ int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name, umode_t mode) */ static void css_killed_work_fn(struct work_struct *work) { - struct cgroup_subsys_state *css = + struct cgroup_subsys_state *css_killed, *css = container_of(work, struct cgroup_subsys_state, destroy_work);
mutex_lock(&cgroup_mutex); + css_killed = css; + css_killed->flags &= ~CSS_KILL_ENQED;
do { offline_css(css); @@ -5557,6 +5575,14 @@ static void css_killed_work_fn(struct work_struct *work) css = css->parent; } while (css && atomic_dec_and_test(&css->online_cnt));
+ spin_lock_bh(&css->lock); + if (css_killed->flags & CSS_REL_LATER) { + /* If css_release work was delayed for the css enqueue it now. */ + INIT_WORK(&css_killed->destroy_work, css_release_work_fn); + queue_work(cgroup_destroy_wq, &css_killed->destroy_work); + css_killed->flags &= ~CSS_REL_LATER; + } + spin_unlock_bh(&css->lock); mutex_unlock(&cgroup_mutex); }
@@ -5566,10 +5592,13 @@ static void css_killed_ref_fn(struct percpu_ref *ref) struct cgroup_subsys_state *css = container_of(ref, struct cgroup_subsys_state, refcnt);
+ spin_lock_bh(&css->lock); if (atomic_dec_and_test(&css->online_cnt)) { + css->flags |= CSS_KILL_ENQED; INIT_WORK(&css->destroy_work, css_killed_work_fn); queue_work(cgroup_destroy_wq, &css->destroy_work); } + spin_unlock_bh(&css->lock); }
/**