Messages in this thread Patch in this message | | | From | Neeraj Upadhyay <> | Subject | [PATCH] cgroup: Fix potential race between cgroup_exit and migrate path | Date | Wed, 30 Aug 2017 18:24:09 +0530 |
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There is a potential race between cgroup_exit() and the migration path. This race happens because cgroup_exit path reads the css_set and does cg_list empty check outside of css_set lock. This can potentially race with the migrate path trying to move the tasks to a different css_set. For instance, below is the interleaved sequence of events, where race is observed:
cpuset_hotplug_workfn() cgroup_transfer_tasks() cgroup_migrate() cgroup_migrate_execute() css_set_move_task() list_del_init(&task->cg_list); <TASK EXIT> cgroup_exit() cset = task_css_set(tsk); if (!list_empty(&tsk->cg_list)) <TASK NOT DISSOCIATED FROM ITS CSS_SET> list_add_tail(&task->cg_list, use_mg_tasks
In above sequence, as cgroup_exit() read the cg_list for the task as empty, it didn't disassociate it from its current css_set, and was moved to new css_set instance css_set_move_task() called from cpuset_hotplug_workfn() path. This eventually can result in use after free scenarios, while accessing the same task_struct again, like in following sequence:
kernfs_seq_start() cgroup_seqfile_start() cgroup_pidlist_start() css_task_iter_next() __put_task_struct() <NULL pointer dereference>
Fix this problem, by moving the css_set and cg_list fetch in cgroup_exit() inside css_set lock.
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org> --- Hi,
We observed this issue for cgroup code corresponding to stable v4.4.85 snapshot 3144d81 ("cgroup, kthread: close race window where new kthreads can be migrated to non-root cgroups"). Can you please tell us, if there are any patches in latest code, which fixes these issue?
kernel/cgroup/cgroup.c | 20 ++++++++++++-------- 1 file changed, 12 insertions(+), 8 deletions(-)
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c index df2e0f1..f746b70 100644 --- a/kernel/cgroup/cgroup.c +++ b/kernel/cgroup/cgroup.c @@ -692,10 +692,10 @@ static void css_set_move_task(struct task_struct *task, if (to_cset) { /* - * We are synchronized through cgroup_threadgroup_rwsem - * against PF_EXITING setting such that we can't race - * against cgroup_exit() changing the css_set to - * init_css_set and dropping the old one. + * We are synchronized through css_set_lock against + * PF_EXITING setting such that we can't race against + * cgroup_exit() disassociating the task from the + * css_set. */ WARN_ON_ONCE(task->flags & PF_EXITING); @@ -4934,20 +4934,24 @@ void cgroup_exit(struct task_struct *tsk) int i; /* - * Unlink from @tsk from its css_set. As migration path can't race - * with us, we can check css_set and cg_list without synchronization. + * Avoid potential race with the migrate path. + */ + spin_lock_irq(&css_set_lock); + + /* + * Unlink from @tsk from its css_set. */ cset = task_css_set(tsk); if (!list_empty(&tsk->cg_list)) { - spin_lock_irq(&css_set_lock); css_set_move_task(tsk, cset, NULL, false); cset->nr_tasks--; - spin_unlock_irq(&css_set_lock); } else { get_css_set(cset); } + spin_unlock_irq(&css_set_lock); + /* see cgroup_post_fork() for details */ do_each_subsys_mask(ss, i, have_exit_callback) { ss->exit(tsk); -- QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, hosted by The Linux Foundation
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