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When cpuset is bound to a v1 hierarchy, is_in_v2_mode() > > dereferences cpuset_cgrp_subsys.root, which is freed via kfree_rcu() > > once that hierarchy is destroyed and cpuset is rebound to the default > > hierarchy. A preemptible caller outside RCU can therefore read the flags > > of a freed root. > > > > The only current caller, fair's group share calculation, runs under the > > rq lock with preemption disabled, so it can't hit this. However, the > > helper already means to protect itself with RCU, and upcoming sched_ext > > support exposes it to sleepable BPF programs. > > > > Take the RCU read lock before is_in_v2_mode() so that the whole lookup > > is protected regardless of the caller's context. > > This feels like mere querying of the mode shouldn't require such > constraints (despite it's needed anyway later down). But it could truly > happen with the novel usage (CONFIG_CPUSET_V1 && unmounting cpuset > hierarchy for some reason, I wonder how you noticed :)). > > Then I'd welcome more structured approach with at least: > > diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h > index 3754d697854b3..7f4d346cfb119 100644 > --- a/include/linux/cgroup-defs.h > +++ b/include/linux/cgroup-defs.h > @@ -841,7 +841,7 @@ struct cgroup_subsys { > const char *legacy_name; > > /* link to parent, protected by cgroup_lock() */ > - struct cgroup_root *root; > + struct cgroup_root __rcu *root; > > /* idr for css->id */ > struct idr css_idr; > diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c > index 227d09704ca59..a718b5f521fb2 100644 > --- a/kernel/cgroup/cgroup.c > +++ b/kernel/cgroup/cgroup.c > @@ -1909,7 +1909,7 @@ int rebind_subsystems(struct cgroup_root *dst_root, u32 ss_mask) > /* rebind */ > RCU_INIT_POINTER(scgrp->subsys[ssid], NULL); > rcu_assign_pointer(dcgrp->subsys[ssid], css); > - ss->root = dst_root; > + rcu_assign_pointer(ss->root, dst_root); About the __rcu annotations of ss->root, I agree it'd be nice, but it makes sparse complain about all the other plain ss->root accesses. Maybe we could do this as a separate cgroup patch/series on top of this one? Thanks, -Andrea