Re: [PATCH] sched/autogroup: Fix task_group UAF during thread creation

[Peter Zijlstra]

On Fri, Oct 30, 2026 at 04:28:54PM -1000, Tejun Heo wrote:
> Hello, Peter.
> 
> On Fri, Oct 30, 2026 at 04:49:40PM +0100, Peter Zijlstra wrote:
> > Now, AUTOGROUP depends on CGROUP, so we can make autogroup_move_group()
> > also use cgroup_threadgroup_rwsem, such that the autogroup and cgroup
> > logic stays the same?
> 
> cgroup_threadgroup_rwsem is a global percpu rwsem and taking it for
> write on a regular basis can be really expensive.

Yeah, but setsid() isn't more critical than moving tasks between
cgroups. Also, AFAIK nearly nobody is using autogroups these days,
because systemd does the cgroup thing.

> > I mean, yes we can move sched_core_fork() into the siglock region, but
> > duplicating most of sched_cgroup_fork() in there just to fix an
> > autogroup thingy seems weird and inconsistent.
> 
> Either way, AFAICS, autogroup doesn't need cgroup_threadgroup_rwsem. The
> cgroup side has to be settled early in fork, but the autogroup side
> doesn't. Can't we just move the autogroup assignment later in fork, to a
> point where it can't race against setsid()?

That is more or less the proposal, but it rather creates more complexity
rather than reducing it.

Having two different rules for p->sched_task_group, depending on cgroup
or autogroup isn't at all appealing. Not to mention adding even more
fork callbacks.