From: Alexei Starovoitov <email@example.com> To: Roman Gushchin <firstname.lastname@example.org> Cc: "email@example.com" <firstname.lastname@example.org>, "email@example.com" <firstname.lastname@example.org>, Alexei Starovoitov <email@example.com>, Daniel Borkmann <firstname.lastname@example.org>, "email@example.com" <firstname.lastname@example.org>, Kernel Team <Kernelemail@example.com>, "firstname.lastname@example.org" <email@example.com>, "firstname.lastname@example.org" <email@example.com> Subject: Re: [PATCH bpf] bpf: cgroup: prevent out-of-order release of cgroup bpf Date: Mon, 6 Jan 2020 14:07:48 -0800 [thread overview] Message-ID: <20200106220746.fm3hp3zynaiaqgly@ast-mbp> (raw) In-Reply-To: <20200104030041.GA12685@localhost.localdomain> On Sat, Jan 04, 2020 at 03:00:46AM +0000, Roman Gushchin wrote: > On Fri, Jan 03, 2020 at 06:31:14PM -0800, Alexei Starovoitov wrote: > > On Sat, Jan 04, 2020 at 01:13:24AM +0000, Roman Gushchin wrote: > > > On Fri, Jan 03, 2020 at 04:35:25PM -0800, Alexei Starovoitov wrote: > > > > On Fri, Dec 27, 2019 at 01:50:34PM -0800, Roman Gushchin wrote: > > > > > Before commit 4bfc0bb2c60e ("bpf: decouple the lifetime of cgroup_bpf > > > > > from cgroup itself") cgroup bpf structures were released with > > > > > corresponding cgroup structures. It guaranteed the hierarchical order > > > > > of destruction: children were always first. It preserved attached > > > > > programs from being released before their propagated copies. > > > > > > > > > > But with cgroup auto-detachment there are no such guarantees anymore: > > > > > cgroup bpf is released as soon as the cgroup is offline and there are > > > > > no live associated sockets. It means that an attached program can be > > > > > detached and released, while its propagated copy is still living > > > > > in the cgroup subtree. This will obviously lead to an use-after-free > > > > > bug. > > > > ... > > > > > @@ -65,6 +65,9 @@ static void cgroup_bpf_release(struct work_struct *work) > > > > > > > > > > mutex_unlock(&cgroup_mutex); > > > > > > > > > > + for (p = cgroup_parent(cgrp); p; p = cgroup_parent(p)) > > > > > + cgroup_bpf_put(p); > > > > > + > > > > > > > > The fix makes sense, but is it really safe to walk cgroup hierarchy > > > > without holding cgroup_mutex? > > > > > > It is, because we're holding a reference to the original cgroup and going > > > towards the root. On each level the cgroup is protected by a reference > > > from their child cgroup. > > > > cgroup_bpf_put(p) can make bpf.refcnt zero which may call cgroup_bpf_release() > > on another cpu which will do cgroup_put() and this cpu p = cgroup_parent(p) > > would be use-after-free? > > May be not due to the way work_queues are implemented. > > But it feels dangerous to have such delicate release logic. > > If I understand your concern correctly: you assume that parent's > cgroup_bpf_release() can be finished prior to the child's one and > the final cgroup_put() will release the parent? > > If so, it's not possible, because the child hold a reference to the > parent (independent to all cgroup bpf stuff), which exists at least > until the final cgroup_put() in cgroup_bpf_release(). Please, look > at css_free_rwork_fn() for details. > > > Why not to move the loop under the mutex and make things obvious? > > Traversing the cgroup tree to the root cgroup without additional > locking seems pretty common to me. You can find a ton of examples in > mm/memcontrol.c. So it doesn't look scary or adventurous to me. > > I think it doesn't matter that much here, so I'm ok with putting it > under the mutex, but IMO it won't make the code any safer. > > > cc Tejun for the second opinion on cgroup locking Checked with TJ offline. This seems fine. I tweaked commit log: - extra 'diff' lines were confusing 'git am' - commit description shouldn't be split into multiline And applied to bpf tree. Thanks
next prev parent reply other threads:[~2020-01-06 22:07 UTC|newest] Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-12-27 21:50 Roman Gushchin 2020-01-03 15:30 ` Roman Gushchin 2020-01-03 17:47 ` Song Liu 2020-01-04 0:35 ` Alexei Starovoitov 2020-01-04 1:13 ` Roman Gushchin 2020-01-04 2:31 ` Alexei Starovoitov 2020-01-04 3:00 ` Roman Gushchin 2020-01-06 22:07 ` Alexei Starovoitov [this message] 2020-01-06 22:20 ` Roman Gushchin
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