From: Roman Gushchin <guro@fb.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: "bpf@vger.kernel.org" <bpf@vger.kernel.org>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Kernel Team <Kernel-team@fb.com>,
"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
"stable@vger.kernel.org" <stable@vger.kernel.org>
Subject: Re: [PATCH bpf] bpf: cgroup: prevent out-of-order release of cgroup bpf
Date: Sat, 4 Jan 2020 01:13:24 +0000 [thread overview]
Message-ID: <20200104011318.GA11376@localhost.localdomain> (raw)
In-Reply-To: <20200104003523.rfte5rw6hbnncjes@ast-mbp>
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.
Thanks!
next prev parent reply other threads:[~2020-01-04 1:13 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-27 21:50 [PATCH bpf] bpf: cgroup: prevent out-of-order release of cgroup bpf 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 [this message]
2020-01-04 2:31 ` Alexei Starovoitov
2020-01-04 3:00 ` Roman Gushchin
2020-01-06 22:07 ` Alexei Starovoitov
2020-01-06 22:20 ` Roman Gushchin
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