On Fri, Dec 27, 2019 at 1:50 PM Roman Gushchin [off-list ref] 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.

[...]

Thanks to Josef Bacik for the debugging and the initial analysis of
the problem.

Fixes: 4bfc0bb2c60e ("bpf: decouple the lifetime of cgroup_bpf from cgroup itself")
Reported-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Roman Gushchin <redacted>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: stable@vger.kernel.org

LGTM. Thanks for the fix!

Acked-by: Song Liu <redacted>