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The check that a sub-cgroup path (as set by runc exec --cgroup, or libcontainer's Process.SubCgroupPaths) does not escape the container's cgroup was too lax: a plain prefix check also accepts a sibling cgroup whose name starts with the container cgroup name (say, "../foo-suffix" for a container cgroup named "foo"). For addIntoCgroupV1 (used for per-controller sub-cgroup paths, which AddPid does not support) the resulting path is passed to WriteCgroupProc as is, so the pid did end up in the sibling cgroup. For prepareCgroupFD the subsequent cgroups.OpenFile call cleans the path, so nothing escaped in practice, but the check was wrong nevertheless. Use the same check as opencontainers/cgroups v0.0.9 does in AddPid, and add test cases for both code paths. Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com> Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> (cherry picked from commit 7820c58) Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
AkihiroSuda
reviewed
Sep 14, 2026
AkihiroSuda
approved these changes
Sep 14, 2026
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Backport of 7820c58 (the second commit) from #5403 to release-1.5.
The opencontainers/cgroups v0.1.0 bump from #5403 was already backported in #5428.
The relevant portion of original description follows.
This PR also improves the
runc exec --cgroupsub-cgroup path check (see thesecond commit).