The Docker terminal backend runs containers with `--cap-drop ALL`
and re-adds only DAC_OVERRIDE, CHOWN, FOWNER. Since commit fee0e0d3
("run as non-root user, use virtualenv") the image entrypoint drops
from root to the `hermes` user via `gosu`, which requires CAP_SETUID
and CAP_SETGID. Without them every sandbox container exits
immediately with:
Dropping root privileges
error: failed switching to 'hermes': operation not permitted
Breaking every terminal/file tool invocation in `terminal.backend: docker`
mode.
Fix: add SETUID and SETGID to the cap-add list. The `no-new-privileges`
security-opt is kept, so gosu still cannot escalate back to root after
the one-way drop — the hardening posture is preserved.
Reproduction
------------
With any image whose ENTRYPOINT calls `gosu <user>`, the container
exits immediately under the pre-fix cap set. Post-fix, the drop
succeeds and the container proceeds normally.
docker run --rm \
--cap-drop ALL \
--cap-add DAC_OVERRIDE --cap-add CHOWN --cap-add FOWNER \
--security-opt no-new-privileges \
--entrypoint /usr/local/bin/gosu \
hermes-claude:latest hermes id
# -> error: failed switching to 'hermes': operation not permitted
# Same command with SETUID+SETGID added:
# -> uid=10000(hermes) gid=10000(hermes) groups=10000(hermes)
Tests
-----
Added `test_security_args_include_setuid_setgid_for_gosu_drop` that
asserts both caps are present and the overall hardening posture
(cap-drop ALL + no-new-privileges) is preserved.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
