Three maintainer decisions from the challenger read on the #1830 unit.
1. DELETE THE CACHE (not just fix its comment).
posix_ancestor_overflow_uid() memoised via pthread_once behind a comment
claiming "immutable /proc state". Both halves were false: unshare(CLONE_NEWUSER)
rewrites /proc/self/uid_map, and pthread_once state survives fork() already
marked done. A process that forked and then entered a namespace kept the
parent verdict and refused a directory it should have accepted.
It was harmless only because every caller today runs in a freshly exec'd
process -- the daemon double-fork execs, workers go through
cbm_subprocess_spawn's fork+exec, worker_pool.c never forks, and src/ contains
no setns or unshare at all. That made a security decision depend on an
invariant nothing enforced. The next fork-without-exec caller would have
inherited a stale verdict silently.
Now derived fresh per call: two small /proc reads against an openat + fstat +
fchmod + ACL check per path component in the same walk.
With a test that would have caught the original bug and needs no namespace.
daemon_ipc_posix_overflow_uid_is_never_cached_issue1830 counts real
derivations through the production entry point and asserts the count rises on
every ancestor check. Re-introducing a cache fails it by name on every
platform. Proven both ways on linux/arm64 as uid 1001: cache re-introduced
49 passed 1 failed; removed 50 passed 0 skipped.
Worth knowing why that test is not redundant: with the re-exec probe in
place, the userns smoke test PASSES even with the cache restored, because the
probe is a fresh process and immune to it by construction. The two tests are
complementary -- the smoke test binds the namespace wiring, this one binds
the absence of the cache.
2. RUN THE GUARD ON THE CORE MATRIX.
Ubuntu 23.10+ ships kernel.apparmor_restrict_unprivileged_userns=1, so the
#1830 smoke test skipped on every 24.04 leg and its only real-namespace
coverage sat on the two ubuntu-22.04 legs. Those are broad-matrix only, so
the test ran in NO pull request, and GitHub retires those images on
2027-04-17 -- after which it would have become a permanent silent skip, and a
skip is not a red. A deliberate relaxation of a security check was guarded by
a test that ran nowhere a change would be reviewed, on a clock.
_test.yml now lifts the restriction with one sysctl on every ubuntu leg
(hosted runners have passwordless sudo), best-effort so a runner that refuses
simply returns to skipping. The O10 skip-whitelist comment no longer presents
the AppArmor restriction as immovable.
3. KEEP THE TOLERANCE, CORRECT THE SAFETY ARGUMENT.
The comment claimed that in a single-uid map "nobody reachable can have
created it or can mutate it". The overflow uid is what EVERY unmapped host
uid maps to, not only host root (user_namespaces(7)), so overflow-owned does
not prove root-created: on a shared host another local user's sticky
directory is indistinguishable from root-owned /tmp from inside the namespace.
The tolerance stays, because there is no in-namespace discriminator that
could tell them apart -- and narrowing it would refuse the legitimate
root-owned 01777 /tmp that #1830 exists to accept, i.e. it would revert the
fix in practice. What changes is the claim: the real guarantee is that no
principal reachable from inside the namespace can create or mutate such an
ancestor, and the leaf is never tolerated as overflow (0700, euid-rechecked),
so a hostile host-side ancestor owner is bounded to denial of service and
socket-path control.
macOS daemon_ipc 53 passed 2 skipped; linux/arm64 50 passed 0 skipped;
lint-ci clean.
Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>
Martin Vogel
d21e6cc762