Completes Phase 1. The gate now asks for FAN_CLOSE_WRITE alongside the
permission events, so a threat written to disk is quarantined and a
threat being executed is refused — one mechanism, one mark, no
watch-descriptor ceiling and no blind spots outside a configured list.
Verified live. The nicest evidence is an error message:
$ chmod +x /tmp/hound-live/malware.sh
chmod: cannot access '/tmp/hound-live/malware.sh': No such file or directory
Hound had already quarantined it. `hound quarantine list` shows the
entry, the clean binary beside it still runs, and CapPrm/CapEff/CapBnd
read 000000000020000e.
Three bugs, each of which looked like working code:
* A file descriptor number is not an identity. The kernel allocates an
fd per event and recycles the number the moment we close it, so one
write arrives as FAN_OPEN_PERM on fd 6 and then FAN_CLOSE_WRITE on fd
6 again. Idempotency keyed on the fd treated the second as a duplicate
of the first and dropped it — detection ran, matched EICAR, and threw
the result away. Events now carry a monotonic seq that is never reused.
* rename(2) fails EXDEV across filesystems, and for quarantine that is
the common case rather than the exotic one: the vault is under
/var/lib while threats land on /home, in a tmpfs, on a USB stick or
in a container overlay. Quarantine now falls back to copy-then-unlink,
unlinking only once the copy is safely down, and seals the stored file
at 0600 with every execute bit cleared.
* The capability set was too small to do the job. CAP_DAC_READ_SEARCH
lets us read a threat but not unlink it, so quarantine failed EACCES
as root. The set is now four capabilities — SYS_ADMIN, DAC_READ_SEARCH,
DAC_OVERRIDE, FOWNER. DAC_OVERRIDE is close to "write anywhere" and
that is worth being honest about; an antivirus that quarantines cannot
avoid it, because the threat is by definition in a directory somebody
else owns. What the reduction still buys is what it excludes, and
there is a test asserting SYS_MODULE, SYS_BOOT, SYS_PTRACE, NET_ADMIN,
NET_RAW, AUDIT_CONTROL and SETUID never creep back in. Narrowing
further means a separate privileged helper for quarantine.
realtime.rs is now documented as the unprivileged fallback and does not
start when the gate is armed — running both would scan everything twice
and quarantine the same file from two threads.
99 tests pass. HOUNDD_GATE_DEBUG=1 dumps every event and decision.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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