The deny rules: the machine itself is never a destination

The way through the extension sandbox's deny rules is an allowlist, and an
allowlist names addresses. The machine's own LAN address is not a fact
anybody can pin: a new DHCP lease can turn a peer's address on some
package's list into the machine's, and with it open the Grbl port or
forgectrl's listeners to that package.

ffx.nft now refuses it structurally. Everything a host sends to one of its
own addresses, the LAN one included, leaves through lo, so chain pool
refuses `oifname "lo"` before it looks at the allow map; the two refusals
(a reset for TCP, a drop for the rest, both counted) move into chain
refuse, which pool jumps to from both places. No kernel option is new:
oifname is in the nf_tables core.

scripts/sandbox-rules-test.py gets a destination that is not the machine:
a second network namespace joined by a veth pair, with listeners of its
own. A pool uid is refused on loopback, IPv6 loopback, its own LAN address,
and the peer; an allow chain opens one port of the peer to one uid and
nothing else; with loopback, IPv6 loopback, and the machine's own address
added to that list the uid is still refused at all three while the peer
still answers; a reload closes it. It needs ip and nsenter now.

exthost.platform reads its counters from chain refuse, holds the rule's
place ahead of the map, and adds the case on the machine: an allow chain
for the last pool uid that names forgectrl on loopback and on the LAN
address opens neither, and the chain is removed.

Proven. The rules test passes with nft 1.0.9, and four controls each fail
it: the range one uid short, the TCP reject turned to accept, the
delete-table line removed, and the lo rule removed (the uid then reaches
all three of the machine's addresses). On the bench reference, image
20260920211625, this rule file loaded from /tmp with nft -f and this suite
file mounted: exthost.platform PASS, uid 831 refused at 127.0.0.1:443 and
172.16.1.97:443 with both on its allowlist, the counters [0, 0] to
[12, 4]. Against the image's own rules the same test fails on the rule's
absence, which is the control. The image's rules were reloaded after. The
unit suite passes (422).

Acceptance. exthost.platform gates the rule on the machine; sandbox-ci
gates the file. The rule file is layer content, in the platform identity
of every fingerprint.

forgetest/forgetest/suite/exthost.py

@@ -205,8 +205,8 @@ def _pids_of(comm):
 
 
 def _pool_counters():
-    """The packet counts of chain pool's two refusals: [tcp reset, drop]."""
-    p = subprocess.run([NFT, "-j", "list", "chain", "inet", "ffx", "pool"], capture_output=True, text=True)
+    """The packet counts of chain refuse's two rules: [tcp reset, drop]."""
+    p = subprocess.run([NFT, "-j", "list", "chain", "inet", "ffx", "refuse"], capture_output=True, text=True)
     if p.returncode != 0:
         return None
     return [e["counter"]["packets"] for c in json.loads(p.stdout)["nftables"] if "rule" in c
@@ -366,6 +366,9 @@ def platform(ctx):
         ev["rules"] = p.stdout
         ctx.check("meta skuid %d-%d jump pool" % (POOL_FIRST, uid) in p.stdout and "hook output" in p.stdout
                   and "policy accept" in p.stdout, "table inet ffx is not the image's: %s", p.stdout[:400])
+        pool = p.stdout[p.stdout.find("chain pool"):].split("}")[0]
+        ctx.check(0 <= pool.find('oifname "lo" jump refuse') < pool.find("vmap @allow"),
+                  "the machine itself is not refused ahead of the allow map: %s", pool)
         lan = lan_ip()
         ctx.check(lan, "the machine has no LAN address to aim at")
         tcp = [[4, "127.0.0.1", 443, False], [4, "127.0.0.1", 80, False], [6, "::1", 443, False],
@@ -398,6 +401,29 @@ def platform(ctx):
         ctx.check(c0 is not None and c1 is not None and len(c1) == 2 and c1[0] - c0[0] >= 2 * len(tcp)
                   and c1[1] - c0[1] >= 2 * len(udp), "the rules did not count the attempts: %s -> %s", c0, c1)
 
+        # The machine itself is not a destination, whatever the allow map says: an
+        # allow chain that names forgectrl on loopback and on the LAN address
+        # opens neither.
+        chain = "u%d" % uid
+        rules = ["add chain inet ffx %s" % chain,
+                 "add rule inet ffx %s ip daddr { 127.0.0.1, %s } tcp dport 443 accept" % (chain, lan),
+                 "add element inet ffx allow { %d : jump %s }" % (uid, chain), ""]
+        opened = subprocess.run([NFT, "-f", "-"], capture_output=True, text=True, input=os.linesep.join(rules))
+        try:
+            ctx.check(opened.returncode == 0, "could not add an allow chain: %s", opened.stderr.strip()[:200])
+            rc, lines, err = _probe("-", uid, "net", json.dumps([[4, "127.0.0.1", 443, False], [4, lan, 443, False]]))
+            res = (lines[-1] if lines else {}).get("results", [])
+            ev["allowlisted_self"] = res
+            ctx.log("uid %d with the machine's own addresses on its allowlist: %s", uid, res)
+            ctx.check(len(res) == 2 and all(r[1][0] == "refused" for r in res),
+                      "an allowlist opened the machine itself to a pool uid: %s %s", res, err)
+        finally:
+            subprocess.run([NFT, "delete", "element", "inet", "ffx", "allow", "{ %d }" % uid], capture_output=True)
+            subprocess.run([NFT, "flush", "chain", "inet", "ffx", chain], capture_output=True)
+            subprocess.run([NFT, "delete", "chain", "inet", "ffx", chain], capture_output=True)
+        left = subprocess.run([NFT, "list", "chain", "inet", "ffx", chain], capture_output=True)
+        ctx.check(left.returncode != 0, "the test's allow chain stayed behind")
+
         # 6. landlock and seccomp, on a root process so that neither the uid nor the rules explain the refusal
         rc, lines, err = _probe("-", 0, "landlock", json.dumps([4, "127.0.0.1", 443, False]))
         ll = lines[-1] if lines else {}