roothelper: pin bridge name + IP + CIDR to a banger-managed shape

priv.ensure_bridge / priv.create_tap accepted the daemon's network
config triple (BridgeName, BridgeIP, CIDR) and forwarded it straight
to `ip link` / `ip addr` / `ip link set master`. Argv-style exec
ruled out shell injection, but the kernel happily honours those
commands against any iface a compromised owner-uid daemon names —
including eth0/docker0/lo. Concretely:

  * priv.ensure_bridge could `ip link set <iface> up` against any
    host interface and `ip addr add` arbitrary IP/CIDR to it.
  * priv.create_tap could `ip link set <new-tap> master <iface>`,
    bridging the per-VM tap into the host's primary LAN so the
    guest sees host-local broadcast traffic.
  * priv.sync_resolver_routing / priv.clear_resolver_routing only
    enforced "name shaped like a Linux iface" — no banger constraint.

New validators (single chokepoint via validateNetworkConfig):
  * validateBangerBridgeName: name must equal "br-fc" or start with
    "br-fc-". Stops a compromised daemon from naming any host iface
    in these RPCs. Users with a custom bridge keep the prefix.
  * validateCIDRPrefix: numeric in [8, 32]. Wider prefixes would
    silently widen the bridge subnet beyond what the daemon intends.
  * validateNetworkConfig bundles bridge-name + validateIPv4 +
    validateCIDRPrefix so every helper RPC that takes the triple
    stays in lockstep.

Wired into methodEnsureBridge, methodCreateTap, and the resolver-
routing pair (replacing the older validateLinuxIfaceName-only check
with the stricter banger-bridge check).

docs/privileges.md updated: the helper-RPC table rows now spell out
the banger-managed bridge constraint, and the trust list includes
the new validators.

Tests: TestValidateBangerBridgeName (default + suffixed accepted,
host ifaces / wrong prefix / oversized rejected), TestValidate
CIDRPrefix (boundary + non-numeric + IPv6-style 64 rejected),
TestValidateNetworkConfig (happy path + each-field-bad cases).
Smoke at JOBS=4 still green — banger's defaults sail through the
new gate.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/privileges.md

@@ -71,11 +71,11 @@ validator before the helper touches the host.
 
 | Method | Effect | Validation gate |
 |---|---|---|
-| `priv.ensure_bridge` | Create the configured Linux bridge if missing; assign the bridge IP. | Bridge name and IP come from owner config; helper does not allow caller to pick `lo` etc. |
+| `priv.ensure_bridge` | Create the configured Linux bridge if missing; assign the bridge IP. | Bridge name must equal `br-fc` or start with `br-fc-` (so a compromised daemon can't drive `ip link` against `eth0` / `docker0` / `lo`). Bridge IP must parse as IPv4. CIDR prefix must be a number in `[8, 32]`. |
-| `priv.create_tap` | `ip link add tap NAME tuntap` and add to bridge, owned by the owner user. | Tap name must match `tap-fc-*` or `tap-pool-*`. |
+| `priv.create_tap` | `ip link add tap NAME tuntap` and add to bridge, owned by the owner user. | Tap name must match `tap-fc-*` or `tap-pool-*`. Bridge config (name + IP + CIDR) passes the same banger-managed check as `priv.ensure_bridge`, otherwise the new tap could be `master`-attached to an arbitrary host iface. |
-| `priv.delete_tap` | `ip link del NAME`. | Same prefix check. |
+| `priv.delete_tap` | `ip link del NAME`. | Same prefix check on the tap name. |
-| `priv.sync_resolver_routing` | `resolvectl dns/domain/default-route` on the configured bridge. | Bridge name passes the kernel iface-name rules (1–15 chars, no `/`/`:`/whitespace, not `.`/`..`). Resolver address must parse via `net.ParseIP`. |
+| `priv.sync_resolver_routing` | `resolvectl dns/domain/default-route` on the configured bridge. | Bridge name must equal `br-fc` or start with `br-fc-` (same banger-managed check). Resolver address must parse via `net.ParseIP`. |
-| `priv.clear_resolver_routing` | `resolvectl revert` on the bridge. | Same iface-name check. |
+| `priv.clear_resolver_routing` | `resolvectl revert` on the bridge. | Same banger-managed bridge-name check. |
 | `priv.ensure_nat` | `iptables -t nat MASQUERADE` for `(guest_ip, tap)` plus matching FORWARD rules; `enable=false` removes them. | Tap must be banger-prefixed. Guest IP must parse as IPv4. |
 | `priv.create_dm_snapshot` | Create a `dmsetup` device-mapper snapshot from `rootfs.ext4` with COW backing file. | Both paths must be inside `/var/lib/banger`; DM name must start with `fc-rootfs-`. |
 | `priv.cleanup_dm_snapshot` | `dmsetup remove` and `losetup -d` for a snapshot the helper itself just created. | Every non-empty `dmsnap.Handles` field is checked: DM name `fc-rootfs-*`, DM device `/dev/mapper/fc-rootfs-*`, loops `/dev/loopN`. |
@@ -271,11 +271,12 @@ If you install banger as root, you are trusting:
    `validateLoopDevicePath`, `validateDMRemoveTarget`,
    `validateDMSnapshotHandles`, `validateRootExecutable`,
    `validateNotSymlink`, `validateExt4ImagePath`,
-   `validateLinuxIfaceName`, `validateIPv4`, `validateResolverAddr`,
+   `validateLinuxIfaceName`, `validateBangerBridgeName`,
-   and `validateFirecrackerPID`. If any of these are bypassed, the
+   `validateNetworkConfig`, `validateCIDRPrefix`, `validateIPv4`,
-   helper would carry out a privileged op against an unmanaged
+   `validateResolverAddr`, `validateSignalName`, and
-   target. They are unit-tested in
+   `validateFirecrackerPID`. If any of these are bypassed, the helper
-   `internal/roothelper/roothelper_test.go`.
+   would carry out a privileged op against an unmanaged target. They
+   are unit-tested in `internal/roothelper/roothelper_test.go`.
 3. The Firecracker binary banger executes. The helper refuses to launch
    anything that isn't a regular, executable, root-owned, not
    world-writable file — but the binary's own behaviour is your

internal/roothelper/roothelper.go

@@ -436,6 +436,13 @@ func (s *Server) dispatch(ctx context.Context, req rpc.Request) rpc.Response {
 		if err != nil {
 			return rpc.NewError("bad_params", err.Error())
 		}
+		// Without these the helper would happily run `ip link add`
+		// against arbitrary names, `ip addr add` with arbitrary
+		// IP/CIDR, and `ip link set <NAME> up` against any host
+		// iface a compromised daemon might pick.
+		if err := validateNetworkConfig(params); err != nil {
+			return rpc.NewError("bad_params", err.Error())
+		}
 		return marshalResultOrError(struct{}{}, s.ensureBridge(ctx, params))
 	case methodCreateTap:
 		params, err := rpc.DecodeParams[struct {
@@ -445,6 +452,12 @@ func (s *Server) dispatch(ctx context.Context, req rpc.Request) rpc.Response {
 		if err != nil {
 			return rpc.NewError("bad_params", err.Error())
 		}
+		// Pin both the bridge config (so the new TAP can't be
+		// attached to e.g. eth0 via `ip link set <tap> master`) and
+		// the tap name itself.
+		if err := validateNetworkConfig(params.NetworkConfig); err != nil {
+			return rpc.NewError("bad_params", err.Error())
+		}
 		return marshalResultOrError(struct{}{}, s.createTap(ctx, params.NetworkConfig, params.TapName))
 	case methodDeleteTap:
 		params, err := rpc.DecodeParams[struct {
@@ -463,11 +476,13 @@ func (s *Server) dispatch(ctx context.Context, req rpc.Request) rpc.Response {
 			return rpc.NewError("bad_params", err.Error())
 		}
 		// syncResolverRouting short-circuits on empty input; only
-		// validate when actually doing something. This stops a
+		// validate when actually doing something. validateBanger
-		// compromised daemon from flapping arbitrary system-managed
+		// BridgeName is stricter than the previous validateLinux
-		// links via resolvectl.
+		// IfaceName: it stops a compromised daemon from pointing
+		// resolvectl at any host interface, not just refusing
+		// obviously-malformed names.
 		if strings.TrimSpace(params.BridgeName) != "" || strings.TrimSpace(params.ServerAddr) != "" {
-			if err := validateLinuxIfaceName(params.BridgeName); err != nil {
+			if err := validateBangerBridgeName(params.BridgeName); err != nil {
 				return rpc.NewError("bad_params", err.Error())
 			}
 			if err := validateResolverAddr(params.ServerAddr); err != nil {
@@ -483,7 +498,7 @@ func (s *Server) dispatch(ctx context.Context, req rpc.Request) rpc.Response {
 			return rpc.NewError("bad_params", err.Error())
 		}
 		if strings.TrimSpace(params.BridgeName) != "" {
-			if err := validateLinuxIfaceName(params.BridgeName); err != nil {
+			if err := validateBangerBridgeName(params.BridgeName); err != nil {
 				return rpc.NewError("bad_params", err.Error())
 			}
 		}
@@ -1105,6 +1120,75 @@ func (s *Server) validateExt4ImagePath(path string) error {
 	return fmt.Errorf("path %q is not a banger-managed ext4 image", path)
 }
 
+// bangerBridgeNamePrefix pins the only iface-name shape the helper
+// will mutate via priv.ensure_bridge / priv.create_tap / the resolver
+// routing RPCs. Anything that doesn't match — host primary interfaces
+// like eth0/wlan0/lo, foreign managed bridges like docker0/virbr0,
+// arbitrary attacker-chosen names — is refused outright. Banger's
+// daemon-config default for BridgeName is "br-fc"; users wanting a
+// different name must keep the "br-fc-" prefix so the helper can
+// recognise it as banger-managed.
+const bangerBridgeNamePrefix = "br-fc"
+
+// validateBangerBridgeName enforces the banger naming convention on
+// any bridge name a helper RPC mutates. Without this, a compromised
+// owner-uid daemon could ask the helper (which runs with
+// CAP_NET_ADMIN) to bring up arbitrary host interfaces, attach
+// per-VM taps to other users' bridges, or flap the host's primary
+// iface — argv-style exec rules out shell injection but the kernel
+// happily honours these requests against any iface the caller
+// names.
+func validateBangerBridgeName(name string) error {
+	if err := validateLinuxIfaceName(name); err != nil {
+		return err
+	}
+	trimmed := strings.TrimSpace(name)
+	if trimmed == bangerBridgeNamePrefix {
+		return nil
+	}
+	if strings.HasPrefix(trimmed, bangerBridgeNamePrefix+"-") {
+		return nil
+	}
+	return fmt.Errorf("bridge name %q is not banger-managed (must equal %q or start with %q)", name, bangerBridgeNamePrefix, bangerBridgeNamePrefix+"-")
+}
+
+// validateCIDRPrefix accepts a numeric IPv4 prefix length in [8, 32].
+// fcproc.EnsureBridge concatenates BridgeIP + "/" + CIDR into the
+// `ip addr add` argument, so anything that doesn't parse as a small
+// integer in that range either errors out (helpful) or, worse,
+// silently widens the bridge subnet beyond what the daemon intends.
+func validateCIDRPrefix(s string) error {
+	trimmed := strings.TrimSpace(s)
+	if trimmed == "" {
+		return errors.New("cidr prefix is required")
+	}
+	n, err := strconv.Atoi(trimmed)
+	if err != nil {
+		return fmt.Errorf("cidr prefix %q is not numeric", s)
+	}
+	if n < 8 || n > 32 {
+		return fmt.Errorf("cidr prefix %d is outside [8, 32]", n)
+	}
+	return nil
+}
+
+// validateNetworkConfig is the single chokepoint for every helper RPC
+// that takes a bridge name + IP + CIDR triple. Bundling the checks
+// here keeps every caller in lockstep on what counts as a
+// well-formed banger network config.
+func validateNetworkConfig(cfg NetworkConfig) error {
+	if err := validateBangerBridgeName(cfg.BridgeName); err != nil {
+		return err
+	}
+	if err := validateIPv4(cfg.BridgeIP); err != nil {
+		return fmt.Errorf("bridge ip: %w", err)
+	}
+	if err := validateCIDRPrefix(cfg.CIDR); err != nil {
+		return fmt.Errorf("bridge cidr: %w", err)
+	}
+	return nil
+}
+
 // validateLoopDevicePath confirms path is `/dev/loopN` for some N≥0.
 // dmsnap.Cleanup detaches loops via `losetup -d <path>`; without this
 // a compromised daemon could ask the helper to detach an arbitrary

internal/roothelper/roothelper_test.go

@@ -397,6 +397,102 @@ func TestValidateManagedPathPassesPlainSubpath(t *testing.T) {
 	}
 }
 
+func TestValidateBangerBridgeName(t *testing.T) {
+	t.Parallel()
+	for _, tc := range []struct {
+		name string
+		arg  string
+		ok   bool
+	}{
+		{name: "default", arg: "br-fc", ok: true},
+		{name: "suffixed", arg: "br-fc-alt", ok: true},
+		{name: "with_whitespace", arg: " br-fc ", ok: true},
+		{name: "wrong_prefix", arg: "br0", ok: false},
+		{name: "host_iface", arg: "eth0", ok: false},
+		{name: "docker", arg: "docker0", ok: false},
+		{name: "loopback", arg: "lo", ok: false},
+		{name: "empty", arg: "", ok: false},
+		{name: "br_dash_only", arg: "br-", ok: false}, // not "br-fc" exactly
+		{name: "almost_match", arg: "br-fcx", ok: false},
+		{name: "with_slash", arg: "br-fc/x", ok: false},
+		{name: "too_long", arg: "br-fc-aaaaaaaaaa", ok: false}, // 16 chars
+	} {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			err := validateBangerBridgeName(tc.arg)
+			if tc.ok && err != nil {
+				t.Fatalf("validateBangerBridgeName(%q) = %v, want nil", tc.arg, err)
+			}
+			if !tc.ok && err == nil {
+				t.Fatalf("validateBangerBridgeName(%q) succeeded, want error", tc.arg)
+			}
+		})
+	}
+}
+
+func TestValidateCIDRPrefix(t *testing.T) {
+	t.Parallel()
+	for _, tc := range []struct {
+		name string
+		arg  string
+		ok   bool
+	}{
+		{name: "default_24", arg: "24", ok: true},
+		{name: "min_8", arg: "8", ok: true},
+		{name: "max_32", arg: "32", ok: true},
+		{name: "with_whitespace", arg: " 16 ", ok: true},
+		{name: "below_min", arg: "7", ok: false},
+		{name: "above_max", arg: "33", ok: false},
+		{name: "non_numeric", arg: "abc", ok: false},
+		{name: "ipv6_prefix", arg: "64", ok: false}, // outside [8, 32]
+		{name: "with_slash", arg: "/24", ok: false},
+		{name: "empty", arg: "", ok: false},
+		{name: "negative", arg: "-1", ok: false},
+	} {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			err := validateCIDRPrefix(tc.arg)
+			if tc.ok && err != nil {
+				t.Fatalf("validateCIDRPrefix(%q) = %v, want nil", tc.arg, err)
+			}
+			if !tc.ok && err == nil {
+				t.Fatalf("validateCIDRPrefix(%q) succeeded, want error", tc.arg)
+			}
+		})
+	}
+}
+
+func TestValidateNetworkConfig(t *testing.T) {
+	t.Parallel()
+	good := NetworkConfig{
+		BridgeName: "br-fc",
+		BridgeIP:   "172.16.0.1",
+		CIDR:       "24",
+	}
+	if err := validateNetworkConfig(good); err != nil {
+		t.Fatalf("validateNetworkConfig(default) = %v, want nil", err)
+	}
+	for _, tc := range []struct {
+		name   string
+		mutate func(NetworkConfig) NetworkConfig
+	}{
+		{name: "bad_bridge", mutate: func(c NetworkConfig) NetworkConfig { c.BridgeName = "eth0"; return c }},
+		{name: "bad_ip", mutate: func(c NetworkConfig) NetworkConfig { c.BridgeIP = "::1"; return c }},
+		{name: "bad_cidr", mutate: func(c NetworkConfig) NetworkConfig { c.CIDR = "/24"; return c }},
+		{name: "missing_ip", mutate: func(c NetworkConfig) NetworkConfig { c.BridgeIP = ""; return c }},
+	} {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			if err := validateNetworkConfig(tc.mutate(good)); err == nil {
+				t.Fatalf("validateNetworkConfig(%s) succeeded, want error", tc.name)
+			}
+		})
+	}
+}
+
 func TestValidateLinuxIfaceName(t *testing.T) {
 	t.Parallel()