apparmor: add signal (receive) peer=/usr/local/bin/rootlesskit,

Fix containerd/nerdctl issue 2730
> [Rootless] `nerdctl rm` fails when AppArmor is loaded:
> `error="unknown error after kill: runc did not terminate successfully: exit status 1:
> unable to signal init: permission denied\n: unknown"`

Caused by:
> kernel: audit: type=1400 audit(1713840662.766:122): apparmor="DENIED" operation="signal" class="signal"
> profile="nerdctl-default" pid=366783 comm="runc" requested_mask="receive" denied_mask="receive" signal=kill
> peer="/usr/local/bin/rootlesskit"

The issue is known to happen on Ubuntu 23.10 and 24.04 LTS.
Doesn't seem to happen on Ubuntu 22.04 LTS.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
(cherry picked from commit eb5a0c0)
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

contrib/apparmor/template.go

@@ -29,6 +29,8 @@ import (
 	"path"
 	"strings"
 	"text/template"
+
+	"github.com/containerd/log"
 )
 
 // NOTE: This code is copied from <github.com/docker/docker/profiles/apparmor>.
@@ -57,6 +59,10 @@ profile {{.Name}} flags=(attach_disconnected,mediate_deleted) {
   signal (receive) peer={{.DaemonProfile}},
   # Container processes may send signals amongst themselves.
   signal (send,receive) peer={{.Name}},
+{{if .RootlessKit}}
+  # https://github.com/containerd/nerdctl/issues/2730
+  signal (receive) peer={{.RootlessKit}},
+{{end}}
 
   deny @{PROC}/* w,   # deny write for all files directly in /proc (not in a subdir)
   # deny write to files not in /proc/<number>/** or /proc/sys/**
@@ -90,6 +96,7 @@ type data struct {
 	Imports       []string
 	InnerImports  []string
 	DaemonProfile string
+	RootlessKit   string
 }
 
 func cleanProfileName(profile string) string {
@@ -125,6 +132,16 @@ func loadData(name string) (*data, error) {
 	}
 	p.DaemonProfile = cleanProfileName(string(currentProfile))
 
+	// If we were running in Rootless mode, we could read `/proc/$(cat ${ROOTLESSKIT_STATE_DIR}/child_pid)/exe`,
+	// but `nerdctl apparmor load` has to be executed as the root.
+	// So, do not check ${ROOTLESSKIT_STATE_DIR} (nor EUID) here.
+	p.RootlessKit, err = exec.LookPath("rootlesskit")
+	if err != nil {
+		log.L.WithError(err).Debug("apparmor: failed to determine the RootlessKit binary path")
+		p.RootlessKit = ""
+	}
+	log.L.Debugf("apparmor: RootlessKit=%q", p.RootlessKit)
+
 	return &p, nil
 }