Add dmz-vs-selinux kludge and a way to disable it

Add a workaround for a problem of older container-selinux not allowing
runc to use dmz feature. If runc sees that SELinux is in enforced mode
and the container's SELinux label is set, it disables dmz.

Add a build tag, runc_dmz_selinux_nocompat, which disables the workaround.
Newer distros that ship container-selinux >= 2.224.0 (currently CentOS
Stream 8 and 9, RHEL 8 and 9, and Fedora 38+) may build runc with this
build tag set to benefit from dmz working with SELinux.

Document the build tag in the top-level and libct/dmz READMEs.

Use the build tag in our CI builds for CentOS Stream 9 and Fedora 38,
as they already has container-selinux 2.224.0 available in updates.

Add a TODO to use the build tag for CentOS Stream 8 once it has
container-selinux updated.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.cirrus.yml

@@ -57,7 +57,7 @@ task:
     mkdir -p -m 0700 /root/.ssh
     vagrant ssh-config >> /root/.ssh/config
   guest_info_script: |
-    ssh default 'sh -exc "uname -a && systemctl --version && df -T && cat /etc/os-release && go version && sestatus"'
+    ssh default 'sh -exc "uname -a && systemctl --version && df -T && cat /etc/os-release && go version && sestatus && rpm -q container-selinux"'
   check_config_script: |
     ssh default /vagrant/script/check-config.sh
   unit_tests_script: |
@@ -159,6 +159,17 @@ task:
     echo -e "Host localhost\n\tStrictHostKeyChecking no\t\nIdentityFile /root/.ssh/id_ed25519\n" >> /root/.ssh/config
     sed -e "s,PermitRootLogin.*,PermitRootLogin prohibit-password,g" -i /etc/ssh/sshd_config
     systemctl restart sshd
+
+    # Disable the dmz-vs-selinux workaround for distros that have container-selinux >= 2.224.0.
+    case $DISTRO in
+    # TODO: remove centos-stream-8.
+    centos-7|centos-stream-8)
+      # Do nothing.
+      ;;
+    *)
+      echo 'export EXTRA_BUILDTAGS=runc_dmz_selinux_nocompat' >> /root/.bashrc
+      ;;
+    esac
   host_info_script: |
     uname -a
     # -----

README.md

@@ -69,13 +69,15 @@ make BUILDTAGS=""
 |---------------|---------------------------------------|--------------------|---------------------|
 | `seccomp`     | Syscall filtering using `libseccomp`. | yes                | `libseccomp`        |
 | `!runc_nodmz` | Reduce memory usage for CVE-2019-5736 protection by using a small C binary, [see `memfd-bind` for more details][contrib-memfd-bind]. `runc_nodmz` disables this feature and causes runc to use a different protection mechanism which will further increases memory usage temporarily during container startup. This feature can also be disabled at runtime by setting the `RUNC_DMZ=legacy` environment variable. | yes ||
+| `runc_dmz_selinux_nocompat` | Disables a SELinux DMZ workaround (new distros should set this). See [dmz README] for details. | no ||
 
 The following build tags were used earlier, but are now obsoleted:
  - **nokmem** (since runc v1.0.0-rc94 kernel memory settings are ignored)
  - **apparmor** (since runc v1.0.0-rc93 the feature is always enabled)
  - **selinux**  (since runc v1.0.0-rc93 the feature is always enabled)
 
  [contrib-memfd-bind]: /contrib/cmd/memfd-bind/README.md
+ [dmz README]: /libcontainer/dmz/README.md
 
 ### Running the test suite
 

Vagrantfile.fedora

@@ -32,6 +32,9 @@ EOF
     # To avoid "avc: denied { nosuid_transition }" from SELinux as we run tests on /tmp.
     mount -o remount,suid /tmp
 
+    # Disable selinux-vs-dmz workaround as Fedora doesn't need it.
+    echo 'export EXTRA_BUILDTAGS=runc_dmz_selinux_nocompat' >> /root/.bashrc
+
     # Prevent the "fatal: unsafe repository" git complain during build.
     git config --global --add safe.directory /vagrant
 

libcontainer/container_linux.go

@@ -457,6 +457,10 @@ func slicesContains[S ~[]E, E comparable](slice S, needle E) bool {
 }
 
 func isDmzBinarySafe(c *configs.Config) bool {
+	if !dmz.WorksWithSELinux(c) {
+		return false
+	}
+
 	// Because we set the dumpable flag in nsexec, the only time when it is
 	// unsafe to use runc-dmz is when the container process would be able to
 	// race against "runc init" and bypass the ptrace_may_access() checks.

libcontainer/dmz/README.md

@@ -15,4 +15,16 @@ It also support all the architectures we support in runc.
 
 If the GOARCH we use for compiling doesn't support nolibc, it fallbacks to using the C stdlib.
 
+## SELinux compatibility issue and a workaround
+
+Older SELinux policy can prevent runc to execute the dmz binary. The issue is
+fixed in [container-selinux v2.224.0]. Yet, some older distributions may not
+have the fix, so runc has a runtime workaround of disabling dmz if it finds
+that SELinux is in enforced mode and the container SELinux label is set.
+
+Distributions that have a sufficiently new container-selinux can disable the
+workaround by building runc with the `runc_dmz_selinux_nocompat` build flag,
+essentially allowing dmz to be used together with SELinux.
+
 [nolibc-upstream]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/include/nolibc?h=v6.6-rc3
+[container-selinux v2.224.0]: https://github.com/containers/container-selinux/releases/tag/v2.224.0

libcontainer/dmz/selinux.go

@@ -0,0 +1,10 @@
+//go:build runc_dmz_selinux_nocompat || !linux
+
+package dmz
+
+import "github.com/opencontainers/runc/libcontainer/configs"
+
+// WorksWithSELinux tells whether runc-dmz can work with SELinux.
+func WorksWithSELinux(*configs.Config) bool {
+	return true
+}

libcontainer/dmz/selinux_compat.go

@@ -0,0 +1,28 @@
+//go:build linux && !runc_dmz_selinux_nocompat
+
+package dmz
+
+import (
+	"github.com/opencontainers/runc/libcontainer/configs"
+	"github.com/opencontainers/selinux/go-selinux"
+)
+
+// WorksWithSELinux tells whether runc-dmz can work with SELinux.
+//
+// Older SELinux policy can prevent runc to execute the dmz binary. The issue is
+// fixed in container-selinux >= 2.224.0:
+//
+//   - https://github.com/containers/container-selinux/issues/274
+//   - https://github.com/containers/container-selinux/pull/280
+//
+// Alas, there is is no easy way to do a runtime check if dmz works with
+// SELinux, so the below workaround is enabled by default. It results in
+// disabling dmz in case container SELinux label is set and the selinux is in
+// enforced mode.
+//
+// Newer distributions that have the sufficiently new container-selinux version
+// can build runc with runc_dmz_selinux_nocompat build flag to disable this
+// workaround (essentially allowing dmz to be used together with SELinux).
+func WorksWithSELinux(c *configs.Config) bool {
+	return c.ProcessLabel == "" || selinux.EnforceMode() != selinux.Enforcing
+}