tests: add regression test for CVE-2019-19921 / CVE-2023-27561

We reintroduced this once already because it is quite easy to miss this
subtle aspect of proc mounting. The recent migration to
securejoin.MkdirAllInRoot could have also inadvertently reintroduced
this (though it didn't).

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

tests/integration/mounts.bats

@@ -199,6 +199,18 @@ function test_mount_order() {
 	[ "$status" -eq 0 ]
 }
 
+# CVE-2023-27561 CVE-2019-19921
+@test "runc run [/proc is a symlink]" {
+	# Make /proc in the container a symlink.
+	rm -rf rootfs/proc
+	mkdir -p rootfs/bad-proc
+	ln -sf /bad-proc rootfs/proc
+	# This should fail.
+	runc run test_busybox
+	[ "$status" -ne 0 ]
+	[[ "$output" == *"must be mounted on ordinary directory"* ]]
+}
+
 @test "runc run [ro /sys/fs/cgroup mounts]" {
 	# Without cgroup namespace.
 	update_config '.linux.namespaces -= [{"type": "cgroup"}]'