Disallow mounting to certain destination /dir paths

When certain directories, like /tmp, get mounted over, FCOS/Linux can
act in unexpected ways.  Added a sanity check for a list of directories
think might be impacted by this.  Also, moved the volume parsing earlier
in the init process so we can catch problems before the expensive
decompression of machine images.

The following destinations are forbidden for volumes:

`/bin`, `/boot`, `/dev`, `/etc`, `/home`, `/proc`, `/root`, `/run`, `/sbin`, `/sys`, `/tmp`, `/usr`, and `/var`. Subdirectories

Fixes: #18230

Signed-off-by: Brent Baude <bbaude@redhat.com>

Author: baude

docs/source/markdown/podman-machine-init.1.md.in

@@ -149,6 +149,12 @@ options are:
 * **rw**: mount volume read/write (default)
 * **security_model=[model]**: specify 9p security model (see below)
 
+Note: The following destinations are forbidden for volumes: `/bin`, `/boot`, `/dev`, `/etc`,
+`/home`, `/proc`, `/root`, `/run`, `/sbin`, `/sys`, `/tmp`, `/usr`, and `/var`. Subdirectories
+of these destinations are allowed but users should be careful to not mount to important directories
+as unexpected results may occur.
+
+
 The 9p security model [determines] https://wiki.qemu.org/Documentation/9psetup#Starting_the_Guest_directly
 if and how the 9p filesystem translates some filesystem operations before
 actual storage on the host.

pkg/machine/e2e/init_test.go

@@ -79,6 +79,23 @@ var _ = Describe("podman machine init", func() {
 		Expect(badMemSession).To(Exit(125))
 	})
 
+	It("init volume check", func() {
+		skipIfWSL("WSL does not use volume mounts")
+		// Check that mounting to certain target directories like /tmp at the / level is NOT ok
+		tmpVol := initMachine{}
+		targetMount := "/tmp"
+		tmpVolSession, err := mb.setCmd(tmpVol.withVolume(fmt.Sprintf("/whatever:%s", targetMount))).run()
+		Expect(err).ToNot(HaveOccurred())
+		Expect(tmpVolSession).To(Exit(125))
+		Expect(tmpVolSession.errorToString()).To(ContainSubstring(fmt.Sprintf("Error: machine mount destination cannot be %q: consider another location or a subdirectory of an existing location", targetMount)))
+
+		// Mounting to /tmp/foo (subdirectory) is OK
+		tmpSubdir := initMachine{}
+		tmpSubDirSession, err := mb.setCmd(tmpSubdir.withVolume("/whatever:/tmp/foo")).run()
+		Expect(err).ToNot(HaveOccurred())
+		Expect(tmpSubDirSession).To(Exit(0))
+	})
+
 	It("simple init", func() {
 		i := new(initMachine)
 		session, err := mb.setCmd(i.withImage(mb.imagePath)).run()

pkg/machine/shim/host.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"path"
 	"path/filepath"
 	"runtime"
 	"strings"
@@ -118,6 +119,18 @@ func Init(opts machineDefine.InitOptions, mp vmconfigs.VMProvider) error {
 		createOpts.UserModeNetworking = *umn
 	}
 
+	// Mounts
+	if mp.VMType() != machineDefine.WSLVirt {
+		mc.Mounts = CmdLineVolumesToMounts(opts.Volumes, mp.MountType())
+	}
+
+	// Issue #18230 ... do not mount over important directories at the / level (subdirs are fine)
+	for _, mnt := range mc.Mounts {
+		if err := validateDestinationPaths(mnt.Target); err != nil {
+			return err
+		}
+	}
+
 	// Get Image
 	// TODO This needs rework bigtime; my preference is most of below of not living in here.
 	// ideally we could get a func back that pulls the image, and only do so IF everything works because
@@ -251,11 +264,6 @@ func Init(opts machineDefine.InitOptions, mp vmconfigs.VMProvider) error {
 	}
 	ignBuilder.WithUnit(readyUnit)
 
-	// Mounts
-	if mp.VMType() != machineDefine.WSLVirt {
-		mc.Mounts = CmdLineVolumesToMounts(opts.Volumes, mp.MountType())
-	}
-
 	// TODO AddSSHConnectionToPodmanSocket could take an machineconfig instead
 	if err := connection.AddSSHConnectionsToPodmanSocket(mc.HostUser.UID, mc.SSH.Port, mc.SSH.IdentityPath, mc.Name, mc.SSH.RemoteUsername, opts); err != nil {
 		return err
@@ -774,3 +782,27 @@ func Reset(mps []vmconfigs.VMProvider, opts machine.ResetOptions) error {
 	}
 	return resetErrors.ErrorOrNil()
 }
+
+func validateDestinationPaths(dest string) error {
+	// illegalMounts are locations at the / level of the podman machine where we do want users mounting directly over
+	illegalMounts := map[string]struct{}{
+		"/bin":  {},
+		"/boot": {},
+		"/dev":  {},
+		"/etc":  {},
+		"/home": {},
+		"/proc": {},
+		"/root": {},
+		"/run":  {},
+		"/sbin": {},
+		"/sys":  {},
+		"/tmp":  {},
+		"/usr":  {},
+		"/var":  {},
+	}
+	mountTarget := path.Clean(dest)
+	if _, ok := illegalMounts[mountTarget]; ok {
+		return fmt.Errorf("machine mount destination cannot be %q: consider another location or a subdirectory of an existing location", mountTarget)
+	}
+	return nil
+}

pkg/machine/shim/host_test.go

@@ -0,0 +1,61 @@
+package shim
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_validateDestinationPaths(t *testing.T) {
+	tests := []struct {
+		name    string
+		dest    string
+		wantErr bool
+	}{
+		{
+			name:    "Expect fail - /tmp",
+			dest:    "/tmp",
+			wantErr: true,
+		},
+		{
+			name:    "Expect fail trailing /",
+			dest:    "/tmp/",
+			wantErr: true,
+		},
+		{
+			name:    "Expect fail double /",
+			dest:    "//tmp",
+			wantErr: true,
+		},
+		{
+			name:    "/var should fail",
+			dest:    "/var",
+			wantErr: true,
+		},
+		{
+			name:    "/etc should fail",
+			dest:    "/etc",
+			wantErr: true,
+		},
+		{
+			name:    "/tmp subdir OK",
+			dest:    "/tmp/foobar",
+			wantErr: false,
+		},
+		{
+			name:    "/foobar OK",
+			dest:    "/foobar",
+			wantErr: false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := validateDestinationPaths(tt.dest)
+			if tt.wantErr {
+				assert.ErrorContainsf(t, err, "onsider another location or a subdirectory of an existing location", "illegal mount target")
+			} else {
+				assert.NoError(t, err, "mounts to subdirs or non-critical dirs should succeed")
+			}
+		})
+	}
+}