Merge pull request #21876 from MichaelEischer/cleanup-mount-validation

mount: refactor mountpoint validation

changelog/unreleased/issue-5234

@@ -0,0 +1,12 @@
+Bugfix: Prevent mounting over the repository directory
+
+Using a local repository directory as the `mount` target — or a path
+that contains it, or that it contains — caused the FUSE server to
+read its own backend files through the new mount, deadlocking the
+kernel and requiring a long reboot to recover.
+
+Restic now resolves both paths and refuses any such overlap with a
+clear error before mounting.
+
+https://github.com/restic/restic/issues/5234
+https://github.com/restic/restic/pull/5348

cmd/restic/cmd_mount.go

@@ -6,6 +6,7 @@ import (
 	"context"
 	"fmt"
 	"os"
+	"path/filepath"
 	"strings"
 	"time"
 
@@ -13,6 +14,8 @@ import (
 	"github.com/spf13/pflag"
 	"golang.org/x/sys/unix"
 
+	"github.com/restic/restic/internal/backend/local"
+	"github.com/restic/restic/internal/backend/location"
 	"github.com/restic/restic/internal/data"
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/errors"
@@ -131,22 +134,8 @@ func runMount(ctx context.Context, opts MountOptions, gopts global.Options, args
 	}
 
 	mountpoint := args[0]
-
-	// Check the existence of the mount point at the earliest stage to
-	// prevent unnecessary computations while opening the repository.
-	stat, err := os.Stat(mountpoint)
-	if errors.Is(err, os.ErrNotExist) {
-		printer.P("Mountpoint %s doesn't exist", mountpoint)
-		return errors.Fatal("invalid mountpoint")
-	} else if !stat.IsDir() {
-		printer.P("Mountpoint %s is not a directory", mountpoint)
-		return errors.Fatal("invalid mountpoint")
-	}
-
-	err = unix.Access(mountpoint, unix.W_OK|unix.X_OK)
-	if err != nil {
-		printer.P("Mountpoint %s is not writeable or not executable", mountpoint)
-		return errors.Fatal("inaccessible mountpoint")
+	if err := validateMountpoint(mountpoint, gopts); err != nil {
+		return err
 	}
 
 	debug.Log("start mount")
@@ -230,3 +219,89 @@ func runMount(ctx context.Context, opts MountOptions, gopts global.Options, args
 
 	return err
 }
+
+func validateMountpoint(mountpoint string, gopts global.Options) error {
+	// Check the existence of the mount point at the earliest stage to
+	// prevent unnecessary computations while opening the repository.
+	stat, err := os.Stat(mountpoint)
+	if errors.Is(err, os.ErrNotExist) {
+		return errors.Fatal(fmt.Sprintf("mountpoint %s does not exist", mountpoint))
+	} else if !stat.IsDir() {
+		return errors.Fatal(fmt.Sprintf("mountpoint %s is not a directory", mountpoint))
+	}
+
+	err = unix.Access(mountpoint, unix.W_OK|unix.X_OK)
+	if err != nil {
+		return errors.Fatal(fmt.Sprintf("mountpoint %s is not writeable or not executable", mountpoint))
+	}
+
+	// Refuse to mount onto (or under, or over) the local repository directory.
+	// Doing so makes the FUSE server read its own backend files through the
+	// mount it just created, deadlocking the kernel (GH #5234).
+	loc, err := location.Parse(gopts.Backends, gopts.Repo)
+	if err != nil {
+		return err
+	}
+	if loc.Scheme == "local" {
+		if err := checkMountpointOverlap(loc.Config.(*local.Config).Path, mountpoint); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// checkMountpointOverlap returns an error.Fatal if the local repository at
+// repoPath and the mountpoint overlap: equal paths, mountpoint nested inside
+// the repo, or the repo nested inside the mountpoint. Any overlap deadlocks
+// the FUSE server (GH #5234).
+func checkMountpointOverlap(repoPath, mountpoint string) error {
+	rp, err := resolvePath(repoPath)
+	if err != nil {
+		return err
+	}
+	mp, err := resolvePath(mountpoint)
+	if err != nil {
+		return err
+	}
+
+	const tail = "; refusing to mount to avoid deadlocking the FUSE server"
+	switch {
+	case rp == mp:
+		return errors.Fatal(fmt.Sprintf("mountpoint %s is the local repository directory%s", mp, tail))
+	case isInside(rp, mp):
+		return errors.Fatal(fmt.Sprintf("mountpoint %s is inside the local repository directory %s%s", mp, rp, tail))
+	case isInside(mp, rp):
+		return errors.Fatal(fmt.Sprintf("local repository directory %s is inside the mountpoint %s%s", rp, mp, tail))
+	}
+	return nil
+}
+
+// resolvePath returns p as an absolute, symlink-resolved path. If EvalSymlinks
+// fails (e.g. the path does not fully exist), it falls back to the absolute
+// form: overlap detection is best-effort and we'd rather refuse a clear
+// overlap than abort on an unrelated stat error.
+func resolvePath(p string) (string, error) {
+	abs, err := filepath.Abs(p)
+	if err != nil {
+		return "", err
+	}
+	resolved, err := filepath.EvalSymlinks(abs)
+	if err != nil {
+		return abs, nil
+	}
+	return resolved, nil
+}
+
+// isInside reports whether child is strictly nested inside parent. Both paths
+// must already be cleaned and absolute. Equal paths return false; the caller
+// handles equality separately so it can produce a distinct error message.
+func isInside(parent, child string) bool {
+	rel, err := filepath.Rel(parent, child)
+	if err != nil {
+		return false
+	}
+	if rel == "." || rel == ".." {
+		return false
+	}
+	return !strings.HasPrefix(rel, ".."+string(filepath.Separator))
+}

cmd/restic/cmd_mount_integration_test.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -212,6 +213,62 @@ func TestMount(t *testing.T) {
 	checkSnapshots(t, env.gopts, env.mountpoint, snapshotIDs, 4)
 }
 
+func TestCheckMountpointOverlap(t *testing.T) {
+	tempdir := t.TempDir()
+	repo := filepath.Join(tempdir, "repo")
+	repoSub := filepath.Join(repo, "sub")
+	sibling := filepath.Join(tempdir, "mnt")
+	for _, d := range []string{repo, repoSub, sibling} {
+		rtest.OK(t, os.MkdirAll(d, 0700))
+	}
+
+	cases := []struct {
+		name    string
+		repo    string
+		mount   string
+		wantSub string // substring of expected error; empty means expect nil
+	}{
+		{"equal", repo, repo, "is the local repository directory"},
+		{"mount inside repo", repo, repoSub, "is inside the local repository directory"},
+		{"repo inside mount", repoSub, repo, "is inside the mountpoint"},
+		{"disjoint", repo, sibling, ""},
+		{"prefix-not-subpath", repo, repo + "-other", ""},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			rtest.OK(t, os.MkdirAll(tc.mount, 0700))
+			err := checkMountpointOverlap(tc.repo, tc.mount)
+			if tc.wantSub == "" {
+				rtest.OK(t, err)
+				return
+			}
+			if err == nil {
+				t.Fatalf("expected error containing %q, got nil", tc.wantSub)
+			}
+			if !strings.Contains(err.Error(), tc.wantSub) {
+				t.Fatalf("error %q does not contain %q", err.Error(), tc.wantSub)
+			}
+		})
+	}
+}
+
+func TestCheckMountpointOverlapSymlink(t *testing.T) {
+	tempdir := t.TempDir()
+	repo := filepath.Join(tempdir, "repo")
+	rtest.OK(t, os.MkdirAll(repo, 0700))
+	link := filepath.Join(tempdir, "link-to-repo")
+	rtest.OK(t, os.Symlink(repo, link))
+
+	err := checkMountpointOverlap(repo, link)
+	if err == nil {
+		t.Fatal("expected overlap error when mountpoint is a symlink to repo, got nil")
+	}
+	if !strings.Contains(err.Error(), "is the local repository directory") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
 func TestMountSameTimestamps(t *testing.T) {
 	if !rtest.RunFuseTest {
 		t.Skip("Skipping fuse tests")

doc/050_restore.rst

@@ -208,6 +208,12 @@ command needs to be in the ``PATH``. On macOS, you need `FUSE-T
 <https://www.fuse-t.org/>`__ or `FUSE for macOS <https://osxfuse.github.io/>`__.
 On FreeBSD, you may need to install FUSE and load the kernel module (``kldload fuse``).
 
+.. note:: The mountpoint must not overlap the local repository directory.
+   Using the repository directory itself, a subdirectory of it, or a parent
+   of it as the mountpoint causes the FUSE server to read its own backend
+   files through the new mount and deadlock the kernel. ``restic mount``
+   detects this and refuses such mountpoints.
+
 Restic supports storage and preservation of hard links. However, since
 hard links exist in the scope of a filesystem by definition, restoring
 hard links from a FUSE mount should be done by a program that preserves