rootless: fix up unprivileged mount opts

Port https://github.com/moby/moby/blob/v23.0.1/daemon/oci_linux.go#L430-L460

> // Get the set of mount flags that are set on the mount that contains the given
> // path and are locked by CL_UNPRIVILEGED. This is necessary to ensure that
> // bind-mounting "with options" will not fail with user namespaces, due to
> // kernel restrictions that require user namespace mounts to preserve
> // CL_UNPRIVILEGED locked flags.

Fix issue 3098

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

cache/refs.go

@@ -14,6 +14,7 @@ import (
 	"github.com/containerd/containerd/images"
 	"github.com/containerd/containerd/leases"
 	"github.com/containerd/containerd/mount"
+	"github.com/containerd/containerd/pkg/userns"
 	"github.com/containerd/containerd/snapshots"
 	"github.com/docker/docker/pkg/idtools"
 	"github.com/hashicorp/go-multierror"
@@ -27,6 +28,7 @@ import (
 	"github.com/moby/buildkit/util/flightcontrol"
 	"github.com/moby/buildkit/util/leaseutil"
 	"github.com/moby/buildkit/util/progress"
+	rootlessmountopts "github.com/moby/buildkit/util/rootless/mountopts"
 	"github.com/moby/buildkit/util/winlayers"
 	"github.com/moby/sys/mountinfo"
 	digest "github.com/opencontainers/go-digest"
@@ -1640,6 +1642,12 @@ func (sm *sharableMountable) Mount() (_ []mount.Mount, _ func() error, retErr er
 				os.Remove(dir)
 			}
 		}()
+		if userns.RunningInUserNS() {
+			mounts, err = rootlessmountopts.FixUp(mounts)
+			if err != nil {
+				return nil, nil, err
+			}
+		}
 		if err := mount.All(mounts, dir); err != nil {
 			return nil, nil, err
 		}

executor/oci/spec.go

@@ -11,12 +11,14 @@ import (
 	"github.com/containerd/containerd/mount"
 	"github.com/containerd/containerd/namespaces"
 	"github.com/containerd/containerd/oci"
+	"github.com/containerd/containerd/pkg/userns"
 	"github.com/containerd/continuity/fs"
 	"github.com/docker/docker/pkg/idtools"
 	"github.com/mitchellh/hashstructure/v2"
 	"github.com/moby/buildkit/executor"
 	"github.com/moby/buildkit/snapshot"
 	"github.com/moby/buildkit/util/network"
+	rootlessmountopts "github.com/moby/buildkit/util/rootless/mountopts"
 	traceexec "github.com/moby/buildkit/util/tracing/exec"
 	specs "github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/opencontainers/selinux/go-selinux"
@@ -193,6 +195,14 @@ func GenerateSpec(ctx context.Context, meta executor.Meta, mounts []executor.Mou
 	}
 
 	s.Mounts = dedupMounts(s.Mounts)
+
+	if userns.RunningInUserNS() {
+		s.Mounts, err = rootlessmountopts.FixUpOCI(s.Mounts)
+		if err != nil {
+			return nil, nil, err
+		}
+	}
+
 	return s, releaseAll, nil
 }
 

snapshot/localmounter_unix.go

@@ -8,6 +8,8 @@ import (
 	"syscall"
 
 	"github.com/containerd/containerd/mount"
+	"github.com/containerd/containerd/pkg/userns"
+	rootlessmountopts "github.com/moby/buildkit/util/rootless/mountopts"
 	"github.com/pkg/errors"
 )
 
@@ -24,6 +26,14 @@ func (lm *localMounter) Mount() (string, error) {
 		lm.release = release
 	}
 
+	if userns.RunningInUserNS() {
+		var err error
+		lm.mounts, err = rootlessmountopts.FixUp(lm.mounts)
+		if err != nil {
+			return "", err
+		}
+	}
+
 	if len(lm.mounts) == 1 && (lm.mounts[0].Type == "bind" || lm.mounts[0].Type == "rbind") {
 		ro := false
 		for _, opt := range lm.mounts[0].Options {

util/rootless/mountopts/mountopts_linux.go

@@ -0,0 +1,88 @@
+package mountopts
+
+import (
+	"github.com/containerd/containerd/mount"
+	"github.com/moby/buildkit/util/strutil"
+	specs "github.com/opencontainers/runtime-spec/specs-go"
+	"github.com/pkg/errors"
+	"golang.org/x/sys/unix"
+)
+
+// UnprivilegedMountFlags gets the set of mount flags that are set on the mount that contains the given
+// path and are locked by CL_UNPRIVILEGED. This is necessary to ensure that
+// bind-mounting "with options" will not fail with user namespaces, due to
+// kernel restrictions that require user namespace mounts to preserve
+// CL_UNPRIVILEGED locked flags.
+//
+// From https://github.com/moby/moby/blob/v23.0.1/daemon/oci_linux.go#L430-L460
+func UnprivilegedMountFlags(path string) ([]string, error) {
+	var statfs unix.Statfs_t
+	if err := unix.Statfs(path, &statfs); err != nil {
+		return nil, err
+	}
+
+	// The set of keys come from https://github.com/torvalds/linux/blob/v4.13/fs/namespace.c#L1034-L1048.
+	unprivilegedFlags := map[uint64]string{
+		unix.MS_RDONLY:     "ro",
+		unix.MS_NODEV:      "nodev",
+		unix.MS_NOEXEC:     "noexec",
+		unix.MS_NOSUID:     "nosuid",
+		unix.MS_NOATIME:    "noatime",
+		unix.MS_RELATIME:   "relatime",
+		unix.MS_NODIRATIME: "nodiratime",
+	}
+
+	var flags []string
+	for mask, flag := range unprivilegedFlags {
+		if uint64(statfs.Flags)&mask == mask {
+			flags = append(flags, flag)
+		}
+	}
+
+	return flags, nil
+}
+
+// FixUp is for https://github.com/moby/buildkit/issues/3098
+func FixUp(mounts []mount.Mount) ([]mount.Mount, error) {
+	for i, m := range mounts {
+		var isBind bool
+		for _, o := range m.Options {
+			switch o {
+			case "bind", "rbind":
+				isBind = true
+			}
+		}
+		if !isBind {
+			continue
+		}
+		unpriv, err := UnprivilegedMountFlags(m.Source)
+		if err != nil {
+			return nil, errors.Wrapf(err, "failed to get unprivileged mount flags for %+v", m)
+		}
+		m.Options = strutil.DedupeSlice(append(m.Options, unpriv...))
+		mounts[i] = m
+	}
+	return mounts, nil
+}
+
+func FixUpOCI(mounts []specs.Mount) ([]specs.Mount, error) {
+	for i, m := range mounts {
+		var isBind bool
+		for _, o := range m.Options {
+			switch o {
+			case "bind", "rbind":
+				isBind = true
+			}
+		}
+		if !isBind {
+			continue
+		}
+		unpriv, err := UnprivilegedMountFlags(m.Source)
+		if err != nil {
+			return nil, errors.Wrapf(err, "failed to get unprivileged mount flags for %+v", m)
+		}
+		m.Options = strutil.DedupeSlice(append(m.Options, unpriv...))
+		mounts[i] = m
+	}
+	return mounts, nil
+}

util/rootless/mountopts/mountopts_others.go

@@ -0,0 +1,21 @@
+//go:build !linux
+// +build !linux
+
+package mountopts
+
+import (
+	"github.com/containerd/containerd/mount"
+	specs "github.com/opencontainers/runtime-spec/specs-go"
+)
+
+func UnprivilegedMountFlags(path string) ([]string, error) {
+	return []string{}, nil
+}
+
+func FixUp(mounts []mount.Mount) ([]mount.Mount, error) {
+	return mounts, nil
+}
+
+func FixUpOCI(mounts []specs.Mount) ([]specs.Mount, error) {
+	return mounts, nil
+}

util/strutil/strutil.go

@@ -0,0 +1,30 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package strutil
+
+// DedupeSlice is from https://github.com/containerd/nerdctl/blob/v1.2.1/pkg/strutil/strutil.go#L72-L82
+func DedupeSlice(in []string) []string {
+	m := make(map[string]struct{})
+	var res []string
+	for _, s := range in {
+		if _, ok := m[s]; !ok {
+			res = append(res, s)
+			m[s] = struct{}{}
+		}
+	}
+	return res
+}