Implement mount(2) and umount2(2) for VFS2.

This is mostly syscall plumbing, VFS2 already implements the internals of
mounts. In addition to the syscall defintions, the following mount-related
mechanisms are updated:

- Implement MS_NOATIME for VFS2, but only for tmpfs and goferfs. The other VFS2
  filesystems don't implement node-level timestamps yet.

- Implement the 'mode', 'uid' and 'gid' mount options for VFS2's tmpfs.

- Plumb mount namespace ownership, which is necessary for checking appropriate
  capabilities during mount(2).

Updates #1035

PiperOrigin-RevId: 315035352

10 files changed, 246 insertions, 19 deletions

diff --git a/pkg/sentry/fsimpl/gofer/time.go b/pkg/sentry/fsimpl/gofer/time.go
index 2608e7e1d..1d5aa82dc 100644
--- a/pkg/sentry/fsimpl/gofer/time.go
+++ b/pkg/sentry/fsimpl/gofer/time.go
@@ -38,6 +38,9 @@ func statxTimestampFromDentry(ns int64) linux.StatxTimestamp {
 
 // Preconditions: fs.interop != InteropModeShared.
 func (d *dentry) touchAtime(mnt *vfs.Mount) {
+	if mnt.Flags.NoATime {
+		return
+	}
 	if err := mnt.CheckBeginWrite(); err != nil {
 		return
 	}
diff --git a/pkg/sentry/fsimpl/tmpfs/tmpfs.go b/pkg/sentry/fsimpl/tmpfs/tmpfs.go
index f0e098702..3777ebdf2 100644
--- a/pkg/sentry/fsimpl/tmpfs/tmpfs.go
+++ b/pkg/sentry/fsimpl/tmpfs/tmpfs.go
@@ -30,6 +30,7 @@ package tmpfs
 import (
 	"fmt"
 	"math"
+	"strconv"
 	"strings"
 	"sync/atomic"
 
@@ -124,14 +125,45 @@ func (fstype FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 	}
 	fs.vfsfs.Init(vfsObj, newFSType, &fs)
 
+	mopts := vfs.GenericParseMountOptions(opts.Data)
+
+	defaultMode := linux.FileMode(0777)
+	if modeStr, ok := mopts["mode"]; ok {
+		mode, err := strconv.ParseUint(modeStr, 8, 32)
+		if err != nil {
+			return nil, nil, fmt.Errorf("Mount option \"mode='%v'\" not parsable: %v", modeStr, err)
+		}
+		defaultMode = linux.FileMode(mode)
+	}
+
+	defaultOwnerCreds := creds.Fork()
+	if uidStr, ok := mopts["uid"]; ok {
+		uid, err := strconv.ParseInt(uidStr, 10, 32)
+		if err != nil {
+			return nil, nil, fmt.Errorf("Mount option \"uid='%v'\" not parsable: %v", uidStr, err)
+		}
+		if err := defaultOwnerCreds.SetUID(auth.UID(uid)); err != nil {
+			return nil, nil, fmt.Errorf("Error using mount option \"uid='%v'\": %v", uidStr, err)
+		}
+	}
+	if gidStr, ok := mopts["gid"]; ok {
+		gid, err := strconv.ParseInt(gidStr, 10, 32)
+		if err != nil {
+			return nil, nil, fmt.Errorf("Mount option \"gid='%v'\" not parsable: %v", gidStr, err)
+		}
+		if err := defaultOwnerCreds.SetGID(auth.GID(gid)); err != nil {
+			return nil, nil, fmt.Errorf("Error using mount option \"gid='%v'\": %v", gidStr, err)
+		}
+	}
+
 	var root *dentry
 	switch rootFileType {
 	case linux.S_IFREG:
-		root = fs.newDentry(fs.newRegularFile(creds, 0777))
+		root = fs.newDentry(fs.newRegularFile(defaultOwnerCreds, defaultMode))
 	case linux.S_IFLNK:
-		root = fs.newDentry(fs.newSymlink(creds, tmpfsOpts.RootSymlinkTarget))
+		root = fs.newDentry(fs.newSymlink(defaultOwnerCreds, tmpfsOpts.RootSymlinkTarget))
 	case linux.S_IFDIR:
-		root = &fs.newDirectory(creds, 01777).dentry
+		root = &fs.newDirectory(defaultOwnerCreds, defaultMode).dentry
 	default:
 		fs.vfsfs.DecRef()
 		return nil, nil, fmt.Errorf("invalid tmpfs root file type: %#o", rootFileType)
@@ -562,6 +594,9 @@ func (i *inode) isDir() bool {
 }
 
 func (i *inode) touchAtime(mnt *vfs.Mount) {
+	if mnt.Flags.NoATime {
+		return
+	}
 	if err := mnt.CheckBeginWrite(); err != nil {
 		return
 	}
diff --git a/pkg/sentry/kernel/auth/credentials.go b/pkg/sentry/kernel/auth/credentials.go
index e057d2c6d..6862f2ef5 100644
--- a/pkg/sentry/kernel/auth/credentials.go
+++ b/pkg/sentry/kernel/auth/credentials.go
@@ -232,3 +232,31 @@ func (c *Credentials) UseGID(gid GID) (KGID, error) {
 	}
 	return NoID, syserror.EPERM
 }
+
+// SetUID translates the provided uid to the root user namespace and updates c's
+// uids to it. This performs no permissions or capabilities checks, the caller
+// is responsible for ensuring the calling context is permitted to modify c.
+func (c *Credentials) SetUID(uid UID) error {
+	kuid := c.UserNamespace.MapToKUID(uid)
+	if !kuid.Ok() {
+		return syserror.EINVAL
+	}
+	c.RealKUID = kuid
+	c.EffectiveKUID = kuid
+	c.SavedKUID = kuid
+	return nil
+}
+
+// SetGID translates the provided gid to the root user namespace and updates c's
+// gids to it. This performs no permissions or capabilities checks, the caller
+// is responsible for ensuring the calling context is permitted to modify c.
+func (c *Credentials) SetGID(gid GID) error {
+	kgid := c.UserNamespace.MapToKGID(gid)
+	if !kgid.Ok() {
+		return syserror.EINVAL
+	}
+	c.RealKGID = kgid
+	c.EffectiveKGID = kgid
+	c.SavedKGID = kgid
+	return nil
+}
diff --git a/pkg/sentry/syscalls/linux/vfs2/BUILD b/pkg/sentry/syscalls/linux/vfs2/BUILD
index 9c8b44f64..c0d005247 100644
--- a/pkg/sentry/syscalls/linux/vfs2/BUILD
+++ b/pkg/sentry/syscalls/linux/vfs2/BUILD
@@ -16,6 +16,7 @@ go_library(
         "ioctl.go",
         "memfd.go",
         "mmap.go",
+        "mount.go",
         "path.go",
         "pipe.go",
         "poll.go",
diff --git a/pkg/sentry/syscalls/linux/vfs2/mount.go b/pkg/sentry/syscalls/linux/vfs2/mount.go
new file mode 100644
index 000000000..adeaa39cc
--- /dev/null
+++ b/pkg/sentry/syscalls/linux/vfs2/mount.go
@@ -0,0 +1,145 @@
+// Copyright 2020 The gVisor 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 vfs2
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/arch"
+	"gvisor.dev/gvisor/pkg/sentry/kernel"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+	"gvisor.dev/gvisor/pkg/usermem"
+)
+
+// Mount implements Linux syscall mount(2).
+func Mount(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
+	sourceAddr := args[0].Pointer()
+	targetAddr := args[1].Pointer()
+	typeAddr := args[2].Pointer()
+	flags := args[3].Uint64()
+	dataAddr := args[4].Pointer()
+
+	// For null-terminated strings related to mount(2), Linux copies in at most
+	// a page worth of data. See fs/namespace.c:copy_mount_string().
+	fsType, err := t.CopyInString(typeAddr, usermem.PageSize)
+	if err != nil {
+		return 0, nil, err
+	}
+	source, err := t.CopyInString(sourceAddr, usermem.PageSize)
+	if err != nil {
+		return 0, nil, err
+	}
+
+	targetPath, err := copyInPath(t, targetAddr)
+	if err != nil {
+		return 0, nil, err
+	}
+
+	data := ""
+	if dataAddr != 0 {
+		// In Linux, a full page is always copied in regardless of null
+		// character placement, and the address is passed to each file system.
+		// Most file systems always treat this data as a string, though, and so
+		// do all of the ones we implement.
+		data, err = t.CopyInString(dataAddr, usermem.PageSize)
+		if err != nil {
+			return 0, nil, err
+		}
+	}
+
+	// Ignore magic value that was required before Linux 2.4.
+	if flags&linux.MS_MGC_MSK == linux.MS_MGC_VAL {
+		flags = flags &^ linux.MS_MGC_MSK
+	}
+
+	// Must have CAP_SYS_ADMIN in the current mount namespace's associated user
+	// namespace.
+	creds := t.Credentials()
+	if !creds.HasCapabilityIn(linux.CAP_SYS_ADMIN, t.MountNamespaceVFS2().Owner) {
+		return 0, nil, syserror.EPERM
+	}
+
+	const unsupportedOps = linux.MS_REMOUNT | linux.MS_BIND |
+		linux.MS_SHARED | linux.MS_PRIVATE | linux.MS_SLAVE |
+		linux.MS_UNBINDABLE | linux.MS_MOVE
+
+	// Silently allow MS_NOSUID, since we don't implement set-id bits
+	// anyway.
+	const unsupportedFlags = linux.MS_NODEV |
+		linux.MS_NODIRATIME | linux.MS_STRICTATIME
+
+	// Linux just allows passing any flags to mount(2) - it won't fail when
+	// unknown or unsupported flags are passed. Since we don't implement
+	// everything, we fail explicitly on flags that are unimplemented.
+	if flags&(unsupportedOps|unsupportedFlags) != 0 {
+		return 0, nil, syserror.EINVAL
+	}
+
+	var opts vfs.MountOptions
+	if flags&linux.MS_NOATIME == linux.MS_NOATIME {
+		opts.Flags.NoATime = true
+	}
+	if flags&linux.MS_NOEXEC == linux.MS_NOEXEC {
+		opts.Flags.NoExec = true
+	}
+	if flags&linux.MS_RDONLY == linux.MS_RDONLY {
+		opts.ReadOnly = true
+	}
+	opts.GetFilesystemOptions.Data = data
+
+	target, err := getTaskPathOperation(t, linux.AT_FDCWD, targetPath, disallowEmptyPath, nofollowFinalSymlink)
+	if err != nil {
+		return 0, nil, err
+	}
+	defer target.Release()
+
+	return 0, nil, t.Kernel().VFS().MountAt(t, creds, source, &target.pop, fsType, &opts)
+}
+
+// Umount2 implements Linux syscall umount2(2).
+func Umount2(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
+	addr := args[0].Pointer()
+	flags := args[1].Int()
+
+	// Must have CAP_SYS_ADMIN in the mount namespace's associated user
+	// namespace.
+	//
+	// Currently, this is always the init task's user namespace.
+	creds := t.Credentials()
+	if !creds.HasCapabilityIn(linux.CAP_SYS_ADMIN, t.MountNamespaceVFS2().Owner) {
+		return 0, nil, syserror.EPERM
+	}
+
+	const unsupported = linux.MNT_FORCE | linux.MNT_EXPIRE
+	if flags&unsupported != 0 {
+		return 0, nil, syserror.EINVAL
+	}
+
+	path, err := copyInPath(t, addr)
+	if err != nil {
+		return 0, nil, err
+	}
+	tpop, err := getTaskPathOperation(t, linux.AT_FDCWD, path, disallowEmptyPath, nofollowFinalSymlink)
+	if err != nil {
+		return 0, nil, err
+	}
+	defer tpop.Release()
+
+	opts := vfs.UmountOptions{
+		Flags: uint32(flags),
+	}
+
+	return 0, nil, t.Kernel().VFS().UmountAt(t, creds, &tpop.pop, &opts)
+}
diff --git a/pkg/sentry/syscalls/linux/vfs2/vfs2.go b/pkg/sentry/syscalls/linux/vfs2/vfs2.go
index ef8358b8a..7b6e7571a 100644
--- a/pkg/sentry/syscalls/linux/vfs2/vfs2.go
+++ b/pkg/sentry/syscalls/linux/vfs2/vfs2.go
@@ -90,8 +90,8 @@ func Override() {
 	s.Table[138] = syscalls.Supported("fstatfs", Fstatfs)
 	s.Table[161] = syscalls.Supported("chroot", Chroot)
 	s.Table[162] = syscalls.Supported("sync", Sync)
-	delete(s.Table, 165) // mount
-	delete(s.Table, 166) // umount2
+	s.Table[165] = syscalls.Supported("mount", Mount)
+	s.Table[166] = syscalls.Supported("umount2", Umount2)
 	delete(s.Table, 187) // readahead
 	s.Table[188] = syscalls.Supported("setxattr", Setxattr)
 	s.Table[189] = syscalls.Supported("lsetxattr", Lsetxattr)
diff --git a/pkg/sentry/vfs/genericfstree/genericfstree.go b/pkg/sentry/vfs/genericfstree/genericfstree.go
index 286510195..8882fa84a 100644
--- a/pkg/sentry/vfs/genericfstree/genericfstree.go
+++ b/pkg/sentry/vfs/genericfstree/genericfstree.go
@@ -43,7 +43,7 @@ type Dentry struct {
 // IsAncestorDentry returns true if d is an ancestor of d2; that is, d is
 // either d2's parent or an ancestor of d2's parent.
 func IsAncestorDentry(d, d2 *Dentry) bool {
-	for {
+	for d2 != nil { // Stop at root, where d2.parent == nil.
 		if d2.parent == d {
 			return true
 		}
@@ -52,6 +52,7 @@ func IsAncestorDentry(d, d2 *Dentry) bool {
 		}
 		d2 = d2.parent
 	}
+	return false
 }
 
 // ParentOrSelf returns d.parent. If d.parent is nil, ParentOrSelf returns d.
diff --git a/pkg/sentry/vfs/mount.go b/pkg/sentry/vfs/mount.go
index 3adb7c97d..32f901bd8 100644
--- a/pkg/sentry/vfs/mount.go
+++ b/pkg/sentry/vfs/mount.go
@@ -55,6 +55,10 @@ type Mount struct {
 	// ID is the immutable mount ID.
 	ID uint64
 
+	// Flags contains settings as specified for mount(2), e.g. MS_NOEXEC, except
+	// for MS_RDONLY which is tracked in "writers". Immutable.
+	Flags MountFlags
+
 	// key is protected by VirtualFilesystem.mountMu and
 	// VirtualFilesystem.mounts.seq, and may be nil. References are held on
 	// key.parent and key.point if they are not nil.
@@ -81,10 +85,6 @@ type Mount struct {
 	// umounted is true. umounted is protected by VirtualFilesystem.mountMu.
 	umounted bool
 
-	// flags contains settings as specified for mount(2), e.g. MS_NOEXEC, except
-	// for MS_RDONLY which is tracked in "writers".
-	flags MountFlags
-
 	// The lower 63 bits of writers is the number of calls to
 	// Mount.CheckBeginWrite() that have not yet been paired with a call to
 	// Mount.EndWrite(). The MSB of writers is set if MS_RDONLY is in effect.
@@ -95,10 +95,10 @@ type Mount struct {
 func newMount(vfs *VirtualFilesystem, fs *Filesystem, root *Dentry, mntns *MountNamespace, opts *MountOptions) *Mount {
 	mnt := &Mount{
 		ID:    atomic.AddUint64(&vfs.lastMountID, 1),
+		Flags: opts.Flags,
 		vfs:   vfs,
 		fs:    fs,
 		root:  root,
-		flags: opts.Flags,
 		ns:    mntns,
 		refs:  1,
 	}
@@ -113,13 +113,12 @@ func (mnt *Mount) Options() MountOptions {
 	mnt.vfs.mountMu.Lock()
 	defer mnt.vfs.mountMu.Unlock()
 	return MountOptions{
-		Flags:    mnt.flags,
+		Flags:    mnt.Flags,
 		ReadOnly: mnt.readOnly(),
 	}
 }
 
-// A MountNamespace is a collection of Mounts.
-//
+// A MountNamespace is a collection of Mounts.//
 // MountNamespaces are reference-counted. Unless otherwise specified, all
 // MountNamespace methods require that a reference is held.
 //
@@ -127,6 +126,9 @@ func (mnt *Mount) Options() MountOptions {
 //
 // +stateify savable
 type MountNamespace struct {
+	// Owner is the usernamespace that owns this mount namespace.
+	Owner *auth.UserNamespace
+
 	// root is the MountNamespace's root mount. root is immutable.
 	root *Mount
 
@@ -163,6 +165,7 @@ func (vfs *VirtualFilesystem) NewMountNamespace(ctx context.Context, creds *auth
 		return nil, err
 	}
 	mntns := &MountNamespace{
+		Owner:       creds.UserNamespace,
 		refs:        1,
 		mountpoints: make(map[*Dentry]uint32),
 	}
@@ -279,6 +282,9 @@ func (vfs *VirtualFilesystem) UmountAt(ctx context.Context, creds *auth.Credenti
 	}
 
 	// MNT_FORCE is currently unimplemented except for the permission check.
+	// Force unmounting specifically requires CAP_SYS_ADMIN in the root user
+	// namespace, and not in the owner user namespace for the target mount. See
+	// fs/namespace.c:SYSCALL_DEFINE2(umount, ...)
 	if opts.Flags&linux.MNT_FORCE != 0 && creds.HasCapabilityIn(linux.CAP_SYS_ADMIN, creds.UserNamespace.Root()) {
 		return syserror.EPERM
 	}
@@ -753,7 +759,10 @@ func (vfs *VirtualFilesystem) GenerateProcMounts(ctx context.Context, taskRootDi
 		if mnt.readOnly() {
 			opts = "ro"
 		}
-		if mnt.flags.NoExec {
+		if mnt.Flags.NoATime {
+			opts = ",noatime"
+		}
+		if mnt.Flags.NoExec {
 			opts += ",noexec"
 		}
 
@@ -838,11 +847,12 @@ func (vfs *VirtualFilesystem) GenerateProcMountInfo(ctx context.Context, taskRoo
 		if mnt.readOnly() {
 			opts = "ro"
 		}
-		if mnt.flags.NoExec {
+		if mnt.Flags.NoATime {
+			opts = ",noatime"
+		}
+		if mnt.Flags.NoExec {
 			opts += ",noexec"
 		}
-		// TODO(gvisor.dev/issue/1193): Add "noatime" if MS_NOATIME is
-		// set.
 		fmt.Fprintf(buf, "%s ", opts)
 
 		// (7) Optional fields: zero or more fields of the form "tag[:value]".
diff --git a/pkg/sentry/vfs/options.go b/pkg/sentry/vfs/options.go
index 53d364c5c..f223aeda8 100644
--- a/pkg/sentry/vfs/options.go
+++ b/pkg/sentry/vfs/options.go
@@ -75,6 +75,10 @@ type MknodOptions struct {
 type MountFlags struct {
 	// NoExec is equivalent to MS_NOEXEC.
 	NoExec bool
+
+	// NoATime is equivalent to MS_NOATIME and indicates that the
+	// filesystem should not update access time in-place.
+	NoATime bool
 }
 
 // MountOptions contains options to VirtualFilesystem.MountAt().
diff --git a/pkg/sentry/vfs/vfs.go b/pkg/sentry/vfs/vfs.go
index 52643a7c5..9acca8bc7 100644
--- a/pkg/sentry/vfs/vfs.go
+++ b/pkg/sentry/vfs/vfs.go
@@ -405,7 +405,7 @@ func (vfs *VirtualFilesystem) OpenAt(ctx context.Context, creds *auth.Credential
 			vfs.putResolvingPath(rp)
 
 			if opts.FileExec {
-				if fd.Mount().flags.NoExec {
+				if fd.Mount().Flags.NoExec {
 					fd.DecRef()
 					return nil, syserror.EACCES
 				}