Add option to panic gofer if writes are attempted over RO mounts

This is used when '--overlay=true' to guarantee writes are not sent to gofer.

PiperOrigin-RevId: 210116288
Change-Id: I7616008c4c0e8d3668e07a205207f46e2144bf30

runsc/cmd/gofer.go

@@ -38,6 +38,8 @@ type Gofer struct {
 	// controllerFD is the file descriptor of a stream socket for the
 	// control server that is donated to this process.
 	controllerFD int
+
+	panicOnWrite bool
 }
 
 // Name implements subcommands.Command.
@@ -61,6 +63,7 @@ func (g *Gofer) SetFlags(f *flag.FlagSet) {
 	f.Var(&g.ioFDs, "io-fds", "list of FDs to connect 9P servers. They must follow this order: root first, then mounts as defined in the spec")
 	f.BoolVar(&g.applyCaps, "apply-caps", true, "if true, apply capabilities to restrict what the Gofer process can do")
 	f.IntVar(&g.controllerFD, "controller-fd", -1, "required FD of a stream socket for the control server that must be donated to this process")
+	f.BoolVar(&g.panicOnWrite, "panic-on-write", false, "if true, panics on attempts to write to RO mounts. RW mounts are unnaffected")
 }
 
 // Execute implements subcommands.Command.
@@ -110,7 +113,8 @@ func (g *Gofer) Execute(_ context.Context, f *flag.FlagSet, args ...interface{})
 	ats := make([]p9.Attacher, 0, len(spec.Mounts)+1)
 	p := absPath(g.bundleDir, spec.Root.Path)
 	ats = append(ats, fsgofer.NewAttachPoint(p, fsgofer.Config{
-		ROMount: spec.Root.Readonly,
+		ROMount:      spec.Root.Readonly,
+		PanicOnWrite: g.panicOnWrite,
 		// Docker uses overlay2 by default for the root mount, and overlay2 does a copy-up when
 		// each file is opened as writable. Thus, we open files lazily to avoid copy-up.
 		LazyOpenForWrite: true,
@@ -123,6 +127,7 @@ func (g *Gofer) Execute(_ context.Context, f *flag.FlagSet, args ...interface{})
 			p = absPath(g.bundleDir, m.Source)
 			ats = append(ats, fsgofer.NewAttachPoint(p, fsgofer.Config{
 				ROMount:          isReadonlyMount(m.Options),
+				PanicOnWrite:     g.panicOnWrite,
 				LazyOpenForWrite: false,
 			}))
 

runsc/fsgofer/fsgofer.go

@@ -76,6 +76,9 @@ type Config struct {
 	// ROMount is set to true if this is a readonly mount.
 	ROMount bool
 
+	// PanicOnWrite panics on attempts to write to RO mounts.
+	PanicOnWrite bool
+
 	// LazyOpenForWrite makes the underlying file to be opened in RDONLY
 	// mode initially and be reopened in case write access is desired.
 	// This is done to workaround the behavior in 'overlay2' that
@@ -375,6 +378,9 @@ func (l *localFile) Open(mode p9.OpenFlags) (*fd.FD, p9.QID, uint32, error) {
 // Create implements p9.File.
 func (l *localFile) Create(name string, mode p9.OpenFlags, perm p9.FileMode, uid p9.UID, gid p9.GID) (*fd.FD, p9.File, p9.QID, uint32, error) {
 	if l.conf.ROMount {
+		if l.conf.PanicOnWrite {
+			panic("attempt to write to RO mount")
+		}
 		return nil, nil, p9.QID{}, 0, syscall.EBADF
 	}
 	if !isNameValid(name) {
@@ -429,6 +435,9 @@ func (l *localFile) Create(name string, mode p9.OpenFlags, perm p9.FileMode, uid
 // Mkdir implements p9.File.
 func (l *localFile) Mkdir(name string, perm p9.FileMode, uid p9.UID, gid p9.GID) (p9.QID, error) {
 	if l.conf.ROMount {
+		if l.conf.PanicOnWrite {
+			panic("attempt to write to RO mount")
+		}
 		return p9.QID{}, syscall.EBADF
 	}
 
@@ -585,6 +594,9 @@ func (l *localFile) GetAttr(_ p9.AttrMask) (p9.QID, p9.AttrMask, p9.Attr, error)
 // an error happens.
 func (l *localFile) SetAttr(valid p9.SetAttrMask, attr p9.SetAttr) error {
 	if l.conf.ROMount {
+		if l.conf.PanicOnWrite {
+			panic("attempt to write to RO mount")
+		}
 		return syscall.EBADF
 	}
 
@@ -722,6 +734,9 @@ func (*localFile) Remove() error {
 // Rename implements p9.File.
 func (l *localFile) Rename(directory p9.File, name string) error {
 	if l.conf.ROMount {
+		if l.conf.PanicOnWrite {
+			panic("attempt to write to RO mount")
+		}
 		return syscall.EBADF
 	}
 	if !isNameValid(name) {
@@ -789,6 +804,9 @@ func (l *localFile) WriteAt(p []byte, offset uint64) (int, error) {
 // Symlink implements p9.File.
 func (l *localFile) Symlink(target, newName string, uid p9.UID, gid p9.GID) (p9.QID, error) {
 	if l.conf.ROMount {
+		if l.conf.PanicOnWrite {
+			panic("attempt to write to RO mount")
+		}
 		return p9.QID{}, syscall.EBADF
 	}
 	if !isNameValid(newName) {
@@ -819,6 +837,9 @@ func (l *localFile) Symlink(target, newName string, uid p9.UID, gid p9.GID) (p9.
 // Link implements p9.File.
 func (l *localFile) Link(target p9.File, newName string) error {
 	if l.conf.ROMount {
+		if l.conf.PanicOnWrite {
+			panic("attempt to write to RO mount")
+		}
 		return syscall.EBADF
 	}
 	if !isNameValid(newName) {
@@ -842,6 +863,9 @@ func (*localFile) Mknod(_ string, _ p9.FileMode, _ uint32, _ uint32, _ p9.UID, _
 // UnlinkAt implements p9.File.
 func (l *localFile) UnlinkAt(name string, flags uint32) error {
 	if l.conf.ROMount {
+		if l.conf.PanicOnWrite {
+			panic("attempt to write to RO mount")
+		}
 		return syscall.EBADF
 	}
 	if !isNameValid(name) {

runsc/fsgofer/fsgofer_test.go

@@ -34,6 +34,15 @@ func init() {
 	allConfs = append(allConfs, roConfs...)
 }
 
+func assertPanic(t *testing.T, f func()) {
+	defer func() {
+		if r := recover(); r == nil {
+			t.Errorf("function did not panic")
+		}
+	}()
+	f()
+}
+
 var (
 	allTypes = []fileType{regular, directory, symlink}
 
@@ -434,6 +443,22 @@ func TestROMountChecks(t *testing.T) {
 	})
 }
 
+func TestROMountPanics(t *testing.T) {
+	conf := Config{ROMount: true, PanicOnWrite: true}
+	runCustom(t, allTypes, []Config{conf}, func(t *testing.T, s state) {
+		assertPanic(t, func() { s.file.Create("..", p9.ReadWrite, 0777, p9.UID(os.Getuid()), p9.GID(os.Getgid())) })
+		assertPanic(t, func() { s.file.Mkdir("..", 0777, p9.UID(os.Getuid()), p9.GID(os.Getgid())) })
+		assertPanic(t, func() { s.file.Rename(s.file, "..") })
+		assertPanic(t, func() { s.file.Symlink("some_place", "..", p9.UID(os.Getuid()), p9.GID(os.Getgid())) })
+		assertPanic(t, func() { s.file.UnlinkAt("..", 0) })
+		assertPanic(t, func() { s.file.Link(s.file, "..") })
+
+		valid := p9.SetAttrMask{Size: true}
+		attr := p9.SetAttr{Size: 0}
+		assertPanic(t, func() { s.file.SetAttr(valid, attr) })
+	})
+}
+
 func TestInvalidName(t *testing.T) {
 	runCustom(t, []fileType{regular}, rwConfs, func(t *testing.T, s state) {
 		if _, _, _, _, err := s.file.Create("..", p9.ReadWrite, 0777, p9.UID(os.Getuid()), p9.GID(os.Getgid())); err != syscall.EINVAL {

runsc/sandbox/sandbox.go

@@ -345,6 +345,10 @@ func (s *Sandbox) createGoferProcess(spec *specs.Spec, conf *boot.Config, bundle
 	setUIDGIDMappings(cmd, spec)
 	nss := filterNS([]specs.LinuxNamespaceType{specs.UserNamespace}, spec)
 
+	if conf.Overlay {
+		args = append(args, "--panic-on-write=true")
+	}
+
 	// Start the gofer in the given namespace.
 	log.Debugf("Starting gofer: %s %v", binPath, args)
 	if err := startInNS(cmd, nss); err != nil {