Add support for mount flags

Plumbs MS_NOEXEC and MS_RDONLY. Others are TODO.

Updates #1623 #1193

PiperOrigin-RevId: 300764669

pkg/sentry/fsbridge/vfs.go

@@ -115,8 +115,6 @@ func NewVFSLookup(mntns *vfs.MountNamespace, root, workingDir vfs.VirtualDentry)
 //
 // remainingTraversals is not configurable in VFS2, all callers are using the
 // default anyways.
-//
-// TODO(gvisor.dev/issue/1623): Check mount has read and exec permission.
 func (l *vfsLookup) OpenPath(ctx context.Context, pathname string, opts vfs.OpenOptions, _ *uint, resolveFinal bool) (File, error) {
 	vfsObj := l.mntns.Root().Mount().Filesystem().VirtualFilesystem()
 	creds := auth.CredentialsFromContext(ctx)

pkg/sentry/vfs/mount.go

@@ -74,6 +74,10 @@ 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.
@@ -81,6 +85,21 @@ type Mount struct {
 	writers int64
 }
 
+func newMount(vfs *VirtualFilesystem, fs *Filesystem, root *Dentry, mntns *MountNamespace, opts *MountOptions) *Mount {
+	mnt := &Mount{
+		vfs:   vfs,
+		fs:    fs,
+		root:  root,
+		flags: opts.Flags,
+		ns:    mntns,
+		refs:  1,
+	}
+	if opts.ReadOnly {
+		mnt.setReadOnlyLocked(true)
+	}
+	return mnt
+}
+
 // A MountNamespace is a collection of Mounts.
 //
 // MountNamespaces are reference-counted. Unless otherwise specified, all
@@ -129,13 +148,7 @@ func (vfs *VirtualFilesystem) NewMountNamespace(ctx context.Context, creds *auth
 		refs:        1,
 		mountpoints: make(map[*Dentry]uint32),
 	}
-	mntns.root = &Mount{
-		vfs:  vfs,
-		fs:   fs,
-		root: root,
-		ns:   mntns,
-		refs: 1,
-	}
+	mntns.root = newMount(vfs, fs, root, mntns, &MountOptions{})
 	return mntns, nil
 }
 
@@ -148,12 +161,7 @@ func (vfs *VirtualFilesystem) NewDisconnectedMount(fs *Filesystem, root *Dentry,
 	if root != nil {
 		root.IncRef()
 	}
-	return &Mount{
-		vfs:  vfs,
-		fs:   fs,
-		root: root,
-		refs: 1,
-	}, nil
+	return newMount(vfs, fs, root, nil /* mntns */, opts), nil
 }
 
 // MountAt creates and mounts a Filesystem configured by the given arguments.
@@ -218,13 +226,7 @@ func (vfs *VirtualFilesystem) MountAt(ctx context.Context, creds *auth.Credentia
 	// are directories, or neither are, and returns ENOTDIR if this is not the
 	// case.
 	mntns := vd.mount.ns
-	mnt := &Mount{
-		vfs:  vfs,
-		fs:   fs,
-		root: root,
-		ns:   mntns,
-		refs: 1,
-	}
+	mnt := newMount(vfs, fs, root, mntns, opts)
 	vfs.mounts.seq.BeginWrite()
 	vfs.connectLocked(mnt, vd, mntns)
 	vfs.mounts.seq.EndWrite()

pkg/sentry/vfs/options.go

@@ -46,8 +46,21 @@ type MknodOptions struct {
 	DevMinor uint32
 }
 
+// MountFlags contains flags as specified for mount(2), e.g. MS_NOEXEC.
+// MS_RDONLY is not part of MountFlags because it's tracked in Mount.writers.
+type MountFlags struct {
+	// NoExec is equivalent to MS_NOEXEC.
+	NoExec bool
+}
+
 // MountOptions contains options to VirtualFilesystem.MountAt().
 type MountOptions struct {
+	// Flags contains flags as specified for mount(2), e.g. MS_NOEXEC.
+	Flags MountFlags
+
+	// ReadOnly is equivalent to MS_RDONLY.
+	ReadOnly bool
+
 	// GetFilesystemOptions contains options to FilesystemType.GetFilesystem().
 	GetFilesystemOptions GetFilesystemOptions
 
@@ -75,7 +88,8 @@ type OpenOptions struct {
 
 	// FileExec is set when the file is being opened to be executed.
 	// VirtualFilesystem.OpenAt() checks that the caller has execute permissions
-	// on the file, and that the file is a regular file.
+	// on the file, that the file is a regular file, and that the mount doesn't
+	// have MS_NOEXEC set.
 	FileExec bool
 }
 

pkg/sentry/vfs/vfs.go

@@ -388,6 +388,11 @@ func (vfs *VirtualFilesystem) OpenAt(ctx context.Context, creds *auth.Credential
 			// TODO(gvisor.dev/issue/1193): Move inside fsimpl to avoid another call
 			// to FileDescription.Stat().
 			if opts.FileExec {
+				if fd.Mount().flags.NoExec {
+					fd.DecRef()
+					return nil, syserror.EACCES
+				}
+
 				// Only a regular file can be executed.
 				stat, err := fd.Stat(ctx, StatOptions{Mask: linux.STATX_TYPE})
 				if err != nil {