3 files changed, 50 insertions, 4 deletions

diff --git a/pkg/sentry/fsimpl/gofer/filesystem.go b/pkg/sentry/fsimpl/gofer/filesystem.go
index c34451269..43c3c5a2d 100644
--- a/pkg/sentry/fsimpl/gofer/filesystem.go
+++ b/pkg/sentry/fsimpl/gofer/filesystem.go
@@ -783,7 +783,15 @@ func (fs *filesystem) LinkAt(ctx context.Context, rp *vfs.ResolvingPath, vd vfs.
 func (fs *filesystem) MkdirAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.MkdirOptions) error {
 	creds := rp.Credentials()
 	return fs.doCreateAt(ctx, rp, true /* dir */, func(parent *dentry, name string, _ **[]*dentry) error {
-		if _, err := parent.file.mkdir(ctx, name, (p9.FileMode)(opts.Mode), (p9.UID)(creds.EffectiveKUID), (p9.GID)(creds.EffectiveKGID)); err != nil {
+		// If the parent is a setgid directory, use the parent's GID
+		// rather than the caller's and enable setgid.
+		kgid := creds.EffectiveKGID
+		mode := opts.Mode
+		if atomic.LoadUint32(&parent.mode)&linux.S_ISGID != 0 {
+			kgid = auth.KGID(atomic.LoadUint32(&parent.gid))
+			mode |= linux.S_ISGID
+		}
+		if _, err := parent.file.mkdir(ctx, name, p9.FileMode(mode), (p9.UID)(creds.EffectiveKUID), p9.GID(kgid)); err != nil {
 			if !opts.ForSyntheticMountpoint || err == syserror.EEXIST {
 				return err
 			}
@@ -1145,7 +1153,15 @@ func (d *dentry) createAndOpenChildLocked(ctx context.Context, rp *vfs.Resolving
 	name := rp.Component()
 	// We only want the access mode for creating the file.
 	createFlags := p9.OpenFlags(opts.Flags) & p9.OpenFlagsModeMask
-	fdobj, openFile, createQID, _, err := dirfile.create(ctx, name, createFlags, (p9.FileMode)(opts.Mode), (p9.UID)(creds.EffectiveKUID), (p9.GID)(creds.EffectiveKGID))
+
+	// If the parent is a setgid directory, use the parent's GID rather
+	// than the caller's.
+	kgid := creds.EffectiveKGID
+	if atomic.LoadUint32(&d.mode)&linux.S_ISGID != 0 {
+		kgid = auth.KGID(atomic.LoadUint32(&d.gid))
+	}
+
+	fdobj, openFile, createQID, _, err := dirfile.create(ctx, name, createFlags, p9.FileMode(opts.Mode), (p9.UID)(creds.EffectiveKUID), p9.GID(kgid))
 	if err != nil {
 		dirfile.close(ctx)
 		return nil, err
diff --git a/pkg/sentry/fsimpl/gofer/gofer.go b/pkg/sentry/fsimpl/gofer/gofer.go
index 71569dc65..692da02c1 100644
--- a/pkg/sentry/fsimpl/gofer/gofer.go
+++ b/pkg/sentry/fsimpl/gofer/gofer.go
@@ -1102,10 +1102,26 @@ func (d *dentry) setStat(ctx context.Context, creds *auth.Credentials, opts *vfs
 
 	d.metadataMu.Lock()
 	defer d.metadataMu.Unlock()
+
+	// As with Linux, if the UID, GID, or file size is changing, we have to
+	// clear permission bits. Note that when set, clearSGID causes
+	// permissions to be updated, but does not modify stat.Mask, as
+	// modification would cause an extra inotify flag to be set.
+	clearSGID := stat.Mask&linux.STATX_UID != 0 && stat.UID != atomic.LoadUint32(&d.uid) ||
+		stat.Mask&linux.STATX_GID != 0 && stat.GID != atomic.LoadUint32(&d.gid) ||
+		stat.Mask&linux.STATX_SIZE != 0
+	if clearSGID {
+		if stat.Mask&linux.STATX_MODE != 0 {
+			stat.Mode = uint16(vfs.ClearSUIDAndSGID(uint32(stat.Mode)))
+		} else {
+			stat.Mode = uint16(vfs.ClearSUIDAndSGID(atomic.LoadUint32(&d.mode)))
+		}
+	}
+
 	if !d.isSynthetic() {
 		if stat.Mask != 0 {
 			if err := d.file.setAttr(ctx, p9.SetAttrMask{
-				Permissions:        stat.Mask&linux.STATX_MODE != 0,
+				Permissions:        stat.Mask&linux.STATX_MODE != 0 || clearSGID,
 				UID:                stat.Mask&linux.STATX_UID != 0,
 				GID:                stat.Mask&linux.STATX_GID != 0,
 				Size:               stat.Mask&linux.STATX_SIZE != 0,
@@ -1140,7 +1156,7 @@ func (d *dentry) setStat(ctx context.Context, creds *auth.Credentials, opts *vfs
 			return nil
 		}
 	}
-	if stat.Mask&linux.STATX_MODE != 0 {
+	if stat.Mask&linux.STATX_MODE != 0 || clearSGID {
 		atomic.StoreUint32(&d.mode, d.fileType()|uint32(stat.Mode))
 	}
 	if stat.Mask&linux.STATX_UID != 0 {
diff --git a/pkg/sentry/fsimpl/gofer/regular_file.go b/pkg/sentry/fsimpl/gofer/regular_file.go
index 283b220bb..4f1ad0c88 100644
--- a/pkg/sentry/fsimpl/gofer/regular_file.go
+++ b/pkg/sentry/fsimpl/gofer/regular_file.go
@@ -266,6 +266,20 @@ func (fd *regularFileFD) pwrite(ctx context.Context, src usermem.IOSequence, off
 			return 0, offset, err
 		}
 	}
+
+	// As with Linux, writing clears the setuid and setgid bits.
+	if n > 0 {
+		oldMode := atomic.LoadUint32(&d.mode)
+		// If setuid or setgid were set, update d.mode and propagate
+		// changes to the host.
+		if newMode := vfs.ClearSUIDAndSGID(oldMode); newMode != oldMode {
+			atomic.StoreUint32(&d.mode, newMode)
+			if err := d.file.setAttr(ctx, p9.SetAttrMask{Permissions: true}, p9.SetAttr{Permissions: p9.FileMode(newMode)}); err != nil {
+				return 0, offset, err
+			}
+		}
+	}
+
 	return n, offset + n, nil
 }