Hold t.mu while calling t.FSContext().

PiperOrigin-RevId: 202562686
Change-Id: I0f5be7cc9098e86fa31d016251c127cb91084b05

2 files changed, 15 insertions, 6 deletions

diff --git a/pkg/sentry/kernel/fs_context.go b/pkg/sentry/kernel/fs_context.go
index d1ca9c09d..dbc097696 100644
--- a/pkg/sentry/kernel/fs_context.go
+++ b/pkg/sentry/kernel/fs_context.go
@@ -66,6 +66,11 @@ func newFSContext(root, cwd *fs.Dirent, umask uint) *FSContext {
 // (that return nil).  This is because valid references may still be held via
 // proc files or other mechanisms.
 func (f *FSContext) destroy() {
+	// Hold f.mu so that we don't race with RootDirectory() and
+	// WorkingDirectory().
+	f.mu.Lock()
+	defer f.mu.Unlock()
+
 	f.root.DecRef()
 	f.root = nil
 
@@ -94,9 +99,9 @@ func (f *FSContext) Fork() *FSContext {
 }
 
 // WorkingDirectory returns the current working directory.
-// You should call DecRef on the returned Dirent when finished.
 //
-// This will return nil if called after destroy().
+// This will return nil if called after destroy(), otherwise it will return a
+// Dirent with a reference taken.
 func (f *FSContext) WorkingDirectory() *fs.Dirent {
 	f.mu.Lock()
 	defer f.mu.Unlock()
@@ -129,13 +134,15 @@ func (f *FSContext) SetWorkingDirectory(d *fs.Dirent) {
 }
 
 // RootDirectory returns the current filesystem root.
-// You should call DecRef on the returned Dirent when finished.
 //
-// This will return nil if called after destroy().
+// This will return nil if called after destroy(), otherwise it will return a
+// Dirent with a reference taken.
 func (f *FSContext) RootDirectory() *fs.Dirent {
 	f.mu.Lock()
 	defer f.mu.Unlock()
-	f.root.IncRef()
+	if f.root != nil {
+		f.root.IncRef()
+	}
 	return f.root
 }
 
diff --git a/pkg/sentry/kernel/task_resources.go b/pkg/sentry/kernel/task_resources.go
index 0389989e8..4ca25664a 100644
--- a/pkg/sentry/kernel/task_resources.go
+++ b/pkg/sentry/kernel/task_resources.go
@@ -123,6 +123,8 @@ func (t *Task) IsChrooted() bool {
 	realRoot := t.k.mounts.Root()
 	defer realRoot.DecRef()
 	root := t.tr.FSContext.RootDirectory()
-	defer root.DecRef()
+	if root != nil {
+		defer root.DecRef()
+	}
 	return root != realRoot
 }