Replace TaskFromContext(ctx).Kernel() with KernelFromContext(ctx)

Panic seen at some code path like control.ExecAsync where
ctx does not have a Task.

Reported-by: syzbot+55ce727161cf94a7b7d6@syzkaller.appspotmail.com
PiperOrigin-RevId: 355960596

pkg/sentry/socket/hostinet/socket_vfs2.go

@@ -80,8 +80,7 @@ func newVFS2Socket(t *kernel.Task, family int, stype linux.SockType, protocol in
 
 // Release implements vfs.FileDescriptionImpl.Release.
 func (s *socketVFS2) Release(ctx context.Context) {
-	t := kernel.TaskFromContext(ctx)
-	t.Kernel().DeleteSocketVFS2(&s.vfsfd)
+	kernel.KernelFromContext(ctx).DeleteSocketVFS2(&s.vfsfd)
 	s.socketOpsCommon.Release(ctx)
 }
 

pkg/sentry/socket/netstack/netstack_vfs2.go

@@ -79,8 +79,7 @@ func NewVFS2(t *kernel.Task, family int, skType linux.SockType, protocol int, qu
 
 // Release implements vfs.FileDescriptionImpl.Release.
 func (s *SocketVFS2) Release(ctx context.Context) {
-	t := kernel.TaskFromContext(ctx)
-	t.Kernel().DeleteSocketVFS2(&s.vfsfd)
+	kernel.KernelFromContext(ctx).DeleteSocketVFS2(&s.vfsfd)
 	s.socketOpsCommon.Release(ctx)
 }
 

pkg/sentry/socket/unix/unix_vfs2.go

@@ -95,8 +95,7 @@ func NewFileDescription(ep transport.Endpoint, stype linux.SockType, flags uint3
 // DecRef implements RefCounter.DecRef.
 func (s *SocketVFS2) DecRef(ctx context.Context) {
 	s.socketVFS2Refs.DecRef(func() {
-		t := kernel.TaskFromContext(ctx)
-		t.Kernel().DeleteSocketVFS2(&s.vfsfd)
+		kernel.KernelFromContext(ctx).DeleteSocketVFS2(&s.vfsfd)
 		s.ep.Close(ctx)
 		if s.abstractNamespace != nil {
 			s.abstractNamespace.Remove(s.abstractName, s)

runsc/container/container_test.go

@@ -777,6 +777,28 @@ func TestExec(t *testing.T) {
 					}
 				})
 			}
+
+			// Test for exec failure with an non-existent file.
+			t.Run("nonexist", func(t *testing.T) {
+				// b/179114837 found by Syzkaller that causes nil pointer panic when
+				// trying to dec-ref an unix socket FD.
+				fds, err := syscall.Socketpair(syscall.AF_UNIX, syscall.SOCK_STREAM, 0)
+				if err != nil {
+					t.Fatal(err)
+				}
+				defer syscall.Close(fds[0])
+
+				_, err = cont.executeSync(&control.ExecArgs{
+					Argv: []string{"/nonexist"},
+					FilePayload: urpc.FilePayload{
+						Files: []*os.File{os.NewFile(uintptr(fds[1]), "sock")},
+					},
+				})
+				want := "failed to load /nonexist"
+				if err == nil || !strings.Contains(err.Error(), want) {
+					t.Errorf("executeSync: want err containing %q; got err = %q", want, err)
+				}
+			})
 		})
 	}
 }