x86/bugs: Flush IBP in ib_prctl_set()

We missed the window between the TIF flag update and the next reschedule.

Signed-off-by: Rodrigo Branco <bsdaemon@google.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: <stable@vger.kernel.org>
(cherry picked from commit a664ec9)
CVE-2023-0045
Signed-off-by: Yuxuan Luo <yuxuan.luo@canonical.com>
Signed-off-by: Timo Aaltonen <timo.aaltonen@canonical.com>

arch/x86/kernel/cpu/bugs.c

@@ -1535,6 +1535,8 @@ static int ib_prctl_set(struct task_struct *task, unsigned long ctrl)
 		if (ctrl == PR_SPEC_FORCE_DISABLE)
 			task_set_spec_ib_force_disable(task);
 		task_update_spec_tif(task);
+		if (task == current)
+			indirect_branch_prediction_barrier();
 		break;
 	default:
 		return -ERANGE;