commit a5f99bc80c9bdacf3a1902f8922ca9b939d52723
Author: Andy Lutomirski <luto@kernel.org>
Date:   Thu Nov 22 16:53:57 2018 -0800

    x86/fsgsbase/64: Fix do_arch_prctl_64() to work correctly wrt save_base_legacy()
    
    Signed-off-by: Andy Lutomirski <luto@kernel.org>

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 74035c2a85b3..fcf528df5b8d 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -733,6 +733,13 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
 		if (task == current) {
 			loadseg(GS, 0);
 			x86_gsbase_write_cpu_inactive(arg2);
+
+			/*
+			 * On non-FSGSBASE systems, save_base_legacy() expects
+			 * that we also fill in thread.gsbase.
+			 */
+			task->thread.gsbase = arg2;
+
 		} else {
 			task->thread.gsindex = 0;
 			x86_gsbase_write_task(task, arg2);
@@ -756,6 +763,12 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
 		if (task == current) {
 			loadseg(FS, 0);
 			x86_fsbase_write_cpu(arg2);
+
+			/*
+			 * On non-FSGSBASE systems, save_base_legacy() expects
+			 * that we also fill in thread.fsbase.
+			 */
+			task->thread.fsbase = arg2;
 		} else {
 			task->thread.fsindex = 0;
 			x86_fsbase_write_task(task, arg2);