x86/process/64: Use FSGSBASE instructions on thread copy and ptrace

When FSGSBASE is enabled, copying threads and reading fsbase and gsbase
using ptrace must read the actual values.

When copying a thread, use save_fsgs() and copy the saved values.  For
ptrace, the bases must be read from memory regardless of the selector if
FSGSBASE is enabled.

[ tglx: Invoke __rdgsbase_inactive() with interrupts disabled ]
[ luto: Massage changelog ]

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/1557309753-24073-9-git-send-email-chang.seok.bae@intel.com
Link: https://lkml.kernel.org/r/20200528201402.1708239-8-sashal@kernel.org

arch/x86/kernel/process.c

@@ -140,10 +140,12 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
 	memset(p->thread.ptrace_bps, 0, sizeof(p->thread.ptrace_bps));
 
 #ifdef CONFIG_X86_64
-	savesegment(gs, p->thread.gsindex);
-	p->thread.gsbase = p->thread.gsindex ? 0 : current->thread.gsbase;
-	savesegment(fs, p->thread.fsindex);
-	p->thread.fsbase = p->thread.fsindex ? 0 : current->thread.fsbase;
+	current_save_fsgs();
+	p->thread.fsindex = current->thread.fsindex;
+	p->thread.fsbase = current->thread.fsbase;
+	p->thread.gsindex = current->thread.gsindex;
+	p->thread.gsbase = current->thread.gsbase;
+
 	savesegment(es, p->thread.es);
 	savesegment(ds, p->thread.ds);
 #else

arch/x86/kernel/process_64.c

@@ -426,7 +426,8 @@ unsigned long x86_fsbase_read_task(struct task_struct *task)
 
 	if (task == current)
 		fsbase = x86_fsbase_read_cpu();
-	else if (task->thread.fsindex == 0)
+	else if (static_cpu_has(X86_FEATURE_FSGSBASE) ||
+		 (task->thread.fsindex == 0))
 		fsbase = task->thread.fsbase;
 	else
 		fsbase = x86_fsgsbase_read_task(task, task->thread.fsindex);
@@ -440,7 +441,8 @@ unsigned long x86_gsbase_read_task(struct task_struct *task)
 
 	if (task == current)
 		gsbase = x86_gsbase_read_cpu_inactive();
-	else if (task->thread.gsindex == 0)
+	else if (static_cpu_has(X86_FEATURE_FSGSBASE) ||
+		 (task->thread.gsindex == 0))
 		gsbase = task->thread.gsbase;
 	else
 		gsbase = x86_fsgsbase_read_task(task, task->thread.gsindex);