RE: [regression in -rc1] Re: [PATCH v6 2/8] x86/fsgsbase/64: Introduce FS/GS base helper functions

[Bae, Chang Seok]

On Wed, Oct 24, 2018 at 12:43 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> I think x86_fsbase_write_task() makes plenty of sense, but I think
> that callers need to be aware that the effect of writing a nonzero
> fsbase *and* a nonzero fsindex is bizarre on non-FSGSBASE systems.  So
> that code should go in the callers.  The oddities involved have little
> to do with whether the caller is writing to current or to something
> else.
> 
> Arguably the code should be entirely split out into the code that
> writes current (arch_prctl()) and the code that writes a stopped task
> (ptrace).  I don't think there are any code paths that genuinely can
> write either.
> 

Can you check this patch is close to what in your mind?

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index d6674a425714..5f986e15842e 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -341,19 +341,11 @@ static unsigned long x86_fsgsbase_read_task(struct task_struct *task,
 
 void x86_fsbase_write_cpu(unsigned long fsbase)
 {
-	/*
-	 * Set the selector to 0 as a notion, that the segment base is
-	 * overwritten, which will be checked for skipping the segment load
-	 * during context switch.
-	 */
-	loadseg(FS, 0);
 	wrmsrl(MSR_FS_BASE, fsbase);
 }
 
 void x86_gsbase_write_cpu_inactive(unsigned long gsbase)
 {
-	/* Set the selector to 0 for the same reason as %fs above. */
-	loadseg(GS, 0);
 	wrmsrl(MSR_KERNEL_GS_BASE, gsbase);
 }
 
@@ -361,9 +353,7 @@ unsigned long x86_fsbase_read_task(struct task_struct *task)
 {
 	unsigned long fsbase;
 
-	if (task == current)
-		fsbase = x86_fsbase_read_cpu();
-	else if (task->thread.fsindex == 0)
+	if (task->thread.fsindex == 0)
 		fsbase = task->thread.fsbase;
 	else
 		fsbase = x86_fsgsbase_read_task(task, task->thread.fsindex);
@@ -375,9 +365,7 @@ unsigned long x86_gsbase_read_task(struct task_struct *task)
 {
 	unsigned long gsbase;
 
-	if (task == current)
-		gsbase = x86_gsbase_read_cpu_inactive();
-	else if (task->thread.gsindex == 0)
+	if (task->thread.gsindex == 0)
 		gsbase = task->thread.gsbase;
 	else
 		gsbase = x86_fsgsbase_read_task(task, task->thread.gsindex);
@@ -396,8 +384,6 @@ int x86_fsbase_write_task(struct task_struct *task, unsigned long fsbase)
 
 	preempt_disable();
 	task->thread.fsbase = fsbase;
-	if (task == current)
-		x86_fsbase_write_cpu(fsbase);
 	task->thread.fsindex = 0;
 	preempt_enable();
 
@@ -411,8 +397,6 @@ int x86_gsbase_write_task(struct task_struct *task, unsigned long gsbase)
 
 	preempt_disable();
 	task->thread.gsbase = gsbase;
-	if (task == current)
-		x86_gsbase_write_cpu_inactive(gsbase);
 	task->thread.gsindex = 0;
 	preempt_enable();
 
@@ -761,20 +745,42 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
 	switch (option) {
 	case ARCH_SET_GS: {
 		ret = x86_gsbase_write_task(task, arg2);
+		if (task == current && ret == 0) {
+			preempt_disable();
+			loadseg(GS, 0);
+			x86_gsbase_write_cpu_inactive();
+			preempt_enable();
+		}
 		break;
 	}
 	case ARCH_SET_FS: {
 		ret = x86_fsbase_write_task(task, arg2);
+		if (task == current && ret == 0) {
+			preempt_disable();
+			loadseg(FS, 0);
+			x86_fsbase_write_cpu();
+			preempt_enable();
+		}
 		break;
 	}
 	case ARCH_GET_FS: {
-		unsigned long base = x86_fsbase_read_task(task);
+		unsigned long base;
+
+		if (task == current)
+			base = x86_fsbase_read_cpu();
+		else
+			base = x86_fsbase_read_task(task);
 
 		ret = put_user(base, (unsigned long __user *)arg2);
 		break;
 	}
 	case ARCH_GET_GS: {
-		unsigned long base = x86_gsbase_read_task(task);
+		unsigned long base;
+
+		if (task == current)
+			base = x86_gsbase_read_cpu_inactive();
+		else
+			base = x86_gsbase_read_task(task);
 
 		ret = put_user(base, (unsigned long __user *)arg2);
 		break;