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 <l...@kernel.org> 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;