Re: [PATCH v11 6/9] x86/cet: Add PTRACE interface for CET

[Yu, Yu-cheng]

On 9/3/2020 7:26 AM, Dave Hansen wrote:
> On 9/2/20 9:35 PM, Andy Lutomirski wrote:
> > > > > > > +       fpu__prepare_read(fpu);
> > > > > > > +       cetregs = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER);
> > > > > > > +       if (!cetregs)
> > > > > > > +               return -EFAULT;
> > > > > > Can this branch ever be hit without a kernel bug? If yes, I think
> > > > > > -EFAULT is probably a weird error code to choose here. If no, this
> > > > > > should probably use WARN_ON(). Same thing in cetregs_set().
> > > > > When a thread is not CET-enabled, its CET state does not exist.  I looked at EFAULT, and it means "Bad address".  Maybe this can be ENODEV, which means "No such device"?
> > Having read the code, I’m unconvinced. It looks like a get_xsave_addr() failure means “state not saved; task sees INIT state”.  So *maybe* it’s reasonable -ENODEV this, but I’m not really convinced. I tend to think we should return the actual INIT state and that we should permit writes and handle them correctly.
>
> PTRACE is asking for access to the values in the *registers*, not for
> the value in the kernel XSAVE buffer.  We just happen to only have the
> kernel XSAVE buffer around.

When get_xsave_addr() returns NULL, there are three possibilities:
- XSAVE is not enabled or not supported;
- The kernel does not support the requested feature;
- The requested feature is in INIT state.

If the debugger is going to write an MSR, only in the third case would 
this make a slight sense.  For example, if the system has CET enabled, 
but the task does not have CET enabled, and GDB is writing to a CET MSR. 
 But still, this is strange to me.

> If we want to really support PTRACE we have to allow the registers to be
> get/set, regardless of what state they are in, INIT state or not.  So,
> yeah I agree with Andy.

GDB does not have a WRMSR mechanism.  If GDB is going to write an MSR, 
it will call arch_prctl or an assembly routine in memory.

Yu-cheng