Re: [PATCH RFC 2/4] x86/arch_prctl: add ARCH_GET_NOPTI and ARCH_SET_NOPTI to enable/disable PTI

[Willy Tarreau <w@1wt.eu>]

On Mon, Jan 08, 2018 at 06:02:35PM +0100, Thomas Gleixner wrote:
> On Mon, 8 Jan 2018, Willy Tarreau wrote:
> > This allows to report the current state of the PTI protection and to
> > enable or disable it for the current task.
> > 
> > Signed-off-by: Willy Tarreau <w@1wt.eu>
> > ---
> >  arch/x86/include/uapi/asm/prctl.h |  3 +++
> >  arch/x86/kernel/process_64.c      | 24 ++++++++++++++++++++++++
> >  2 files changed, 27 insertions(+)
> > 
> > diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
> > index 5a6aac9..1f1b5bc 100644
> > --- a/arch/x86/include/uapi/asm/prctl.h
> > +++ b/arch/x86/include/uapi/asm/prctl.h
> > @@ -10,6 +10,9 @@
> >  #define ARCH_GET_CPUID		0x1011
> >  #define ARCH_SET_CPUID		0x1012
> >  
> > +#define ARCH_GET_NOPTI		0x1021
> > +#define ARCH_SET_NOPTI		0x1022
> > +
> >  #define ARCH_MAP_VDSO_X32	0x2001
> >  #define ARCH_MAP_VDSO_32	0x2002
> >  #define ARCH_MAP_VDSO_64	0x2003
> > diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> > index c754662..1686d3d 100644
> > --- a/arch/x86/kernel/process_64.c
> > +++ b/arch/x86/kernel/process_64.c
> > @@ -654,6 +654,30 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
> >  		ret = put_user(base, (unsigned long __user *)arg2);
> >  		break;
> >  	}
> > +	case ARCH_GET_NOPTI: {
> > +		unsigned long flag;
> > +
> > +		printk(KERN_DEBUG "get1: task=%p ti=%p fl=%16lx\n", task, task_thread_info(task), task_thread_info(task)->flags);
> > +		flag = !!(task_thread_info(task)->flags & _TIF_NOPTI);
> > +		ret = put_user(flag, (unsigned long __user *)arg2);
> > +		break;
> 
> Per task is really an odd choice. That should be per process I think, but
> that of course needs synchronization of some form. Aside of that we need to
> think about fork().

I also wondered how to do it and had no idea, but I wanted to start with
something, also keeping in mind that I didn't want to risk losing in
extra checks what we managed to previously save. I agree that it's not
the best we can have. That said, other features seem to work like this,
like TIF_NOTSC and TIF_NOCPUID, so it didn't look too odd at first glance.

Willy