RE: [PATCH 2/5] x86, mpx: hook #BR exception handler to allocatebound tables

[Ren, Qiaowei]

> -----Original Message-----
> From: Steven Rostedt [mailto:rostedt@xxxxxxxxxxx]
> Sent: Saturday, January 18, 2014 1:15 AM
> To: H. Peter Anvin
> Cc: Borislav Petkov; Ren, Qiaowei; Thomas Gleixner; Ingo Molnar;
> x86@xxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH 2/5] x86, mpx: hook #BR exception handler to allocate
> bound tables
> 
> On Fri, 17 Jan 2014 08:51:03 -0800
> "H. Peter Anvin" <hpa@xxxxxxxxx> wrote:
> 
> > On 01/17/2014 08:47 AM, Borislav Petkov wrote:
> > >
> > > Right, so Steve and I played a couple of scenarios in IRC with this.
> > > So #BR is comparable with #PF, AFAICT, and as expected we don't take
> > > any locks when handling page faults in kernel space as we might deadlock.
> > >
> > > Now, what happens if a thread is sleeping on some lock down that
> > > GFP_KERNEL allocation path and another thread gets a #BR and goes
> > > that same mmap_pgoff path and tries to grab that same lock?
> >
> > It goes to sleep.  Same as if we take a page fault and have to page
> > something in.
> 
> Yep, which is what I was explaining to Boris on IRC.
> 
> >
> > > Also, what happens if you take a #BR in NMI context, say the NMI
> > > handler?
> >
> > You should never, ever do that.  We should never take a #BR in the
> > kernel, full stop -- if we do it is panic time.
> 
> Right. It should actually do what a page fault does too. If we page fault in NMI,
> it reports it and crashes.
> 
> >
> > > All I'm trying to say is, it might not be such a good idea to sleep
> > > in a fault handler...
> >
> > A fault handler from user space is really nothing other than a
> > different kind of system call.  It is nothing magic about it.
> 
> Exactly. I was saying that #BR should be just like #PF, as it can detect bugs in
> the kernel too. The first thing the handler should do is check to see if the fault
> occurred in userspace or kernel space. If it is userspace, then there's no
> restrictions. If it is kernel space then we should do the bare minimum to report
> the bug and then kill whatever task happened to do it.
> 
Yes. I guess I know what you mean and this detection is necessary.

The following check should be added into at the beginning of this handler.

        if (!user_mode(regs)) {
                if (!fixup_exception(regs)) {
                        tsk->thread.error_code = error_code;
                        tsk->thread.trap_nr = trapnr;
                        die(str, regs, error_code);
                }
        }

Thanks,
Qiaowei

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