Re: [PATCH] x86 : Ensure X86_FLAGS_NT is cleared on syscall entry

[Thomas Gleixner]

On Mon, 29 Sep 2014, Andy Lutomirski wrote:
> On Mon, Sep 29, 2014 at 12:41 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > On Mon, 29 Sep 2014, Andy Lutomirski wrote:
> >> On Mon, Sep 29, 2014 at 11:59 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> >> > On Mon, 29 Sep 2014, Andy Lutomirski wrote:
> >> >> Presumably interrupt delivery clears NT.  I haven't spotted where that's
> >> >> documented yet.
> >> >
> >> > Nope, that's unrelated.
> >>
> >> If it weren't the case, then we'd be totally screwed.  Fortunately, it
> >> is.  I found it: SDM Volume 3 6.12.1.2 says:
> >>
> >> (On calls to exception and interrupt
> >> handlers, the processor also clears the VM, RF, and NT flags in the
> >> EFLAGS register,
> >> after they are saved on the stack.)
> >
> > Sorry, I misunderstood your question.
> >
> > And yes on exception and interrupt entry it is cleared. Otherwise the
> > whole feature would not work at all ...
> >
> > But that's why I'm really not worried about it. While we can mask out
> > the stupid bit easily, it does not provide any value except protecting
> > silly userspace from rightfully raised exceptions.
> >
> > When I first saw that patch, I was worried about the security impact,
> > but after staring long enough at the SDM and the code, the only way it
> > can explode is when returning to user space. It cannot explode in the
> > kernel.
> 
> This is only true as long as the only use of lret from a system call
> (or kernel thread started from a system call) is to return to
> userspace.
> 
> For example, __efi64_thunk uses lretq, so mixed-mode EFI doesn't
> violate this assumption, but __efi64_thunk could just as easily have
> used iret.

And if __efi64_thunk would use iret, it would be wrong to begin with,
really. I'd rather see it die right there.
 
> IOW, I don't think there's any vulnerability here, but this makes me
> nervous.

I was pretty relaxed until you mentioned EFI ....

Thanks,

	tglx
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