Re: [RFC PATCH v9 03/13] mm: Add support for eXclusive Page Frame Ownership (XPFO)

[Thomas Gleixner]

On Wed, 17 Apr 2019, Linus Torvalds wrote:
> On Wed, Apr 17, 2019 at 4:42 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > On Wed, 17 Apr 2019, Linus Torvalds wrote:
> > > With SMEP, user space pages are always NX.
> >
> > We talk past each other. The user space page in the ring3 valid virtual
> > address space (non negative) is of course protected by SMEP.
> >
> > The attack utilizes the kernel linear mapping of the physical
> > memory. I.e. user space address 0x43210 has a kernel equivalent at
> > 0xfxxxxxxxxxx. So if the attack manages to trick the kernel to that valid
> > kernel address and that is mapped X --> game over. SMEP does not help
> > there.
> 
> Oh, agreed.
> 
> But that would simply be a kernel bug. We should only map kernel pages
> executable when we have kernel code in them, and we should certainly
> not allow those pages to be mapped writably in user space.
> 
> That kind of "executable in kernel, writable in user" would be a
> horrendous and major bug.
> 
> So i think it's a non-issue.

Pretty much.

> > From the top of my head I'd say this is a non issue as those kernel address
> > space mappings _should_ be NX, but we got bitten by _should_ in the past:)
> 
> I do agree that bugs can happen, obviously, and we might have missed something.
>
> But in the context of XPFO, I would argue (*very* strongly) that the
> likelihood of the above kind of bug is absolutely *miniscule* compared
> to the likelihood that we'd have something wrong in the software
> implementation of XPFO.
> 
> So if the argument is "we might have bugs in software", then I think
> that's an argument _against_ XPFO rather than for it.

No argument from my side. We better spend time to make sure that a bogus
kernel side X mapping is caught, like we catch other things.

Thanks,

	tglx