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

[Andy Lutomirski]

On Wed, Apr 17, 2019 at 5:00 PM Linus Torvalds
<torvalds at linux-foundation.org> wrote:
>
> On Wed, Apr 17, 2019 at 4:42 PM Thomas Gleixner <tglx at linutronix.de> 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.
>
> > 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.
>

I don't think this type of NX goof was ever the argument for XPFO.
The main argument I've heard is that a malicious user program writes a
ROP payload into user memory (regular anonymous user memory) and then
gets the kernel to erroneously set RSP (*not* RIP) to point there.

I find this argument fairly weak for a couple reasons.  First, if
we're worried about this, let's do in-kernel CFI, not XPFO, to
mitigate it.  Second, I don't see why the exact same attack can't be
done using, say, page cache, and unless I'm missing something, XPFO
doesn't protect page cache.  Or network buffers, or pipe buffers, etc.