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

From: Ingo Molnar
Date: Wed Apr 17 2019 - 13:26:42 EST

* Nadav Amit <nadav.amit@xxxxxxxxx> wrote:

> > On Apr 17, 2019, at 10:09 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> >
> > * Khalid Aziz <khalid.aziz@xxxxxxxxxx> wrote:
> >
> >>> I.e. the original motivation of the XPFO patches was to prevent execution
> >>> of direct kernel mappings. Is this motivation still present if those
> >>> mappings are non-executable?
> >>>
> >>> (Sorry if this has been asked and answered in previous discussions.)
> >>
> >> Hi Ingo,
> >>
> >> That is a good question. Because of the cost of XPFO, we have to be very
> >> sure we need this protection. The paper from Vasileios, Michalis and
> >> Angelos - <>,
> >> does go into how ret2dir attacks can bypass SMAP/SMEP in sections 6.1
> >> and 6.2.
> >
> > So it would be nice if you could generally summarize external arguments
> > when defending a patchset, instead of me having to dig through a PDF
> > which not only causes me to spend time that you probably already spent
> > reading that PDF, but I might also interpret it incorrectly. ;-)
> >
> > The PDF you cited says this:
> >
> > "Unfortunately, as shown in Table 1, the W^X prop-erty is not enforced
> > in many platforms, including x86-64. In our example, the content of
> > user address 0xBEEF000 is also accessible through kernel address
> > 0xFFFF87FF9F080000 as plain, executable code."
> >
> > Is this actually true of modern x86-64 kernels? We've locked down W^X
> > protections in general.
> As I was curious, I looked at the paper. Here is a quote from it:
> "In x86-64, however, the permissions of physmap are not in sane state.
> Kernels up to v3.8.13 violate the W^X property by mapping the entire region
> as âreadable, writeable, and executableâ (RWX)âonly very recent kernels
> (âv3.9) use the more conservative RW mapping.â

But v3.8.13 is a 5+ years old kernel, it doesn't count as a "modern"
kernel in any sense of the word. For any proposed patchset with
significant complexity and non-trivial costs the benchmark version
threshold is the "current upstream kernel".

So does that quote address my followup questions:

> Is this actually true of modern x86-64 kernels? We've locked down W^X
> protections in general.
> I.e. this conclusion:
> "Therefore, by simply overwriting kfptr with 0xFFFF87FF9F080000 and
> triggering the kernel to dereference it, an attacker can directly
> execute shell code with kernel privileges."
> ... appears to be predicated on imperfect W^X protections on the x86-64
> kernel.
> Do such holes exist on the latest x86-64 kernel? If yes, is there a
> reason to believe that these W^X holes cannot be fixed, or that any fix
> would be more expensive than XPFO?


What you are proposing here is a XPFO patch-set against recent kernels
with significant runtime overhead, so my questions about the W^X holes
are warranted.