* 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 - <http://www.cs.columbia.edu/~vpk/papers/ret2dir.sec14.pdf>, > >> 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 “readable, writeable, and executable” (RWX)—only very recent kernels > (≥v3.9) use the more conservative RW mapping.” 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. Thanks, Ingo
