On 2017/9/12 15:40, Juerg Haefliger wrote: > > > On 09/12/2017 09:07 AM, Yisheng Xie wrote: >> Hi Tycho, >> >> On 2017/9/11 23:02, Tycho Andersen wrote: >>> Hi Yisheng, >>> >>> On Mon, Sep 11, 2017 at 06:34:45PM +0800, Yisheng Xie wrote: >>>> Hi Tycho , >>>> >>>> On 2017/9/8 1:35, Tycho Andersen wrote: >>>>> Hi all, >>>>> >>>>> Here is v6 of the XPFO set; see v5 discussion here: >>>>> https://lkml.org/lkml/2017/8/9/803 >>>>> >>>>> Changelogs are in the individual patch notes, but the highlights are: >>>>> * add primitives for ensuring memory areas are mapped (although these are quite >>>>> ugly, using stack allocation; I'm open to better suggestions) >>>>> * instead of not flushing caches, re-map pages using the above >>>>> * TLB flushing is much more correct (i.e. we're always flushing everything >>>>> everywhere). I suspect we may be able to back this off in some cases, but I'm >>>>> still trying to collect performance numbers to prove this is worth doing. >>>>> >>>>> I have no TODOs left for this set myself, other than fixing whatever review >>>>> feedback people have. Thoughts and testing welcome! >>>> >>>> According to the paper of Vasileios P. Kemerlis et al, the mainline kernel >>>> will not set the Pro. of physmap(direct map area) to RW(X), so do we really >>>> need XPFO to protect from ret2dir attack? >>> >>> I guess you're talking about section 4.3? >> Yes >> >>> They mention that that x86 >>> only gets rw, but that aarch64 is rwx still. >> IIRC, the in kernel of v4.13 the aarch64 is not rwx, I will check it. >> >>> >>> But in either case this still provides access protection, similar to >>> SMAP. Also, if I understand things correctly the protections are >>> unmanaged, so a page that had the +x bit set at some point, it could >>> be used for ret2dir. >> So you means that the Pro. of direct map area maybe changed to +x, then ret2dir attack can use it? > > XPFO protects against malicious reads from userspace (potentially > accessing sensitive data). This sounds reasonable to me. > I've also been told by a security expert that > ROP attacks are still possible even if user space memory is > non-executable. XPFO is supposed to prevent that but I haven't been able > to confirm this. It's way out of my comfort zone. It also quite out of knowledge, and I just try hard to understand it. Thanks so much for your kind explain. And hope some security expert can give some more detail explain? Thanks Yisheng Xie > > ...Juerg -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
