Date: Tue, 12 Sep 2017 09:40:14 +0200 From: Juerg Haefliger <juerg.haefliger@...onical.com> To: Yisheng Xie <xieyisheng1@...wei.com>, Tycho Andersen <tycho@...ker.com> Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, kernel-hardening@...ts.openwall.com, Marco Benatto <marco.antonio.780@...il.com> Subject: Re: [PATCH v6 00/11] Add support for eXclusive Page Frame Ownership 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). 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. ...Juerg > Thanks > Yisheng Xie > >
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