Messages in this thread | | | From | Nadav Amit <> | Subject | Re: [RFC] x86: Avoid CR3 load on compatibility mode with PTI | Date | Mon, 15 Jan 2018 17:50:32 +0000 |
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Andy Lutomirski <luto@amacapital.net> wrote:
> > >> On Jan 15, 2018, at 9:42 AM, Nadav Amit <namit@vmware.com> wrote: >> >> Andy Lutomirski <luto@amacapital.net> wrote: >> >>>> On Jan 14, 2018, at 12:13 PM, Nadav Amit <namit@vmware.com> wrote: >>>> >>>> Currently, when page-table isolation is on to prevent the Meltdown bug >>>> (CVE-2017-5754), CR3 is always loaded on system-call and interrupt. >>>> >>>> However, it appears that this is an unnecessary measure when programs >>>> run in compatibility mode. In this mode only 32-bit registers are >>>> available, which means that there *should* be no way for the CPU to >>>> access, even speculatively, memory that belongs to the kernel, which >>>> sits in high addresses. >>> >>> You're assuming that TIF_IA32 prevents the execution of 64-bit code. It doesn't. >>> >>> I've occasionally considered adding an opt-in hardening mechanism to enforce 32-bit or 64-bit execution, but we don't have this now. >> >> I noticed it doesn’t. I thought the removing/restoring the __USER_CS >> descriptor on context switch, based on TIF_IA32, would be enough. >> modify_ldt() always keeps the descriptor l-bit clear. I will review the >> other GDT descriptors, and if needed, create two GDTs. Let me know if I >> missed anything else. > > There world need to be some opt-in control, I think, for CRIU if nothing else. > > Also, on Xen PV, it's a complete nonstarter. We don't have enough control over the GDT unless someone knows otherwise. But there's no PTI on Xen PV either. > >>> Anything like this would also need to spend on SMEP, I think -- the pseudo-SMEP granted by PTI is too valuable to give up on old boxes, I think. >> >> If SMEP is not supported, compatibility mode would still require page-table >> isolation. >> >> Thanks for the feedback. I still look for an ack for the basic idea of >> disabling page-table isolation on compatibility mode. > > I'm still not really convinced this is worth it. It will send a bad message and get people to run critical stuff compiled for 32-bit, which has its own downsides.
I can handle #GP gracefully if __USER_CS is loaded so PTI would be required again. Doing so would eliminate the need for an opt-in, and preserve the current semantics.
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