Andy Lutomirski <[email protected]> wrote: > > >> On Jan 15, 2018, at 9:42 AM, Nadav Amit <[email protected]> wrote: >> >> Andy Lutomirski <[email protected]> wrote: >> >>>> On Jan 14, 2018, at 12:13 PM, Nadav Amit <[email protected]> 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.
