> On Jan 15, 2018, at 9:50 AM, Nadav Amit <[email protected]> wrote: > > 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. >
Not if someone used LAR, a la the sigreturn_32 test. Not necessarily a showstopper, though. You'd also have to figure out how to do PTI per-thread, which Linus doesn't like. See Willy's PTI opt-out thread.
