On 07/11/2018 04:00 PM, Yu-cheng Yu wrote: > On Wed, 2018-07-11 at 15:40 -0700, Dave Hansen wrote: >> On 07/11/2018 03:10 PM, Yu-cheng Yu wrote: >>> >>> On Tue, 2018-07-10 at 17:11 -0700, Dave Hansen wrote: >>>> >>>> Is this feature *integral* to shadow stacks? Or, should it just >>>> be >>>> in a >>>> different series? >>> The whole CET series is mostly about SHSTK and only a minority for >>> IBT. >>> IBT changes cannot be applied by itself without first applying >>> SHSTK >>> changes. Would the titles help, e.g. x86/cet/ibt, x86/cet/shstk, >>> etc.? >> That doesn't really answer what I asked, though. >> >> Do shadow stacks *require* IBT? Or, should we concentrate on merging >> shadow stacks themselves first and then do IBT at a later time, in a >> different patch series? >> >> But, yes, better patch titles would help, although I'm not sure >> that's >> quite the format that Ingo and Thomas prefer. > > Shadow stack does not require IBT, but they complement each other. If > we can resolve the legacy bitmap, both features can be merged at the > same time. As large as this patch set is, I'd really prefer to see you get shadow stacks merged and then move on to IBT. I say separate them. > GLIBC does the bitmap setup. It sets bits in there. > I thought you wanted a smaller bitmap? One way is forcing legacy libs > to low address, or not having the bitmap at all, i.e. turn IBT off. I'm concerned with two things: 1. the virtual address space consumption, especially the *default* case which will be apps using 4-level address space amounts, but having 5-level-sized tables. 2. the driving a truck-sized hole in the address space limits You can force legacy libs to low addresses, but you can't stop anyone from putting code into a high address *later*, at least with the code we have today. >>>>> + rdmsrl(MSR_IA32_U_CET, r); >>>>> + r &= ~(MSR_IA32_CET_ENDBR_EN | MSR_IA32_CET_LEG_IW_EN >>>>> | >>>>> + MSR_IA32_CET_NO_TRACK_EN); >>>>> + wrmsrl(MSR_IA32_U_CET, r); >>>>> + current->thread.cet.ibt_enabled = 0; >>>>> +} >>>> What's the locking for current->thread.cet? >>> Now CET is not locked until the application calls ARCH_CET_LOCK. >> No, I mean what is the in-kernel locking for the current->thread.cet >> data structure? Is there none because it's only every modified via >> current->thread and it's entirely thread-local? > > Yes, that is the case.
