On Fri, 2019-06-07 at 13:43 -0700, Andy Lutomirski wrote: > > On Jun 7, 2019, at 12:49 PM, Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote: > > > > On Fri, 2019-06-07 at 11:29 -0700, Andy Lutomirski wrote: > > > > On Jun 7, 2019, at 10:59 AM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote: > > > > > > > > > On 6/7/19 10:43 AM, Peter Zijlstra wrote: > > > > > I've no idea what the kernel should do; since you failed to answer the > > > > > question what happens when you point this to garbage. > > > > > > > > > > Does it then fault or what? > > > > > > > > Yeah, I think you'll fault with a rather mysterious CR2 value since > > > > you'll go look at the instruction that faulted and not see any > > > > references to the CR2 value. > > > > > > > > I think this new MSR probably needs to get included in oops output when > > > > CET is enabled. > > > > > > This shouldn’t be able to OOPS because it only happens at CPL 3, > > > right? We > > > should put it into core dumps, though. > > > > > > > > > > > Why don't we require that a VMA be in place for the entire bitmap? > > > > Don't we need a "get" prctl function too in case something like a JIT is > > > > running and needs to find the location of this bitmap to set bits > > > > itself? > > > > > > > > Or, do we just go whole-hog and have the kernel manage the bitmap > > > > itself. Our interface here could be: > > > > > > > > prctl(PR_MARK_CODE_AS_LEGACY, start, size); > > > > > > > > and then have the kernel allocate and set the bitmap for those code > > > > locations. > > > > > > Given that the format depends on the VA size, this might be a good > > > idea. I > > > bet we can reuse the special mapping infrastructure for this — the VMA > > > could > > > be a MAP_PRIVATE special mapping named [cet_legacy_bitmap] or similar, and > > > we > > > can even make special rules to core dump it intelligently if needed. And > > > we > > > can make mremap() on it work correctly if anyone (CRIU?) cares. > > > > > > Hmm. Can we be creative and skip populating it with zeros? The CPU > > > should > > > only ever touch a page if we miss an ENDBR on it, so, in normal operation, > > > we > > > don’t need anything to be there. We could try to prevent anyone from > > > *reading* it outside of ENDBR tracking if we want to avoid people > > > accidentally > > > wasting lots of memory by forcing it to be fully populated when the read > > > it. > > > > > > The one downside is this forces it to be per-mm, but that seems like a > > > generally reasonable model anyway. > > > > > > This also gives us an excellent opportunity to make it read-only as seen > > > from > > > userspace to prevent exploits from just poking it full of ones before > > > redirecting execution. > > > > GLIBC sets bits only for legacy code, and then makes the bitmap read- > > only. That > > avoids most issues: > > How does glibc know the linear address space size? We don’t want LA64 to > break old binaries because the address calculation changed. When an application starts, its highest stack address is determined. It uses that as the maximum the bitmap needs to cover. Yu-cheng
