On Mon, Jun 18, 2018 at 3:03 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: > On Tue, Jun 12, 2018 at 12:34 PM H.J. Lu <hjl.tools@xxxxxxxxx> wrote: >> >> On Tue, Jun 12, 2018 at 11:59 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: >> > On Tue, 12 Jun 2018, H.J. Lu wrote: >> >> On Tue, Jun 12, 2018 at 9:34 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: >> >> > On Tue, Jun 12, 2018 at 9:05 AM H.J. Lu <hjl.tools@xxxxxxxxx> wrote: >> >> >> On Tue, Jun 12, 2018 at 9:01 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: >> >> >> > On Tue, Jun 12, 2018 at 4:43 AM H.J. Lu <hjl.tools@xxxxxxxxx> wrote: >> >> >> >> On Tue, Jun 12, 2018 at 3:03 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: >> >> >> >> > That works for stuff which loads all libraries at start time, but what >> >> >> >> > happens if the program uses dlopen() later on? If CET is force locked and >> >> >> >> > the library is not CET enabled, it will fail. >> >> >> >> >> >> >> >> That is to prevent disabling CET by dlopening a legacy shared library. >> >> >> >> >> >> >> >> > I don't see the point of trying to support CET by magic. It adds complexity >> >> >> >> > and you'll never be able to handle all corner cases correctly. dlopen() is >> >> >> >> > not even a corner case. >> >> >> >> >> >> >> >> That is a price we pay for security. To enable CET, especially shadow >> >> >> >> shack, the program and all of shared libraries it uses should be CET >> >> >> >> enabled. Most of programs can be enabled with CET by compiling them >> >> >> >> with -fcf-protection. >> >> >> > >> >> >> > If you charge too high a price for security, people may turn it off. >> >> >> > I think we're going to need a mode where a program says "I want to use >> >> >> > the CET, but turn it off if I dlopen an unsupported library". There >> >> >> > are programs that load binary-only plugins. >> >> >> >> >> >> You can do >> >> >> >> >> >> # export GLIBC_TUNABLES=glibc.tune.hwcaps=-SHSTK >> >> >> >> >> >> which turns off shadow stack. >> >> >> >> >> > >> >> > Which exactly illustrates my point. By making your security story too >> >> > absolute, you'll force people to turn it off when they don't need to. >> >> > If I'm using a fully CET-ified distro and I'm using a CET-aware >> >> > program that loads binary plugins, and I may or may not have an old >> >> > (binary-only, perhaps) plugin that doesn't support CET, then the >> >> > behavior I want is for CET to be on until I dlopen() a program that >> >> > doesn't support it. Unless there's some ABI reason why that can't be >> >> > done, but I don't think there is. >> >> >> >> We can make it opt-in via GLIBC_TUNABLES. But by default, the legacy >> >> shared object is disallowed when CET is enabled. >> > >> > That's a bad idea. Stuff has launchers which people might not be able to >> > change. So they will simply turn of CET completely or it makes them hack >> > horrible crap into init, e.g. the above export. >> > >> > Give them sane kernel options: >> > >> > cet = off, relaxed, forced >> > >> > where relaxed allows to run binary plugins. Then let dlopen() call into the >> > kernel with the filepath of the library to check for CET and that will tell >> > you whether its ok or or not and do the necessary magic in the kernel when >> > CET has to be disabled due to a !CET library/application. >> > >> > That's also making the whole thing independent of magic glibc environment >> > options and allows it to be used all over the place in the same way. >> >> This is very similar to our ARCH_CET_EXEC proposal which controls how >> CET should be enforced. But Andy thinks it is a bad idea. > > I do think it's a bad idea to have a new piece of state that survives > across exec(). It's going to have nasty usability problems and nasty > security problems. > > We may need a mode by which glibc can turn CET *back off* even after a > program had it on if it dlopens() an old binary. Or maybe there won't > be demand. I can certainly understand why the CET_LOCK feature is > there, although I think we need a way to override it using something > like ptrace(). I'm not convinced that CET_LOCK is really needed, but > someone who understand the thread model should chime in. > > Kees, do you know anyone who has a good enough understanding of > usermode exploits and how they'll interact with CET? Adding Florian to CC, but if something gets CET enabled, it really shouldn't have a way to turn it off. If there's a way to turn it off, all the ROP research will suddenly turn to exactly one gadget before doing the rest of the ROP: turning off CET. Right now ROP is: use stack-pivot gadget, do everything else. Allowed CET to turn off will just add one step: use CET-off gadget, use stack-pivot gadget, do everything else. :P Following Linus's request for "slow introduction" of new security features, likely the best approach is to default to "relaxed" (with a warning about down-grades), and allow distros/end-users to pick "forced" if they know their libraries are all CET-enabled. -Kees -- Kees Cook Pixel Security
