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? --Andy
