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: >> >> > On Thu, 7 Jun 2018, H.J. Lu wrote: >> >> >> On Thu, Jun 7, 2018 at 2:01 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: >> >> >> > Why is the lockout necessary? If user code enables CET and tries to >> >> >> > run code that doesn't support CET, it will crash. I don't see why we >> >> >> > need special code in the kernel to prevent a user program from calling >> >> >> > arch_prctl() and crashing itself. There are already plenty of ways to >> >> >> > do that :) >> >> >> >> >> >> On CET enabled machine, not all programs nor shared libraries are >> >> >> CET enabled. But since ld.so is CET enabled, all programs start >> >> >> as CET enabled. ld.so will disable CET if a program or any of its shared >> >> >> libraries aren't CET enabled. ld.so will lock up CET once it is done CET >> >> >> checking so that CET can't no longer be disabled afterwards. >> >> > >> >> > 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. > I'm concerned that the entire concept of locking CET is there to solve > a security problem that doesn't actually exist. We don't know that. -- H.J.
