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.
