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    SubjectRe: [PATCH 06/10] x86/cet: Add arch_prctl functions for shadow stack
    On Tue, Jun 12, 2018 at 11:59 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
    > On Tue, 12 Jun 2018, H.J. Lu wrote:
    >> On Tue, Jun 12, 2018 at 9:34 AM, Andy Lutomirski <luto@kernel.org> wrote:
    >> > On Tue, Jun 12, 2018 at 9:05 AM H.J. Lu <hjl.tools@gmail.com> wrote:
    >> >> On Tue, Jun 12, 2018 at 9:01 AM, Andy Lutomirski <luto@kernel.org> wrote:
    >> >> > On Tue, Jun 12, 2018 at 4:43 AM H.J. Lu <hjl.tools@gmail.com> wrote:
    >> >> >> On Tue, Jun 12, 2018 at 3:03 AM, Thomas Gleixner <tglx@linutronix.de> 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.


    --
    H.J.

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