Re: [RFC PATCH v9 01/27] Documentation/x86: Add CET description

H.J. Lu
2020-03-10T00:09:11Z

On Mon, Mar 9, 2020 at 4:59 PM Andy Lutomirski <luto@amacapital.net> wrote:
>
> On Mon, Mar 9, 2020 at 4:52 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > On Mon, Mar 9, 2020 at 4:21 PM Dave Hansen <dave.hansen@intel.com> wrote:
> > >
> > > On 3/9/20 4:11 PM, H.J. Lu wrote:
> > > > A threaded application is loaded from disk. The object file on disk is
> > > > either CET enabled or not CET enabled.
> > >
> > > Huh. Are you saying that all instructions executed on userspace on
> > > Linux come off of object files on the disk? That's an interesting
> > > assertion. You might want to go take a look at the processes on your
> > > systems. Here's my browser for example:
> > >
> > > # for p in $(ps aux | grep chromium | awk '{print $2}' ); do cat
> > > /proc/$p/maps; done | grep ' r-xp 00000000 00:00 0'
> > > ...
> > > 202f00082000-202f000bf000 r-xp 00000000 00:00 0
> > > 202f000c2000-202f000c3000 r-xp 00000000 00:00 0
> > > 202f00102000-202f00103000 r-xp 00000000 00:00 0
> > > 202f00142000-202f00143000 r-xp 00000000 00:00 0
> > > 202f00182000-202f001bf000 r-xp 00000000 00:00 0
> > >
> > > Lots of funny looking memory areas which are anonymous and executable!
> > > Those didn't come off the disk. Same thing in firefox. Weird. Any
> > > idea what those are?
> > >
> > > One guess: https://en.wikipedia.org/wiki/Just-in-time_compilation
> >
> > jitted code belongs to a process loaded from disk. Enable CET in
> > an application which uses JIT engine means to also enable CET in
> > JIT engine. Take git as an example, "git grep" crashed for me on Tiger
> > Lake. It turned out that git itself was compiled with -fcf-protection and
> > git was linked against libpcre2-8.so.0 also compiled with -fcf-protection,
> > which has a JIT, sljit, which was not CET enabled. git crashed in the
> > jitted codes due to missing ENDBR. I had to enable CET in sljit to make
> > git working on CET enabled Tiger Lake. So we need to enable CET in
> > JIT engine before enabling CET in applications which use JIT engine.
>
> This could presumably have been fixed by having libpcre or sljit
> disable IBT before calling into JIT code or by running the JIT code in
> another thread. In the other direction, a non-CET libpcre build could
> build IBT-capable JITted code and enable JIT (by syscall if we allow
> that or by creating a thread?) when calling it. And IBT has this

This is not how thread in user space works.

> fancy legacy bitmap to allow non-instrumented code to run with IBT on,
> although SHSTK doesn't have hardware support for a similar feature.

All these changes are called CET enabing.

> So, sure, the glibc-linked ELF ecosystem needs some degree of CET
> coordination, but it is absolutely not the case that a process MUST
> have all CET or no CET. Let's please support the complicated cases in
> the kernel and the ABI too. If glibc wants to make it annoying to do
> complicated things, so be it. People work behind glibc's back all the
> time.

CET is no different from NX in this regard.

--
H.J.