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Lu" Date: Mon, 9 Mar 2020 17:08:29 -0700 Message-ID: Subject: Re: [RFC PATCH v9 01/27] Documentation/x86: Add CET description To: Andy Lutomirski Cc: Dave Hansen , Yu-cheng Yu , "the arch/x86 maintainers" , "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , LKML , "open list:DOCUMENTATION" , Linux-MM , linux-arch , Linux API , Arnd Bergmann , Andy Lutomirski , Balbir Singh , Borislav Petkov , Cyrill Gorcunov , Dave Hansen , Eugene Syromiatnikov , Florian Weimer , Jann Horn , Jonathan Corbet , Kees Cook , Mike Kravetz , Nadav Amit , Oleg Nesterov , Pavel Machek , Peter Zijlstra , Randy Dunlap , "Ravi V. Shankar" , Vedvyas Shanbhogue , Dave Martin , x86-patch-review@intel.com Content-Type: text/plain; charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Mar 9, 2020 at 4:59 PM Andy Lutomirski wrote: > > On Mon, Mar 9, 2020 at 4:52 PM H.J. Lu wrote: > > > > On Mon, Mar 9, 2020 at 4:21 PM Dave Hansen 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.