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

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Date:   Mon, 9 Mar 2020 18:21:01 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     "H.J. Lu" <hjl.tools@...il.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>, x86-patch-review@...el.com
Subject: Re: [RFC PATCH v9 01/27] Documentation/x86: Add CET description

I am baffled by this discussion.

>> On Mar 9, 2020, at 5:09 PM, H.J. Lu <hjl.tools@...il.com> wrote:
>> 
>> On Mon, Mar 9, 2020 at 4:59 PM Andy Lutomirski <luto@...capital.net> wrote:
> 
>>>> .
>> 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.

void create_cet_thread(void (*func)(), unsigned int cet_flags);

I could implement this using clone() if the kernel provides the requisite support. Sure, creating threads behind libc’s back like this is perilous, but it can be done.

> 
>> 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.

What does that mean?  If program A loads library B, and library B very carefully loads CET-mismatched code, program A may be blissfully unaware.

> 
>> 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.

NX is in the page tables, and CET, mostly, is not.  Also, we seriously flubbed READ_IMPLIES_EXEC and made it affect far more mappings than ever should have been affected.

If a legacy program (non-NX-aware) loads a newer library, and the library opens a device node and mmaps it PROT_READ, it gets RX.  This is not a good design. In fact, it’s actively problematic.

Let us please not take Linux’s NX legacy support as an example of good design.