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

Andy Lutomirski
2020-03-10T01:21:07Z

I am baffled by this discussion.

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