Re: [PATCH v11 9/9] x86: Disallow vsyscall emulation when CET is enabled

[Florian Weimer]

* Andy Lutomirski:

> On Mon, Aug 24, 2020 at 5:30 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
>>
>> From: "H.J. Lu" <hjl.tools@xxxxxxxxx>
>>
>> Emulation of the legacy vsyscall page is required by some programs built
>> before 2013.  Newer programs after 2013 don't use it.  Disallow vsyscall
>> emulation when Control-flow Enforcement (CET) is enabled to enhance
>> security.
>
> NAK.
>
> By all means disable execute emulation if CET-IBT is enabled at the
> time emulation is attempted, and maybe even disable the vsyscall page
> entirely if you can magically tell that CET-IBT will be enabled when a
> process starts, but you don't get to just disable it outright on a
> CET-enabled kernel.

Yeah, we definitely would have to revert/avoid this downstream.  People
definitely want to run glibc-2.12-era workloads on current kernels.
Thanks for catching it.

Florian