Re: [PATCH v12 8/8] x86: Disallow vsyscall emulation when CET is enabled

[H.J. Lu]

On Fri, Sep 18, 2020 at 2:17 PM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>
> On 9/18/20 2:06 PM, H.J. Lu wrote:
> > On Fri, Sep 18, 2020 at 2:00 PM Pavel Machek <pavel@xxxxxx> wrote:
> >> On Fri 2020-09-18 12:32:57, Dave Hansen wrote:
> >>> On 9/18/20 12:23 PM, Yu-cheng Yu wrote:
> >>>> Emulation of the legacy vsyscall page is required by some programs
> >>>> built before 2013.  Newer programs after 2013 don't use it.
> >>>> Disable vsyscall emulation when Control-flow Enforcement (CET) is
> >>>> enabled to enhance security.
> >>> How does this "enhance security"?
> >>>
> >>> What is the connection between vsyscall emulation and CET?
> >> Boom.
> >>
> >> We don't break compatibility by default, and you should not tell
> >> people to enable CET by default if you plan to do this.
> >>
> > Nothing will be broken.   CET enabled applications don't use/need
> > vsyscall emulation.
>
> Hi H.J.,
>
> Could you explain your logic a bit more thoroughly, please?

Here is my CET slides for LPC 2020:

https://gitlab.com/cet-software/cet-smoke-test/-/wikis/uploads/09431a51248858e6f716a59065d732e2/CET-LPC-2020.pdf

which may have answers for most questions.

> I also suspect that Pavel was confused by your changelog where you said
> that you do this when "CET is enabled".  Does enabled in this context mean:
> 1. Just CET support compiled in, or
> 2. Compiled in and on CET hardware, or
> 3. Compiled in to the kernel enabled in the app and running on CET
>    hardware?

CET is enabled only in a process if

1. All components are CET enabled, and
2. CPU supports CET, and
3. Kernel supports CET.

-- 
H.J.