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

[Yu-cheng Yu]

On Mon, 2020-09-21 at 09:22 -0700, Yu, Yu-cheng wrote:
> On 9/18/2020 5:11 PM, Andy Lutomirski wrote:
> > On Fri, Sep 18, 2020 at 12:23 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> 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.
> > > 
> > > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
[...]
> > 
> > Nope, try again.  Having IBT on does *not* mean that every library in
> > the process knows that we have indirect branch tracking.  The legacy
> > bitmap exists for a reason.  Also, I want a way to flag programs as
> > not using the vsyscall page, but that flag should not be called CET.
> > And a process with vsyscalls off should not be able to read the
> > vsyscall page, and /proc/self/maps should be correct.
> > 
> > So you have some choices:
> > 
> > 1. Drop this patch and make it work.
> > 
> > 2. Add a real per-process vsyscall control.  Either make it depend on
> > vsyscall=xonly and wire it up correctly or actually make it work
> > correctly with vsyscall=emulate.
> > 
> > NAK to any hacks in this space.  Do it right or don't do it at all.
> > 
> 
> We can drop this patch, and bring back the previous patch that fixes up 
> shadow stack and ibt.  That makes vsyscall emulation work correctly, and 
> does not force the application to do anything different from what is 
> working now.  I will post the previous patch as a reply to this thread 
> so that people can make comments on it.
> 
> Yu-cheng

Here is the patch:

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