linux-kernel - Re: [PATCH] x86/shstk: Enable shadow stack for x32

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Date: Fri, 22 Mar 2024 15:58:19 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "hjl.tools@...il.com" <hjl.tools@...il.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH] x86/shstk: Enable shadow stack for x32

On Fri, 2024-03-22 at 08:06 -0700, H.J. Lu wrote:
> On Fri, Mar 22, 2024 at 7:07 AM Edgecombe, Rick P
> <rick.p.edgecombe@...el.com> wrote:
> > 
> > On Fri, 2024-03-15 at 07:34 -0700, H.J. Lu wrote:
> > > > How many people do you think will use this?
> > 
> > I'm concerned that the only use of this will ever be exercise via
> > the
> > glibc unit tests, but will still require work to support.
> 
>  Correct.  A small glibc change is needed.  Will post it after
> my kernel change is merged.

I mean it will require kernel work in the future to maintain support.
That we will have to think about x32 effects when making other shadow
stack changes.

I'll paste my other comment in this thread:

The main usage of shadow stack is security, and comes with some
overhead. IIUC the main usage of x32 is performance benchmarking type
stuff. Why would someone want to use shadow stack and x32 together?

> 
> 
> > > > 
> > > > I would have thought it would require more changes for basic
> > > > x32
> > > 
> > > This is all needed.
> > > 
> > > > operation. What was the testing exactly?
> > > 
> > > I configured x32 glibc with --enable-cet, build glibc and
> > > run all glibc tests with shadow stack enabled.  There are
> > > no regressions.  I verified that shadow stack is enabled
> > > via /proc/pid/status.
> > 
> > The shadow stack is supposed to be mapped above 4G, so how is this
> > supposed to work for x32?
> 
> This is not what I see:
> 
> (gdb) info reg
> ...
> pl3_ssp        0xf7dcbfe8          0xf7dcbfe8

The mapping above 4G was because Peterz raised the possibility that a
64 bit process could far call into a 32 bit segment and start doing
signal stuff that would encounter undefined behavior. He wanted it
cleanly blocked. So by keeping the shadow stack above 4GB, existing
processes that turned on shadow stack would be preventing from 
transitioning to 32 bit and encountering the missing 32 bit signal
support (because the CPU would #GP during the 32 bit transition if SSP
is above 4GB).

Probably there is some interplay between the x32 mmap logic and shadow
stacks mapping, where it then becomes possible to get below 4GB. Since
x32 needs the shadow stack to be below 4GB, it's incompatible with that
solution. So this patch is not sufficient to enable x32 without side
effects that were previously considered bad.

I see this is in tip now. I don't think it's a good idea to support
upstream. The implications need more discussion, and there doesn't seem
to be any real end user value.