linux-kernel - Re: [PATCH v6 29/41] x86/shstk: Add user-mode shadow stack support

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Message-ID: <06ade29966bbb8bd47cd05b04b5523aa0d4192f0.camel@intel.com>
Date:   Fri, 24 Feb 2023 18:25:57 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH v6 29/41] x86/shstk: Add user-mode shadow stack support

On Fri, 2023-02-24 at 13:22 +0100, Borislav Petkov wrote:
> On Sat, Feb 18, 2023 at 01:14:21PM -0800, Rick Edgecombe wrote:
> > Do not support IA32 emulation or x32.
> 
> Because? Simplicity?
> 
> No one cares about 32-bit?

Yea a little of both. Originally shadow stack 32 bit emulation was
supported for 64 bit kernels, but then Andy Lutomirski asked for the
signal ABI to flexible enough to support alt shadow stacks in the
future. This involved stuffing things on the shadow stack that didn't
work well for 32 bit. Resolving this wasn't exhaustively explored, but
there weren't any obvious things that jumped out.

Then there was the question of how much 32 bit CET apps running on 64
bit kernels would get used. Since shadow stack needs a re-compile this
would only be for newly build 32 bit binaries, not old legacy binaries
that seems to be the thing often using legacy emulation. So it was kind
of not expected to be used much or at all, so any kind of complications
tipped the scales toward dropping it. PeterZ brought up WINE running 32
bit Windows apps, but apparently Windows doesn't support 32 bit CET
either. And then there is that we can always add it later if a big use
shows up.

I'll add a little more info in the commit log about this.