linux-kernel - Re: [PATCH v6 28/41] x86: Introduce userspace API for shadow stack

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Message-ID: <ef5086cd2480ac1a8d0d752f8a019148c3d712b4.camel@intel.com>
Date:   Tue, 28 Feb 2023 22:35:38 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH v6 28/41] x86: Introduce userspace API for shadow stack

On Tue, 2023-02-28 at 11:58 +0100, Borislav Petkov wrote:
> On Fri, Feb 24, 2023 at 06:37:57PM +0000, Edgecombe, Rick P wrote:
> > In the first patch:
> > 
> > 
https://lore.kernel.org/lkml/20230218211433.26859-2-rick.p.edgecombe@intel.com/
> > 
> > Then some more documentation is added about ARCH_SHSTK_UNLOCK and
> > ARCH_SHSTK_STATUS (which are for CRIU) in those patches.
> 
> Right, I was thinking more about ARCH_PRCTL(2), the man page.
> 
> But you can send that to the manpages folks later. I.e., it should be
> nearly impossible to be missed. :)

Sure I can add something. Somewhere I have a man page for
map_shadow_stack written up as well.

> 
> > There are glibc patches prepared by HJ to use the new interface and
> > it's my understanding that he has discussed the changes with the
> > other
> > glibc folks. Those glibc patches are used for testing these kernel
> > patches, but will not get upstream until the kernel patches to
> > avoid
> > repeating the past problems. So I think it's as prepared as it can
> > be.
> 
> Good.
> 
> > One future thing that might come up... Glibc has this mode called
> > "permissive mode". When glibc is configured this way (compile time
> > or
> > env var), it is supposed to disable shadow stack when dlopen()ing
> > any
> > DSO that doesn't have the shadow stack elf header bit.
> 
> Maybe I don't understand all the possible use cases but if I were
> interested in using shadow stack, then I'd enable it for all objects.

Enabling for all objects is the ideal, but in practice distros don't
have that.

> And if I want permissive, I'd disable it for all. A mixed thing
> sounds
> like a mixed can of worms waiting to be opened.

It is definitely a can of worms.