linux-kernel - Re: [PATCH 25/35] x86/cet/shstk: Add user-mode shadow stack support

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Message-ID: <5a2778096ca847273964fe174b0fbfd818a966cb.camel@intel.com>
Date:   Sat, 12 Feb 2022 00:12:09 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
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CC:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Subject: Re: [PATCH 25/35] x86/cet/shstk: Add user-mode shadow stack support

On Fri, 2022-02-11 at 15:37 -0800, Dave Hansen wrote:
> On 1/30/22 13:18, Rick Edgecombe wrote:
> > Add the user shadow stack MSRs to the xsave helpers, so they can be
> > used
> > to implement the functionality.
> 
> Do these MSRs ever affect kernel-mode operation?
> 
> If so, we might need to switch them more aggressively at context-
> switch
> time like PKRU.
> 
> If not, they can continue to be context-switched with the PASID state
> which does not affect kernel-mode operation.
> 
> Either way, it would be nice to have some changelog material to that
> effect.

The only special shadow stack thing the kernel does is WRUSS, which per
the SDM only needs the CR4 bit set to work (unlike WRSS). So I think
the lazy restore is ok.