linux-kernel - Re: [PATCH v3 00/12] Enable Linear Address Space Separation support

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Date:   Tue, 1 Aug 2023 20:38:20 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "Lutomirski, Andy" <luto@...nel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Mehta, Sohil" <sohil.mehta@...el.com>
Subject: Re: [PATCH v3 00/12] Enable Linear Address Space Separation support

On Tue, 2023-08-01 at 12:50 -0700, Sohil Mehta wrote:
> On 7/31/2023 3:36 PM, Edgecombe, Rick P wrote:
> > CET introduces this unusual instruction called WRUSS. It allows you
> > to
> > make user memory accesses while executing in the kernel. Because of
> > this special property, the CET shadow stack patches don't toggle
> > stac/clac while executing this instruction. So I think LASS will
> > need
> > it to behave more like a normal userspace access from the kernel.
> > Shadow stack is not upstream yet, so just something to keep in mind
> > for
> > the future.
> > 
> 
> This is a good point. We should definitely test this out to confirm.
> 
> But, isn't WRUSS already defined as a user-mode access? So, in
> theory, a
> user-mode access to a user address space *should* not be blocked by
> LASS
> (even with CPL=0).
> 
> Are you suggesting that we might need to do something special for
> WRUSS
> with LASS enabled?

I was, but reading the docs, I think you are right. It looks like it
will be treated like a user access as far as LASS is concerned. Thanks.