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

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Message-ID: <24f0902d-3375-b0de-e985-6174c70b394c@intel.com>
Date:   Tue, 1 Aug 2023 12:50:56 -0700
From:   Sohil Mehta <sohil.mehta@...el.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Luck, Tony" <tony.luck@...el.com>
Subject: Re: [PATCH v3 00/12] Enable Linear Address Space Separation support

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?

Sohil