linux-kernel - Re: [PATCH 3/6] x86/sev-es: Use __put_user()/__get

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Message-ID: <YKT3mpYlPqNHo1QU@8bytes.org>
Date:   Wed, 19 May 2021 13:33:46 +0200
From:   'Joerg Roedel' <joro@...tes.org>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     Juergen Gross <jgross@...e.com>,
        David Laight <David.Laight@...lab.com>,
        "x86@...nel.org" <x86@...nel.org>,
        Hyunwook Baek <baekhw@...gle.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "hpa@...or.com" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Kees Cook <keescook@...omium.org>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Martin Radev <martin.b.radev@...il.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH 3/6] x86/sev-es: Use __put_user()/__get_user

On Wed, May 12, 2021 at 11:32:35AM +0200, Joerg Roedel wrote:
> On Wed, May 12, 2021 at 10:58:20AM +0200, Juergen Gross wrote:
> > No, those were used before, but commit 9da3f2b7405440 broke Xen's use
> > case. That is why I did commit 1457d8cf7664f.
>
> [...]
>
> Having the distinction between user and kernel memory accesses
> explicitly in the code seems to be the most robust solution.

On the other hand, as I found out today, 9da3f2b7405440 had a short
life-time and got reverted upstream. So using __get_user()/__put_user()
should be fine in this code path. It just deserves a comment explaining
its use here and why pagefault_enable()/disable() is not needed.
Even the get_kernel* helpers use __get_user_size() internally.

Regards,

	Joerg