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

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Date:   Wed, 12 May 2021 08:57:53 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Joerg Roedel <joro@...tes.org>, x86@...nel.org,
        Hyunwook Baek <baekhw@...gle.com>
Cc:     Joerg Roedel <jroedel@...e.de>, stable@...r.kernel.org,
        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>,
        Juergen Gross <jgross@...e.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-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 3/6] x86/sev-es: Use __put_user()/__get_user

On 5/12/21 12:54 AM, Joerg Roedel wrote:
> The put_user() and get_user() functions do checks on the address which is
> passed to them. They check whether the address is actually a user-space
> address and whether its fine to access it. They also call might_fault()
> to indicate that they could fault and possibly sleep.
> 
> All of these checks are neither wanted nor required in the #VC exception
> handler, which can be invoked from almost any context and also for MMIO
> instructions from kernel space on kernel memory. All the #VC handler
> wants to know is whether a fault happened when the access was tried.
> 
> This is provided by __put_user()/__get_user(), which just do the access
> no matter what.

The changelog _helps_, but using a "user" function to handle kernel MMIO
for its error handling properties seems like it's begging for a comment.

__put_user() also seems to have fun stuff like __chk_user_ptr().  It all
seems sketchy to me.