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

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Message-ID: <fcb2c501-70ca-1a54-4a75-8ab05c21ee30@suse.com>
Date:   Wed, 12 May 2021 10:16:12 +0200
From:   Juergen Gross <jgross@...e.com>
To:     David Laight <David.Laight@...LAB.COM>,
        'Joerg Roedel' <joro@...tes.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Hyunwook Baek <baekhw@...gle.com>
Cc:     Joerg Roedel <jroedel@...e.de>,
        "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 12.05.21 10:04, David Laight wrote:
> From: Joerg
>> Sent: 12 May 2021 08:55
>>
>> From: Joerg Roedel <jroedel@...e.de>
>>
>> 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.
> 
> That can't be right at all.
> __put/get_user() are only valid on user addresses and will try to
> fault in a missing page - so can sleep.
> 
> At best this is abused the calls.

You want something like xen_safe_[read|write]_ulong().


Juergen

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