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

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Message-ID: <8cdbb6928c7144c6b065c3ff68f03aee@AcuMS.aculab.com>
Date:   Wed, 12 May 2021 09:31:10 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Juergen Gross' <jgross@...e.com>, 'Joerg Roedel' <joro@...tes.org>
CC:     "x86@...nel.org" <x86@...nel.org>,
        Hyunwook Baek <baekhw@...gle.com>,
        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

From: Juergen Gross
> Sent: 12 May 2021 09:58
> 
> On 12.05.21 10:50, 'Joerg Roedel' wrote:
> > On Wed, May 12, 2021 at 10:16:12AM +0200, Juergen Gross wrote:
> >> You want something like xen_safe_[read|write]_ulong().
> >
> >  From a first glance I can't see it, what is the difference between the
> > xen_safe_*_ulong() functions and __get_user()/__put_user()? The only
> > difference I can see is that __get/__put_user() support different access
> > sizes, but neither of those disables page-faults by itself, for example.
> >
> > Couldn't these xen-specific functions not also be replaces by
> > __get_user()/__put_user()?
> 
> No, those were used before, but commit 9da3f2b7405440 broke Xen's use
> case. That is why I did commit 1457d8cf7664f.

I've just looked at 9da3f2b7405440.

It doesn't look right to me - wrong return value for a lot of cases.
OTOH it isn't in my newest tree at all.

If bogus_uaccess() is now elsewhere I can't see how (without changes)
it would allow through these faults.

	David

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