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From: 'Joerg Roedel' <joro@8bytes.org>
To: David Laight <David.Laight@aculab.com>
Cc: "x86@kernel.org" <x86@kernel.org>,
	Hyunwook Baek <baekhw@google.com>, Joerg Roedel <jroedel@suse.de>,
	"stable@vger.kernel.org" <stable@vger.kernel.org>,
	"hpa@zytor.com" <hpa@zytor.com>,
	Andy Lutomirski <luto@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Jiri Slaby <jslaby@suse.cz>,
	Dan Williams <dan.j.williams@intel.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Juergen Gross <jgross@suse.com>,
	Kees Cook <keescook@chromium.org>,
	David Rientjes <rientjes@google.com>,
	Cfir Cohen <cfir@google.com>, Erdem Aktas <erdemaktas@google.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mike Stunes <mstunes@vmware.com>,
	Sean Christopherson <seanjc@google.com>,
	Martin Radev <martin.b.radev@gmail.com>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	"linux-coco@lists.linux.dev" <linux-coco@lists.linux.dev>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"virtualization@lists.linux-foundation.org"
	<virtualization@lists.linux-foundation.org>
Subject: Re: [PATCH 3/6] x86/sev-es: Use __put_user()/__get_user
Date: Wed, 12 May 2021 10:37:34 +0200	[thread overview]
Message-ID: <YJuTzhSp2XAJIYlv@8bytes.org> (raw)
In-Reply-To: <0496626f018d4d27a8034a4822170222@AcuMS.aculab.com>

On Wed, May 12, 2021 at 08:04:33AM +0000, David Laight wrote:
> 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.

Yes, in general these functions can sleep, but not in this context. They
are called in atomic context and the page-fault handler will notice that
and goes down the __bad_area_nosemaphore() path and only do the fixup.

I also thought about adding page_fault_disable()/page_fault_enable()
calls, but being in atomic context is enough according to the
faulthandler_disabled() implementation.

This is exactly what is needed here. All I want to know is whether a
fault happened or not, the page-fault handler must not try to fix the
fault in any way. If a fault happens it is later fixed up in
vc_forward_exception().

> At best this is abused the calls.

Yes, but that is only due to the naming of these functions. In this case
they do exactly what is needed.

Regards,

	Joerg

WARNING: multiple messages have this Message-ID (diff)
From: 'Joerg Roedel' <joro@8bytes.org>
To: David Laight <David.Laight@aculab.com>
Cc: "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"virtualization@lists.linux-foundation.org"
	<virtualization@lists.linux-foundation.org>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	"hpa@zytor.com" <hpa@zytor.com>, Jiri Slaby <jslaby@suse.cz>,
	"x86@kernel.org" <x86@kernel.org>,
	David Rientjes <rientjes@google.com>,
	Martin Radev <martin.b.radev@gmail.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Joerg Roedel <jroedel@suse.de>, Kees Cook <keescook@chromium.org>,
	Cfir Cohen <cfir@google.com>, Hyunwook Baek <baekhw@google.com>,
	"linux-coco@lists.linux.dev" <linux-coco@lists.linux.dev>,
	Andy Lutomirski <luto@kernel.org>,
	Dan Williams <dan.j.williams@intel.com>,
	Juergen Gross <jgross@suse.com>, Mike Stunes <mstunes@vmware.com>,
	Sean Christopherson <seanjc@google.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"stable@vger.kernel.org" <stable@vger.kernel.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Erdem Aktas <erdemaktas@google.com>
Subject: Re: [PATCH 3/6] x86/sev-es: Use __put_user()/__get_user
Date: Wed, 12 May 2021 10:37:34 +0200	[thread overview]
Message-ID: <YJuTzhSp2XAJIYlv@8bytes.org> (raw)
In-Reply-To: <0496626f018d4d27a8034a4822170222@AcuMS.aculab.com>

On Wed, May 12, 2021 at 08:04:33AM +0000, David Laight wrote:
> 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.

Yes, in general these functions can sleep, but not in this context. They
are called in atomic context and the page-fault handler will notice that
and goes down the __bad_area_nosemaphore() path and only do the fixup.

I also thought about adding page_fault_disable()/page_fault_enable()
calls, but being in atomic context is enough according to the
faulthandler_disabled() implementation.

This is exactly what is needed here. All I want to know is whether a
fault happened or not, the page-fault handler must not try to fix the
fault in any way. If a fault happens it is later fixed up in
vc_forward_exception().

> At best this is abused the calls.

Yes, but that is only due to the naming of these functions. In this case
they do exactly what is needed.

Regards,

	Joerg
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

  parent reply	other threads:[~2021-05-12  8:37 UTC|newest]

Thread overview: 51+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-12  7:54 [PATCH 0/6] x86/sev-es: Fixes for SEV-ES guest support Joerg Roedel
2021-05-12  7:54 ` Joerg Roedel
2021-05-12  7:54 ` [PATCH 1/6] x86/sev-es: Don't return NULL from sev_es_get_ghcb() Joerg Roedel
2021-05-12  7:54   ` Joerg Roedel
2021-05-12  7:54 ` [PATCH 2/6] x86/sev-es: Forward page-faults which happen during emulation Joerg Roedel
2021-05-12  7:54   ` Joerg Roedel
2021-05-12 17:31   ` Sean Christopherson
2021-05-19 13:16     ` Joerg Roedel
2021-05-19 13:16       ` Joerg Roedel
2021-05-12  7:54 ` [PATCH 3/6] x86/sev-es: Use __put_user()/__get_user Joerg Roedel
2021-05-12  7:54   ` Joerg Roedel
2021-05-12  8:04   ` David Laight
2021-05-12  8:04     ` David Laight
2021-05-12  8:04     ` David Laight
2021-05-12  8:16     ` Juergen Gross
2021-05-12  8:16       ` Juergen Gross via Virtualization
2021-05-12  8:16       ` Juergen Gross
2021-05-12  8:50       ` 'Joerg Roedel'
2021-05-12  8:50         ` 'Joerg Roedel'
2021-05-12  8:50         ` 'Joerg Roedel'
2021-05-12  8:58         ` Juergen Gross
2021-05-12  8:58           ` Juergen Gross via Virtualization
2021-05-12  8:58           ` Juergen Gross
2021-05-12  9:31           ` David Laight
2021-05-12  9:31             ` David Laight
2021-05-12  9:31             ` David Laight
2021-05-12  9:32           ` Joerg Roedel
2021-05-12  9:32             ` Joerg Roedel
2021-05-12  9:32             ` Joerg Roedel
2021-05-19 11:33             ` 'Joerg Roedel'
2021-05-19 11:33               ` 'Joerg Roedel'
2021-05-19 11:33               ` 'Joerg Roedel'
2021-05-12  8:37     ` 'Joerg Roedel' [this message]
2021-05-12  8:37       ` 'Joerg Roedel'
2021-05-12  8:37       ` 'Joerg Roedel'
2021-05-12 15:59       ` Dave Hansen
2021-05-12 15:59         ` Dave Hansen
2021-05-12 15:59         ` Dave Hansen
2021-05-12 15:57   ` Dave Hansen
2021-05-12 15:57     ` Dave Hansen
2021-05-12 16:00     ` Joerg Roedel
2021-05-12 16:00       ` Joerg Roedel
2021-05-12  7:54 ` [PATCH 4/6] Revert "x86/sev-es: Handle string port IO to kernel memory properly" Joerg Roedel
2021-05-12  7:54   ` Joerg Roedel
2021-05-12 17:38   ` Sean Christopherson
2021-05-19 12:22     ` Joerg Roedel
2021-05-19 12:22       ` Joerg Roedel
2021-05-12  7:54 ` [PATCH 5/6] x86/sev-es: Fix error message in runtime #VC handler Joerg Roedel
2021-05-12  7:54   ` Joerg Roedel
2021-05-12  7:54 ` [PATCH 6/6] x86/sev-es: Leave NMI-mode before sending signals Joerg Roedel
2021-05-12  7:54   ` Joerg Roedel

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