linux-kernel - Re: [PATCH v9 1/2] x86/kexec: VMCLEAR VMCSs loaded on all cpus if necessary

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Date:	Tue, 04 Dec 2012 12:14:15 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Zhang Yanfei <zhangyanfei@...fujitsu.com>
Cc:	"x86\@kernel.org" <x86@...nel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	"kexec\@lists.infradead.org" <kexec@...ts.infradead.org>,
	"kvm\@vger.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 1/2] x86/kexec: VMCLEAR VMCSs loaded on all cpus if necessary

Zhang Yanfei <zhangyanfei@...fujitsu.com> writes:

> This patch provides a way to VMCLEAR VMCSs related to guests
> on all cpus before executing the VMXOFF when doing kdump. This
> is used to ensure the VMCSs in the vmcore updated and
> non-corrupted.

Apologies for the delay I have been travelling, and I wanted
to at least read through the code.

Overall I think this is good but I have one nit, and I see one real
problem with this code.

> +/*
> + * This is used to VMCLEAR all VMCSs loaded on the
> + * processor. And when loading kvm_intel module, the
> + * callback function pointer will be assigned.
> + */
> +void (*crash_vmclear_loaded_vmcss)(void) = NULL;
> +EXPORT_SYMBOL_GPL(crash_vmclear_loaded_vmcss);
> +
> +static inline void cpu_emergency_vmclear_loaded_vmcss(void)
> +{
> +	if (crash_vmclear_loaded_vmcss)
> +		crash_vmclear_loaded_vmcss();
> +}

The nit is the use of emergency instead of crash in the name.

The problem is that this is potentially a NULL pointer dereference if
kvm-intel is removed.  The easist fix would be in your second patch to
just make it impossible to unload the kvm-intel module.  Otherwise
there the deference of crash_vmclear_loaded_vmcss needs to be rcu
protected, with a syncrhonize_rcu after the pointer is set to NULL in
the unload path.

Otherwise I have no objections to this code.

Eric
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