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

Zhang Yanfei [off-list ref] 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.

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

于 2012年12月05日 04:14, Eric W. Biederman 写道:

Zhang Yanfei [off-list ref] writes:

quoted

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.

quoted

+/*
+ * 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.

ok, emergency -> crash

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.

Ah, thanks for this comment.

I think I will use the rcu machanism to solve the problem.

Otherwise I have no objections to this code.

Thanks for your review. I will update the patch and resend it.

Thanks
Zhang Yanfei