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Date: Fri, 09 May 2014 10:14:59 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [RFC] vhost: Can we change synchronize_rcu to
 call_rcu in vhost_set_memory() in vhost kernel module?
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To: "Gonglei (Arei)" <arei.gonglei@huawei.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>
Cc: "Huangweidong (C)" <weidong.huang@huawei.com>, "Herongguang (Stephen)" <herongguang.he@huawei.com>, "mst@redhat.com" <mst@redhat.com>

Il 09/05/2014 03:57, Gonglei (Arei) ha scritto:
> Hi,
>
> Vhost devices need to do VHOST_SET_MEM_TABLE ioctl in vhost_dev_start()
> to tell vhost kernel modules GPA to HVA memory mappings, which consume is expensively.
> The reason is same as KVM_SET_GSI_ROUTING ioctl. That is, in ioctl processing,
> kmod and vhost calls synchronize_rcu() to wait for grace period to free old memory.
>
> In KVM_SET_GSI_ROUTING case, we cannot simply change synchronize_rcu to call_rcu,
> since this may leads to DOS attacks if guest VM keeps setting IRQ affinity.
>
> In VHOST_SET_MEM_TABLE case, I wonder if we can change synchronize_rcu() to call_rcu(),
> i.e., is it possible to trigger DOS attack in guest? There are some cases QEMU would do
> VHOST_SET_MEM_TABLE ioctl, like VM start/reboot/attach vhost devices, and RAM memory
> regions in system memory address space change.
>
> And I'd like to know if guest activities could lead to RAM memory regions change?

Yes, for example enabling/disabling PCI BARs would have that effect.

Paolo