Re: [Qemu-devel] [RFC] vhost: Can we change synchronize_rcu to call_rcu in vhost_set_memory() in vhost kernel module?

[Paolo Bonzini <pbonzini@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