Re: [PATCH v2 0/7] KVM: arm64: Forward FFA_NOTIFICATION* calls to TrustZone

[Vincent Donnefort <vdonnefort@google.com>]

On Mon, Jun 08, 2026 at 04:55:42PM +0000, Sebastian Ene wrote:
> Remove the FFA_NOTIFICATION* calls from the blocklist used by the pKVM
> FF-A proxy. This restriction was preventing the use of asynchronous
> signaling mechanisms defined by the Arm FF-A specification to
> communicate with the secure services.
> While these calls are markes as optional, there is no reason why the
> hypervisor proxy would block them because:
> 
> 1. Host is the Sole Non-Secure Endpoint: The Host operates as the
>    only Non-Secure VM ID (VM ID 0) recognized by the Secure World.
>    Because all forwarded notifications are inherently attributed to
>    the Host by the SPMC, there is no risk of VM ID spoofing
>    originating from the Normal World.
> 
> 2. No Memory Pointers or Addresses: The FFA_NOTIFICATION_* ABIs
>    operate strictly via register-based parameters, passing only
>    VM IDs, VCPU IDs, flags, and bitmaps. Because these calls do
>    not contain memory addresses, offsets, or pointers, forwarding
>    them doesn't pose a risk of memory-based confused deputy attack
>    (e.g., tricking the SPMC into overwriting protected memory).
> 
> While the pKVM proxy behaves as a relayer, it doesn't currently have its
> own FF-A ID(only the host has the ID 0). The behavior of the setup
> flow is covered by the spec in the: '10.9 Notification support without
> a Hypervisor'.

As it is only a relayer. Is it really important to check SBZ arguments and
fields on behalf of Trustzone? It doesn't feel it brings any security. If the
host passes broken arguments, I don't believe this puts pKVM at risk. Does it? 

> 
> ---
> Changes in v2:
> - enforce the MBZ/SBZ fields
> - split the calls into separate patches
> - rebase on 7.1-rc7
> 
> Link to v1:
> https://lore.kernel.org/all/20260501114447.2389222-2-sebastianene@google.com/
> 
> Sebastian Ene (7):
>   KVM: arm64: Support FFA_NOTIFICATION_BITMAP_CREATE in host handler
>   KVM: arm64: Support FFA_NOTIFICATION_BITMAP_DESTROY in host handler
>   KVM: arm64: Support FFA_NOTIFICATION_BIND in host handler
>   KVM: arm64: Support FFA_NOTIFICATION_UNBIND in host handler
>   KVM: arm64: Support FFA_NOTIFICATION_SET in host handler
>   KVM: arm64: Support FFA_NOTIFICATION_GET in host handler
>   KVM: arm64: Support FFA_NOTIFICATION_INFO_GET in host handler
> 
>  arch/arm64/kvm/hyp/nvhe/ffa.c | 190 ++++++++++++++++++++++++++++++++--
>  1 file changed, 182 insertions(+), 8 deletions(-)
> 
> -- 
> 2.54.0.1064.gd145956f57-goog
>