Re: [PATCH 0/2] KVM: arm64: Fix host/hyp tracking on share/unshare hypercall failure

[Vincent Donnefort <vdonnefort@google.com>]

On Fri, May 29, 2026 at 08:43:39AM +0100, tabba@google.com wrote:
> Hi folks,
> 
> Yet another bug I found while testing Sashiko locally with fixes to
> review-prompts.
> 
> share_pfn_hyp() and unshare_pfn_hyp() in arch/arm64/kvm/mmu.c
> maintain a host-side RB-tree mirroring the set of pages shared with
> EL2. Both invoke a hypercall that can fail (page-state mismatch,
> EL2 refcount still held), but neither cleans up on failure:
> 
> - share_pfn_hyp() inserts the tracking node before the hypercall
>   and leaves it in the tree on failure, leaking the allocation and
>   presenting a phantom share to a later unshare.
> 
> - unshare_pfn_hyp() erases the tracking node before the hypercall;
>   on failure the host loses its record while EL2 still owns the
>   share, breaking later operations on the same pfn.
> 
> Severity is low (no isolation impact) and the failure paths are rare
> in practice, but the desync is real. Both patches are independent and
> apply cleanly to current mainline. In other words, this can wait for
> 7.2.


I believe I fixed that here lore.kernel.org/all/acyKhZL2di_QQ9xm@google.com but
as Quentin pointed-out, there's absolutely no reason for the hypercall to fail.
So I haven't sent a v2.

> 
> Cheers,
> /fuad
> 
> Fuad Tabba (2):
>   KVM: arm64: Free hyp-share tracking node when share hypercall fails
>   KVM: arm64: Avoid host/hyp share desync on unshare hypercall failure
> 
>  arch/arm64/kvm/mmu.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> -- 
> 2.54.0.929.g9b7fa37559-goog
>