Re: [PATCH v2 0/4] Enforce host page-size alignment for shared buffers

[Jason Gunthorpe <jgg@ziepe.ca>]

On Sun, Dec 21, 2025 at 09:39:16PM +0530, Aneesh Kumar K.V (Arm) wrote:
> Hi all,
> 
> This patch series addresses alignment requirements for buffers shared between
> private-memory guests and the host.
> 
> When running private-memory guests, the guest kernel must apply additional
> constraints when allocating buffers that are shared with the hypervisor. These
> shared buffers are also accessed by the host kernel and therefore must be
> aligned to the host’s page size.
> 
> Architectures such as Arm can tolerate realm physical address space PFNs being
> mapped as shared memory, as incorrect accesses are detected and reported as GPC
> faults. However, relying on this mechanism alone is unsafe and can still lead to
> kernel crashes.
> 
> This is particularly likely when guest_memfd allocations are mmapped and
> accessed from userspace. Once exposed to userspace, it is not possible to
> guarantee that applications will only access the intended 4K shared region
> rather than the full 64K page mapped into their address space. Such userspace
> addresses may also be passed back into the kernel and accessed via the linear
> map, potentially resulting in a GPC fault and a kernel crash.
> 
> To address this, the series introduces a new helper, `mem_encrypt_align()`,
> which allows callers to enforce the required alignment for shared buffers.

This explanation makes sense, but to maybe bottom line the requirement
to something very simple..

 In ARM64 the guest shared/private granule size must be >= the
 hypervisor PAGE_SIZE, which may be larger than the VM's natural
 PAGE_SIZE.

Meaning we have to go through an change all the places doing
shared/private stuff to work on a shared/private granual size. I think
this is not just alignment, but allocation size as well?

Jason