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

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Jason Gunthorpe <jgg@ziepe.ca> writes:

> 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?
>

That is correct. I updated the commit message to

These shared buffers are also accessed by the host kernel and therefore
must be aligned to the host’s page size, and have a size that is a
multiple of the host page size.

-aneesh