Re: [PATCH v9 3/6] KVM: arm64: Block cacheable PFNMAP mapping

Will Deacon
2025-06-27T14:36:30Z

On Sat, Jun 21, 2025 at 04:21:08AM +0000, ankita@nvidia.com wrote:
> From: Ankit Agrawal <ankita@nvidia.com>
>
> Fixes a security bug due to mismatched attributes between S1 and
> S2 mapping.
>
> Currently, it is possible for a region to be cacheable in the userspace
> VMA, but mapped non cached in S2. This creates a potential issue where
> the VMM may sanitize cacheable memory across VMs using cacheable stores,
> ensuring it is zeroed. However, if KVM subsequently assigns this memory
> to a VM as uncached, the VM could end up accessing stale, non-zeroed data
> from a previous VM, leading to unintended data exposure. This is a security
> risk.
>
> Block such mismatch attributes case by returning EINVAL when userspace
> try to map PFNMAP cacheable. Only allow NORMAL_NC and DEVICE_*.
>
> CC: Oliver Upton <oliver.upton@linux.dev>
> CC: Catalin Marinas <catalin.marinas@arm.com>
> CC: Sean Christopherson <seanjc@google.com>
> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Ankit Agrawal <ankita@nvidia.com>
> ---
> arch/arm64/kvm/mmu.c | 34 +++++++++++++++++++++++++++++++++-
> 1 file changed, 33 insertions(+), 1 deletion(-)

Sorry for the drive-by comment, but I was looking at this old series from
Paolo (look at the cover letter and patch 5):

https://lore.kernel.org/r/20250109133817.314401-1-pbonzini@redhat.com

in which he points out that the arm64 get_vma_page_shift() function
incorrectly assumes that a VM_PFNMAP VMA is physically contiguous, which
may not be the case if a driver calls remap_pfn_range() to mess around
with mappings within the VMA. I think that implies that the optimisation
in 2aa53d68cee6 ("KVM: arm64: Try stage2 block mapping for host device
MMIO") is unsound.

But it got me thinking -- given that remap_pfn_range() also takes a 'prot'
argument, how do we ensure that this is reflected in the guest? It feels
a bit dodgy to rely on drivers always passing 'vma->vm_page_prot'.

Will