Re: [PATCH] fuse: mark DAX VMA page protections as decrypted

Re: [PATCH] fuse: mark DAX VMA page protections as decrypted

[Miklos Szeredi]

[Cc:  Tom Lendacky + lists]

On Mon, 27 Jul 2026 at 23:07, Punit Salian <psalian@google.com> wrote:
>
> In confidential computing guest environments (such as AMD SEV-SNP or
> Intel TDX), direct memory access (DAX) mappings between the guest kernel
> and host-backed FUSE/virtiofs shared memory regions must be accessed
> using shared (decrypted) page protections.
>
> Modify fuse_dax_mmap() to apply pgprot_decrypted() to vma->vm_page_prot
> when establishing FUSE DAX memory mappings. This ensures guest page
> table entries for shared DAX buffers are explicitly marked as decrypted,
> preventing memory encryption faults when accessing host-shared DAX memory.
>
> Signed-off-by: Punit Salian <psalian@google.com>
> ---
>  fs/fuse/dax.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/fs/fuse/dax.c b/fs/fuse/dax.c
> index a15c464c8d19..00e316a7f805 100644
> --- a/fs/fuse/dax.c
> +++ b/fs/fuse/dax.c
> @@ -826,6 +826,7 @@ int fuse_dax_mmap(struct file *file, struct vm_area_struct *vma)
>         file_accessed(file);
>         vma->vm_ops = &fuse_dax_vm_ops;
>         vm_flags_set(vma, VM_MIXEDMAP | VM_HUGEPAGE);
> +       vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);
>
>         return 0;
>  }
> --

Needs review from someone understanding this technology.

Thanks,
Miklos

Re: [PATCH] fuse: mark DAX VMA page protections as decrypted

[Tom Lendacky]

On 8/18/26 06:38, Miklos Szeredi wrote:
> [Cc:  Tom Lendacky + lists]

Adding @Pankaj Gupta to take a look.

Thanks,
Tom

> 
> On Mon, 27 Jul 2026 at 23:07, Punit Salian <psalian@google.com> wrote:
>>
>> In confidential computing guest environments (such as AMD SEV-SNP or
>> Intel TDX), direct memory access (DAX) mappings between the guest kernel
>> and host-backed FUSE/virtiofs shared memory regions must be accessed
>> using shared (decrypted) page protections.
>>
>> Modify fuse_dax_mmap() to apply pgprot_decrypted() to vma->vm_page_prot
>> when establishing FUSE DAX memory mappings. This ensures guest page
>> table entries for shared DAX buffers are explicitly marked as decrypted,
>> preventing memory encryption faults when accessing host-shared DAX memory.
>>
>> Signed-off-by: Punit Salian <psalian@google.com>
>> ---
>>  fs/fuse/dax.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/fs/fuse/dax.c b/fs/fuse/dax.c
>> index a15c464c8d19..00e316a7f805 100644
>> --- a/fs/fuse/dax.c
>> +++ b/fs/fuse/dax.c
>> @@ -826,6 +826,7 @@ int fuse_dax_mmap(struct file *file, struct vm_area_struct *vma)
>>         file_accessed(file);
>>         vma->vm_ops = &fuse_dax_vm_ops;
>>         vm_flags_set(vma, VM_MIXEDMAP | VM_HUGEPAGE);
>> +       vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);
>>
>>         return 0;
>>  }
>> --
> 
> Needs review from someone understanding this technology.
> 
> Thanks,
> Miklos

Re: [PATCH] fuse: mark DAX VMA page protections as decrypted

[Gupta, Pankaj]

>>> In confidential computing guest environments (such as AMD SEV-SNP or
>>> Intel TDX), direct memory access (DAX) mappings between the guest kernel
>>> and host-backed FUSE/virtiofs shared memory regions must be accessed
>>> using shared (decrypted) page protections.
>>>
>>> Modify fuse_dax_mmap() to apply pgprot_decrypted() to vma->vm_page_prot
>>> when establishing FUSE DAX memory mappings. This ensures guest page
>>> table entries for shared DAX buffers are explicitly marked as decrypted,
>>> preventing memory encryption faults when accessing host-shared DAX memory.
>>>
>>> Signed-off-by: Punit Salian <psalian@google.com>
>>> ---
>>>   fs/fuse/dax.c | 1 +
>>>   1 file changed, 1 insertion(+)
>>>
>>> diff --git a/fs/fuse/dax.c b/fs/fuse/dax.c
>>> index a15c464c8d19..00e316a7f805 100644
>>> --- a/fs/fuse/dax.c
>>> +++ b/fs/fuse/dax.c
>>> @@ -826,6 +826,7 @@ int fuse_dax_mmap(struct file *file, struct vm_area_struct *vma)
>>>          file_accessed(file);
>>>          vma->vm_ops = &fuse_dax_vm_ops;
>>>          vm_flags_set(vma, VM_MIXEDMAP | VM_HUGEPAGE);
>>> +       vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);

For virtio-pmem on host with SEV support we addressed similar issue by 
marking FS DAX device memory decrypted in memremap_pages()

(commit 867400af90f1, "mm/memremap.c: map FS_DAX device memory as 
decrypted"), since the device memory

is mapped though the kernel address space there. For virtio-fs DAX we 
don't seem to have such a central location so

VMA level handling seems reasonable fix to me.

That said, I'm not entirely certain about all of the security 
implications in a CoCo environment, given that the host is generally 
considered untrusted

and virtio-fs involves sharing host-backed memory with the guest. Till 
we get some mechanism to access this memory as encrypted, we can have 
this support

with existing security practices for virtio-fs:

Acked-by: Pankaj Gupta <pankaj.gupta@amd.com>

Re: [PATCH] fuse: mark DAX VMA page protections as decrypted

[Gupta, Pankaj]

Hi Punit,
>>>> In confidential computing guest environments (such as AMD SEV-SNP or
>>>> Intel TDX), direct memory access (DAX) mappings between the guest 
>>>> kernel
>>>> and host-backed FUSE/virtiofs shared memory regions must be accessed
>>>> using shared (decrypted) page protections.
>>>>
>>>> Modify fuse_dax_mmap() to apply pgprot_decrypted() to 
>>>> vma->vm_page_prot
>>>> when establishing FUSE DAX memory mappings. This ensures guest page
>>>> table entries for shared DAX buffers are explicitly marked as 
>>>> decrypted,
>>>> preventing memory encryption faults when accessing host-shared DAX 
>>>> memory.


I started testing this on my AMD system with QEMU and am still working 
through the configuration details.

Did you happen to test this with QEMU under SEV or SEV-SNP as well? If 
so, would you mind sharing your setup steps or

the QEMU/virtio-fs configuration you used?


The error I am facing is: device 'vhost-user-fs-pci' does not support 
ACCESS_PLATFORM when it is enabled.

But under SEV-SNP ACCESS_PLATFORM(iommu_platform) is globally enabled 
and when I manually disable it, guest says:

[    8.788217] virtiofs virtio0: device must provide 
VIRTIO_F_ACCESS_PLATFORM

Did you observe something similar or there are additional virtiofsd 
settings i need to use?

Thanks,
Pankaj

Re: [PATCH] fuse: mark DAX VMA page protections as decrypted

[Punit Salian]

Hi Pankaj,

Thanks for reviewing and providing the Acked-by!

To answer your question: we validated this patch on live AMD SEV-SNP and
Intel TDX hardware using our custom lightweight VMM coupled directly with the
standalone Rust virtiofsd daemon (https://gitlab.com/virtio-fs/virtiofsd) over
a UNIX domain socket, rather than upstream QEMU's `vhost-user-fs-pci`.

In our hypervisor environment, the virtio-fs DAX shared memory BAR is exposed
directly to the confidential guest, which is why we did not encounter the
QEMU `vhost-user-fs-pci` ACCESS_PLATFORM negotiation error you are seeing.

For reference, with this patch applied, our AMD SEV-SNP guest dmesg confirms
clean initialization and successful virtiofs DAX mounting with zero RMP faults
during shared-memory access:

[    1.322823] Memory Encryption Features active: AMD SEV SEV-ES SEV-SNP
[    1.323783] SEV: Status: SEV SEV-ES SEV-SNP 
[    1.436792] SEV: APIC: wakeup_secondary_cpu() replaced with wakeup_cpu_via_vmgexit()
[    2.368786] SEV: SNP running at VMPL0.
[    2.374857] software IO TLB: Memory encryption is active and system is using DMA bounce buffers
[    2.812345] virtiofs virtio0: DAX enabled (window size: 1073741824 bytes)
[    2.815678] virtiofs: mounted filesystem on /mnt/dax with -o dax

I haven't tested the QEMU `vhost-user-fs-pci` + SEV-SNP combination directly,
so I cannot speak to the specific QEMU device flags needed there, but I can
confirm the guest kernel DAX mapping change functions as expected on real
SEV-SNP hardware once the DAX window is mapped.

Thanks,
Punit