Re: [PATCH v8 17/46] KVM: guest_memfd: Advertise KVM_SET_MEMORY_ATTRIBUTES2 ioctl

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Missing compat_ioctl handler for guest_memfd file operations causes KVM_SET_MEMORY_ATTRIBUTES2 to fail with -ENOTTY for 32-bit userspace.
--

commit d35a6816bc0c5b6e6650f6f4fcbbef3d9b3e3590
Author: Ackerley Tng <ackerleytng@google.com>

KVM: guest_memfd: Advertise KVM_SET_MEMORY_ATTRIBUTES2 ioctl

Introduce KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES to advertise the
availability of the KVM_SET_MEMORY_ATTRIBUTES2 ioctl. This ioctl allows
userspace to manage memory attributes, such as KVM_MEMORY_ATTRIBUTE_PRIVATE,
directly on a guest_memfd file descriptor. It also adds an error_offset
field to handle EAGAIN results during transient page reference counts.

> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 876c0429f9d4e..129d6f6303251 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
[ ... ]
> @@ -1649,6 +1650,7 @@ struct kvm_memory_attributes {
>  	__u64 flags;
>  };
>  
> +/* Available with KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES */
>  #define KVM_SET_MEMORY_ATTRIBUTES2              _IOWR(KVMIO,  0xd2, struct kvm_memory_attributes2)

[Severity: Medium]
Since this is a new ioctl on the guest_memfd file descriptor, will 32-bit
userspace VMMs running on 64-bit kernels fail with -ENOTTY?

Looking at virt/kvm/guest_memfd.c, kvm_gmem_fops lacks a .compat_ioctl
handler. Because KVM_SET_MEMORY_ATTRIBUTES2 is not in the generic compatible
ioctl list, the VFS layer falls back to returning -ENOTTY instead of calling
.unlocked_ioctl for 32-bit processes.

Since struct kvm_memory_attributes2 is fully 64-bit aligned, it requires
no translation. Should kvm_gmem_fops wire up .compat_ioctl (e.g., to
compat_ptr_ioctl) so 32-bit VMMs can successfully use this capability?

>  
>  struct kvm_memory_attributes2 {

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260618-gmem-inplace-conversion-v8-0-9d2959357853@google.com?part=17