Re: [PATCH v6 0/8] KVM: mm: fd-based approach for supporting KVM guest private memory

[Vishal Annapurve <vannapurve@google.com>]

>
> Private memory map/unmap and conversion
> ---------------------------------------
> Userspace's map/unmap operations are done by fallocate() ioctl on the
> backing store fd.
>   - map: default fallocate() with mode=0.
>   - unmap: fallocate() with FALLOC_FL_PUNCH_HOLE.
> The map/unmap will trigger above memfile_notifier_ops to let KVM map/unmap
> secondary MMU page tables.
>
....
>    QEMU: https://github.com/chao-p/qemu/tree/privmem-v6
>
> An example QEMU command line for TDX test:
> -object tdx-guest,id=tdx \
> -object memory-backend-memfd-private,id=ram1,size=2G \
> -machine q35,kvm-type=tdx,pic=no,kernel_irqchip=split,memory-encryption=tdx,memory-backend=ram1
>

There should be more discussion around double allocation scenarios
when using the private fd approach. A malicious guest or buggy
userspace VMM can cause physical memory getting allocated for both
shared (memory accessible from host) and private fds backing the guest
memory.
Userspace VMM will need to unback the shared guest memory while
handling the conversion from shared to private in order to prevent
double allocation even with malicious guests or bugs in userspace VMM.

Options to unback shared guest memory seem to be:
1) madvise(.., MADV_DONTNEED/MADV_REMOVE) - This option won't stop
kernel from backing the shared memory on subsequent write accesses
2) fallocate(..., FALLOC_FL_PUNCH_HOLE...) - For file backed shared
guest memory, this option still is similar to madvice since this would
still allow shared memory to get backed on write accesses
3) munmap - This would give away the contiguous virtual memory region
reservation with holes in the guest backing memory, which might make
guest memory management difficult.
4) mprotect(... PROT_NONE) - This would keep the virtual memory
address range backing the guest memory preserved

ram_block_discard_range_fd from reference implementation:
https://github.com/chao-p/qemu/tree/privmem-v6 seems to be relying on
fallocate/madvise.

Any thoughts/suggestions around better ways to unback the shared
memory in order to avoid double allocation scenarios?

Regards,
Vishal