[RFCv2 00/16] KVM protected memory extension

[Kirill A. Shutemov]

== Background / Problem ==

There are a number of hardware features (MKTME, SEV) which protect guest
memory from some unauthorized host access. The patchset proposes a purely
software feature that mitigates some of the same host-side read-only
attacks.


== What does this set mitigate? ==

 - Host kernel ”accidental” access to guest data (think speculation)

 - Host kernel induced access to guest data (write(fd, &guest_data_ptr, len))

 - Host userspace access to guest data (compromised qemu)

 - Guest privilege escalation via compromised QEMU device emulation

== What does this set NOT mitigate? ==

 - Full host kernel compromise.  Kernel will just map the pages again.

 - Hardware attacks


The second RFC revision addresses /most/ of the feedback.

I still didn't found a good solution to reboot and kexec. Unprotect all
the memory on such operations defeat the goal of the feature. Clearing up
most of the memory before unprotecting what is required for reboot (or
kexec) is tedious and error-prone.
Maybe we should just declare them unsupported?

== Series Overview ==

The hardware features protect guest data by encrypting it and then
ensuring that only the right guest can decrypt it.  This has the
side-effect of making the kernel direct map and userspace mapping
(QEMU et al) useless.  But, this teaches us something very useful:
neither the kernel or userspace mappings are really necessary for normal
guest operations.

Instead of using encryption, this series simply unmaps the memory. One
advantage compared to allowing access to ciphertext is that it allows bad
accesses to be caught instead of simply reading garbage.

Protection from physical attacks needs to be provided by some other means.
On Intel platforms, (single-key) Total Memory Encryption (TME) provides
mitigation against physical attacks, such as DIMM interposers sniffing
memory bus traffic.

The patchset modifies both host and guest kernel. The guest OS must enable
the feature via hypercall and mark any memory range that has to be shared
with the host: DMA regions, bounce buffers, etc. SEV does this marking via a
bit in the guest’s page table while this approach uses a hypercall.

For removing the userspace mapping, use a trick similar to what NUMA
balancing does: convert memory that belongs to KVM memory slots to
PROT_NONE: all existing entries converted to PROT_NONE with mprotect() and
the newly faulted in pages get PROT_NONE from the updated vm_page_prot.
The new VMA flag -- VM_KVM_PROTECTED -- indicates that the pages in the
VMA must be treated in a special way in the GUP and fault paths. The flag
allows GUP to return the page even though it is mapped with PROT_NONE, but
only if the new GUP flag -- FOLL_KVM -- is specified. Any userspace access
to the memory would result in SIGBUS. Any GUP access without FOLL_KVM
would result in -EFAULT.

Removing userspace mapping of the guest memory from QEMU process can help
to address some guest privilege escalation attacks. Consider the case when
unprivileged guest user exploits bug in a QEMU device emulation to gain
access to data it cannot normally have access within the guest.

Any anonymous page faulted into the VM_KVM_PROTECTED VMA gets removed from
the direct mapping with kernel_map_pages(). Note that kernel_map_pages() only
flushes local TLB. I think it's a reasonable compromise between security and
perfromance.

Zapping the PTE would bring the page back to the direct mapping after clearing.
At least for now, we don't remove file-backed pages from the direct mapping.
File-backed pages could be accessed via read/write syscalls. It adds
complexity.

Occasionally, host kernel has to access guest memory that was not made
shared by the guest. For instance, it happens for instruction emulation.
Normally, it's done via copy_to/from_user() which would fail with -EFAULT
now. We introduced a new pair of helpers: copy_to/from_guest(). The new
helpers acquire the page via GUP, map it into kernel address space with
kmap_atomic()-style mechanism and only then copy the data.

For some instruction emulation copying is not good enough: cmpxchg
emulation has to have direct access to the guest memory. __kvm_map_gfn()
is modified to accommodate the case.

The patchset is on top of v5.9

Kirill A. Shutemov (16):
  x86/mm: Move force_dma_unencrypted() to common code
  x86/kvm: Introduce KVM memory protection feature
  x86/kvm: Make DMA pages shared
  x86/kvm: Use bounce buffers for KVM memory protection
  x86/kvm: Make VirtIO use DMA API in KVM guest
  x86/kvmclock: Share hvclock memory with the host
  x86/realmode: Share trampoline area if KVM memory protection enabled
  KVM: Use GUP instead of copy_from/to_user() to access guest memory
  KVM: mm: Introduce VM_KVM_PROTECTED
  KVM: x86: Use GUP for page walk instead of __get_user()
  KVM: Protected memory extension
  KVM: x86: Enabled protected memory extension
  KVM: Rework copy_to/from_guest() to avoid direct mapping
  KVM: Handle protected memory in __kvm_map_gfn()/__kvm_unmap_gfn()
  KVM: Unmap protected pages from direct mapping
  mm: Do not use zero page for VM_KVM_PROTECTED VMAs

 arch/powerpc/kvm/book3s_64_mmu_hv.c    |   2 +-
 arch/powerpc/kvm/book3s_64_mmu_radix.c |   2 +-
 arch/s390/include/asm/pgtable.h        |   2 +-
 arch/x86/Kconfig                       |  11 +-
 arch/x86/include/asm/cpufeatures.h     |   1 +
 arch/x86/include/asm/io.h              |   6 +-
 arch/x86/include/asm/kvm_para.h        |   5 +
 arch/x86/include/asm/pgtable_types.h   |   1 +
 arch/x86/include/uapi/asm/kvm_para.h   |   3 +-
 arch/x86/kernel/kvm.c                  |  20 +++
 arch/x86/kernel/kvmclock.c             |   2 +-
 arch/x86/kernel/pci-swiotlb.c          |   3 +-
 arch/x86/kvm/Kconfig                   |   1 +
 arch/x86/kvm/cpuid.c                   |   3 +-
 arch/x86/kvm/mmu/mmu.c                 |   6 +-
 arch/x86/kvm/mmu/paging_tmpl.h         |  10 +-
 arch/x86/kvm/x86.c                     |   9 +
 arch/x86/mm/Makefile                   |   2 +
 arch/x86/mm/ioremap.c                  |  16 +-
 arch/x86/mm/mem_encrypt.c              |  51 ------
 arch/x86/mm/mem_encrypt_common.c       |  62 +++++++
 arch/x86/mm/pat/set_memory.c           |   7 +
 arch/x86/realmode/init.c               |   7 +-
 drivers/virtio/virtio_ring.c           |   4 +
 include/linux/kvm_host.h               |  11 +-
 include/linux/kvm_types.h              |   1 +
 include/linux/mm.h                     |  21 ++-
 include/uapi/linux/kvm_para.h          |   5 +-
 mm/gup.c                               |  20 ++-
 mm/huge_memory.c                       |  31 +++-
 mm/ksm.c                               |   2 +
 mm/memory.c                            |  18 +-
 mm/mmap.c                              |   3 +
 mm/rmap.c                              |   4 +
 virt/kvm/Kconfig                       |   3 +
 virt/kvm/async_pf.c                    |   2 +-
 virt/kvm/kvm_main.c                    | 238 +++++++++++++++++++++----
 virt/lib/Makefile                      |   1 +
 virt/lib/mem_protected.c               | 193 ++++++++++++++++++++
 39 files changed, 666 insertions(+), 123 deletions(-)
 create mode 100644 arch/x86/mm/mem_encrypt_common.c
 create mode 100644 virt/lib/mem_protected.c

-- 
2.26.2