Re: [RFC 09/16] KVM: Protected memory extension

["Huang, Kai" <kai.huang@intel.com>]

On Mon, 2020-05-25 at 18:34 +0300, Kirill A. Shutemov wrote:
> On Mon, May 25, 2020 at 05:26:37PM +0200, Vitaly Kuznetsov wrote:
> > "Kirill A. Shutemov" <kirill@shutemov.name> writes:
> > 
> > > Add infrastructure that handles protected memory extension.
> > > 
> > > Arch-specific code has to provide hypercalls and define non-zero
> > > VM_KVM_PROTECTED.
> > > 
> > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > > ---
> > >  include/linux/kvm_host.h |   4 ++
> > >  mm/mprotect.c            |   1 +
> > >  virt/kvm/kvm_main.c      | 131 +++++++++++++++++++++++++++++++++++++++
> > >  3 files changed, 136 insertions(+)
> > > 
> > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > > index bd0bb600f610..d7072f6d6aa0 100644
> > > --- a/include/linux/kvm_host.h
> > > +++ b/include/linux/kvm_host.h
> > > @@ -700,6 +700,10 @@ void kvm_arch_flush_shadow_all(struct kvm *kvm);
> > >  void kvm_arch_flush_shadow_memslot(struct kvm *kvm,
> > >  				   struct kvm_memory_slot *slot);
> > >  
> > > +int kvm_protect_all_memory(struct kvm *kvm);
> > > +int kvm_protect_memory(struct kvm *kvm,
> > > +		       unsigned long gfn, unsigned long npages, bool protect);
> > > +
> > >  int gfn_to_page_many_atomic(struct kvm_memory_slot *slot, gfn_t gfn,
> > >  			    struct page **pages, int nr_pages);
> > >  
> > > diff --git a/mm/mprotect.c b/mm/mprotect.c
> > > index 494192ca954b..552be3b4c80a 100644
> > > --- a/mm/mprotect.c
> > > +++ b/mm/mprotect.c
> > > @@ -505,6 +505,7 @@ mprotect_fixup(struct vm_area_struct *vma, struct
> > > vm_area_struct **pprev,
> > >  	vm_unacct_memory(charged);
> > >  	return error;
> > >  }
> > > +EXPORT_SYMBOL_GPL(mprotect_fixup);
> > >  
> > >  /*
> > >   * pkey==-1 when doing a legacy mprotect()
> > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > > index 530af95efdf3..07d45da5d2aa 100644
> > > --- a/virt/kvm/kvm_main.c
> > > +++ b/virt/kvm/kvm_main.c
> > > @@ -155,6 +155,8 @@ static void kvm_uevent_notify_change(unsigned int
> > > type, struct kvm *kvm);
> > >  static unsigned long long kvm_createvm_count;
> > >  static unsigned long long kvm_active_vms;
> > >  
> > > +static int protect_memory(unsigned long start, unsigned long end, bool
> > > protect);
> > > +
> > >  __weak int kvm_arch_mmu_notifier_invalidate_range(struct kvm *kvm,
> > >  		unsigned long start, unsigned long end, bool blockable)
> > >  {
> > > @@ -1309,6 +1311,14 @@ int __kvm_set_memory_region(struct kvm *kvm,
> > >  	if (r)
> > >  		goto out_bitmap;
> > >  
> > > +	if (mem->memory_size && kvm->mem_protected) {
> > > +		r = protect_memory(new.userspace_addr,
> > > +				   new.userspace_addr + new.npages * PAGE_SIZE,
> > > +				   true);
> > > +		if (r)
> > > +			goto out_bitmap;
> > > +	}
> > > +
> > >  	if (old.dirty_bitmap && !new.dirty_bitmap)
> > >  		kvm_destroy_dirty_bitmap(&old);
> > >  	return 0;
> > > @@ -2652,6 +2662,127 @@ void kvm_vcpu_mark_page_dirty(struct kvm_vcpu
> > > *vcpu, gfn_t gfn)
> > >  }
> > >  EXPORT_SYMBOL_GPL(kvm_vcpu_mark_page_dirty);
> > >  
> > > +static int protect_memory(unsigned long start, unsigned long end, bool
> > > protect)
> > > +{
> > > +	struct mm_struct *mm = current->mm;
> > > +	struct vm_area_struct *vma, *prev;
> > > +	int ret;
> > > +
> > > +	if (down_write_killable(&mm->mmap_sem))
> > > +		return -EINTR;
> > > +
> > > +	ret = -ENOMEM;
> > > +	vma = find_vma(current->mm, start);
> > > +	if (!vma)
> > > +		goto out;
> > > +
> > > +	ret = -EINVAL;
> > > +	if (vma->vm_start > start)
> > > +		goto out;
> > > +
> > > +	if (start > vma->vm_start)
> > > +		prev = vma;
> > > +	else
> > > +		prev = vma->vm_prev;
> > > +
> > > +	ret = 0;
> > > +	while (true) {
> > > +		unsigned long newflags, tmp;
> > > +
> > > +		tmp = vma->vm_end;
> > > +		if (tmp > end)
> > > +			tmp = end;
> > > +
> > > +		newflags = vma->vm_flags;
> > > +		if (protect)
> > > +			newflags |= VM_KVM_PROTECTED;
> > > +		else
> > > +			newflags &= ~VM_KVM_PROTECTED;
> > > +
> > > +		/* The VMA has been handled as part of other memslot */
> > > +		if (newflags == vma->vm_flags)
> > > +			goto next;
> > > +
> > > +		ret = mprotect_fixup(vma, &prev, start, tmp, newflags);
> > > +		if (ret)
> > > +			goto out;
> > > +
> > > +next:
> > > +		start = tmp;
> > > +		if (start < prev->vm_end)
> > > +			start = prev->vm_end;
> > > +
> > > +		if (start >= end)
> > > +			goto out;
> > > +
> > > +		vma = prev->vm_next;
> > > +		if (!vma || vma->vm_start != start) {
> > > +			ret = -ENOMEM;
> > > +			goto out;
> > > +		}
> > > +	}
> > > +out:
> > > +	up_write(&mm->mmap_sem);
> > > +	return ret;
> > > +}
> > > +
> > > +int kvm_protect_memory(struct kvm *kvm,
> > > +		       unsigned long gfn, unsigned long npages, bool protect)
> > > +{
> > > +	struct kvm_memory_slot *memslot;
> > > +	unsigned long start, end;
> > > +	gfn_t numpages;
> > > +
> > > +	if (!VM_KVM_PROTECTED)
> > > +		return -KVM_ENOSYS;
> > > +
> > > +	if (!npages)
> > > +		return 0;
> > > +
> > > +	memslot = gfn_to_memslot(kvm, gfn);
> > > +	/* Not backed by memory. It's okay. */
> > > +	if (!memslot)
> > > +		return 0;
> > > +
> > > +	start = gfn_to_hva_many(memslot, gfn, &numpages);
> > > +	end = start + npages * PAGE_SIZE;
> > > +
> > > +	/* XXX: Share range across memory slots? */
> > > +	if (WARN_ON(numpages < npages))
> > > +		return -EINVAL;
> > > +
> > > +	return protect_memory(start, end, protect);
> > > +}
> > > +EXPORT_SYMBOL_GPL(kvm_protect_memory);
> > > +
> > > +int kvm_protect_all_memory(struct kvm *kvm)
> > > +{
> > > +	struct kvm_memslots *slots;
> > > +	struct kvm_memory_slot *memslot;
> > > +	unsigned long start, end;
> > > +	int i, ret = 0;;
> > > +
> > > +	if (!VM_KVM_PROTECTED)
> > > +		return -KVM_ENOSYS;
> > > +
> > > +	mutex_lock(&kvm->slots_lock);
> > > +	kvm->mem_protected = true;
> > 
> > What will happen upon guest reboot? Do we need to unprotect everything
> > to make sure we'll be able to boot? Also, after the reboot how will the
> > guest know that it is protected and needs to unprotect things? -> see my
> > idea about converting KVM_HC_ENABLE_MEM_PROTECTED to a stateful MSR (but
> > we'll likely have to reset it upon reboot anyway).
> 
> That's extremely good question. I have not considered reboot. I tend to use
> -no-reboot in my setup.
> 
> I'll think how to deal with reboot. I don't know how it works now to give
> a good answer.
> 
> The may not be a good solution: unprotecting memory on reboot means we
> expose user data. We can wipe the data before unprotecting, but we should
> not wipe BIOS and anything else that is required on reboot. I donno.

If you let Qemu to protect guest memory when creating the vm, but not ask guest
kernel to enable when it boots, you won't have this problem. And guest kernel
*queries* whether its memory is protected or not during boot. This is consistent
to SEV as well.