Re: [RFC 16/16] KVM: Unmap protected pages from direct mapping

[Kirill A. Shutemov]

On Tue, May 26, 2020 at 09:16:38AM +0300, Mike Rapoport wrote:
> On Fri, May 22, 2020 at 03:52:14PM +0300, Kirill A. Shutemov wrote:
> > If the protected memory feature enabled, unmap guest memory from
> > kernel's direct mappings.
> > 
> > Migration and KSM is disabled for protected memory as it would require a
> > special treatment.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/mm/pat/set_memory.c |  1 +
> >  include/linux/kvm_host.h     |  3 ++
> >  mm/huge_memory.c             |  9 +++++
> >  mm/ksm.c                     |  3 ++
> >  mm/memory.c                  | 13 +++++++
> >  mm/rmap.c                    |  4 ++
> >  virt/kvm/kvm_main.c          | 74 ++++++++++++++++++++++++++++++++++++
> >  7 files changed, 107 insertions(+)
> > 
> > diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> > index 6f075766bb94..13988413af40 100644
> > --- a/arch/x86/mm/pat/set_memory.c
> > +++ b/arch/x86/mm/pat/set_memory.c
> > @@ -2227,6 +2227,7 @@ void __kernel_map_pages(struct page *page, int numpages, int enable)
> >  
> >  	arch_flush_lazy_mmu_mode();
> >  }
> > +EXPORT_SYMBOL_GPL(__kernel_map_pages);
> >  
> >  #ifdef CONFIG_HIBERNATION
> >  bool kernel_page_present(struct page *page)
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index b6944f88033d..e1d7762b615c 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -705,6 +705,9 @@ int kvm_protect_all_memory(struct kvm *kvm);
> >  int kvm_protect_memory(struct kvm *kvm,
> >  		       unsigned long gfn, unsigned long npages, bool protect);
> >  
> > +void kvm_map_page(struct page *page, int nr_pages);
> > +void kvm_unmap_page(struct page *page, int nr_pages);
> > +
> >  int gfn_to_page_many_atomic(struct kvm_memory_slot *slot, gfn_t gfn,
> >  			    struct page **pages, int nr_pages);
> >  
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index c3562648a4ef..d8a444a401cc 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -33,6 +33,7 @@
> >  #include <linux/oom.h>
> >  #include <linux/numa.h>
> >  #include <linux/page_owner.h>
> > +#include <linux/kvm_host.h>
> 
> This does not seem right... 

I agree. I try to find a more clean way to deal with it.

> >  #include <asm/tlb.h>
> >  #include <asm/pgalloc.h>
> > @@ -650,6 +651,10 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
> >  		spin_unlock(vmf->ptl);
> >  		count_vm_event(THP_FAULT_ALLOC);
> >  		count_memcg_events(memcg, THP_FAULT_ALLOC, 1);
> > +
> > +		/* Unmap page from direct mapping */
> > +		if (vma_is_kvm_protected(vma))
> > +			kvm_unmap_page(page, HPAGE_PMD_NR);
> 
> ... and neither does this.
> 
> I think the map/unmap primitives shoud be a part of the generic mm and
> not burried inside KVM.

Well, yes. Except, kvm_map_page() also clears the page before bringing it
back to direct mappings. Not sure yet how to deal with it.

> >  	return 0;
> > @@ -1886,6 +1891,10 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
> >  			page_remove_rmap(page, true);
> >  			VM_BUG_ON_PAGE(page_mapcount(page) < 0, page);
> >  			VM_BUG_ON_PAGE(!PageHead(page), page);
> > +
> > +			/* Map the page back to the direct mapping */
> > +			if (vma_is_kvm_protected(vma))
> > +				kvm_map_page(page, HPAGE_PMD_NR);
> >  		} else if (thp_migration_supported()) {
> >  			swp_entry_t entry;
> >  
> > diff --git a/mm/ksm.c b/mm/ksm.c
> > index 281c00129a2e..942b88782ac2 100644
> > --- a/mm/ksm.c
> > +++ b/mm/ksm.c
> > @@ -527,6 +527,9 @@ static struct vm_area_struct *find_mergeable_vma(struct mm_struct *mm,
> >  		return NULL;
> >  	if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
> >  		return NULL;
> > +	/* TODO */
> 
> Probably this is not something that should be done. For a security
> sensitive environment that wants protected memory, KSM woudn't be
> relevant anyway...

Hm. True.

> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 71aac117357f..defc33d3a124 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -51,6 +51,7 @@
> >  #include <linux/io.h>
> >  #include <linux/lockdep.h>
> >  #include <linux/kthread.h>
> > +#include <linux/pagewalk.h>
> >  
> >  #include <asm/processor.h>
> >  #include <asm/ioctl.h>
> > @@ -2718,6 +2719,72 @@ void kvm_vcpu_mark_page_dirty(struct kvm_vcpu *vcpu, gfn_t gfn)
> >  }
> >  EXPORT_SYMBOL_GPL(kvm_vcpu_mark_page_dirty);
> >  
> > +void kvm_map_page(struct page *page, int nr_pages)
> > +{
> > +	int i;
> > +
> > +	/* Clear page before returning it to the direct mapping */
> > +	for (i = 0; i < nr_pages; i++) {
> > +		void *p = map_page_atomic(page + i);
> > +		memset(p, 0, PAGE_SIZE);
> > +		unmap_page_atomic(p);
> > +	}
> > +
> > +	kernel_map_pages(page, nr_pages, 1);
> > +}
> > +EXPORT_SYMBOL_GPL(kvm_map_page);
> > +
> > +void kvm_unmap_page(struct page *page, int nr_pages)
> > +{
> > +	kernel_map_pages(page, nr_pages, 0);
> > +}
> > +EXPORT_SYMBOL_GPL(kvm_unmap_page);
> > +
> > +static int adjust_direct_mapping_pte_range(pmd_t *pmd, unsigned long addr,
> > +					   unsigned long end,
> > +					   struct mm_walk *walk)
> > +{
> > +	bool protect = (bool)walk->private;
> > +	pte_t *pte;
> > +	struct page *page;
> > +
> > +	if (pmd_trans_huge(*pmd)) {
> > +		page = pmd_page(*pmd);
> > +		if (is_huge_zero_page(page))
> > +			return 0;
> > +		VM_BUG_ON_PAGE(total_mapcount(page) != 1, page);
> > +		/* XXX: Would it fail with direct device assignment? */
> > +		VM_BUG_ON_PAGE(page_count(page) != 1, page);
> > +		kernel_map_pages(page, HPAGE_PMD_NR, !protect);
> > +		return 0;
> > +	}
> > +
> > +	pte = pte_offset_map(pmd, addr);
> > +	for (; addr != end; pte++, addr += PAGE_SIZE) {
> > +		pte_t entry = *pte;
> > +
> > +		if (!pte_present(entry))
> > +			continue;
> > +
> > +		if (is_zero_pfn(pte_pfn(entry)))
> > +			continue;
> > +
> > +		page = pte_page(entry);
> > +
> > +		VM_BUG_ON_PAGE(page_mapcount(page) != 1, page);
> > +		/* XXX: Would it fail with direct device assignment? */
> > +		VM_BUG_ON_PAGE(page_count(page) !=
> > +			       total_mapcount(compound_head(page)), page);
> > +		kernel_map_pages(page, 1, !protect);
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > +static const struct mm_walk_ops adjust_direct_mapping_ops = {
> > +	.pmd_entry	= adjust_direct_mapping_pte_range,
> > +};
> > +
> 
> All this seem to me an addition to set_memory APIs rather then KVM.

Emm?.. I don't think walking userspace mapping is set_memory thing.
And kernel_map_pages() is VMM interface already.

-- 
 Kirill A. Shutemov