[PATCH] x86, kvm: Handle PFNs outside of kernel reach when touching GPTEs

[Filippo Sironi]

cmpxchg_gpte() calls get_user_pages_fast() to retrieve the number of
pages and the respective struct pages for mapping in the kernel virtual
address space.
This doesn't work if get_user_pages_fast() is invoked with a userspace
virtual address that's backed by PFNs outside of kernel reach (e.g.,
when limiting the kernel memory with mem= in the command line and using
/dev/mem to map memory).

If get_user_pages_fast() fails, look up the VMA that backs the userspace
virtual address, compute the PFN and the physical address, and map it in
the kernel virtual address space with memremap().

Signed-off-by: Filippo Sironi <sir...@amazon.de>
Cc: Anthony Liguori <aligu...@amazon.com>
Cc: k...@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
 arch/x86/kvm/paging_tmpl.h | 39 ++++++++++++++++++++++++++++++---------
 1 file changed, 30 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index a01105485315..ab4d6617238c 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -147,15 +147,36 @@ static int FNAME(cmpxchg_gpte)(struct kvm_vcpu *vcpu, 
struct kvm_mmu *mmu,
        struct page *page;
 
        npages = get_user_pages_fast((unsigned long)ptep_user, 1, 1, &page);
-       /* Check if the user is doing something meaningless. */
-       if (unlikely(npages != 1))
-               return -EFAULT;
-
-       table = kmap_atomic(page);
-       ret = CMPXCHG(&table[index], orig_pte, new_pte);
-       kunmap_atomic(table);
-
-       kvm_release_page_dirty(page);
+       if (likely(npages == 1)) {
+               table = kmap_atomic(page);
+               ret = CMPXCHG(&table[index], orig_pte, new_pte);
+               kunmap_atomic(table);
+
+               kvm_release_page_dirty(page);
+       } else {
+               struct vm_area_struct *vma;
+               unsigned long vaddr = (unsigned long)ptep_user & PAGE_MASK;
+               unsigned long pfn;
+               unsigned long paddr;
+
+               down_read(&current->mm->mmap_sem);
+               vma = find_vma_intersection(current->mm, vaddr,
+                                           vaddr + PAGE_SIZE);
+               if (!vma || !(vma->vm_flags & VM_PFNMAP)) {
+                       up_read(&current->mm->mmap_sem);
+                       return -EFAULT;
+               }
+               pfn = ((vaddr - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
+               paddr = pfn << PAGE_SHIFT;
+               table = memremap(paddr, PAGE_SIZE, MEMREMAP_WB);
+               if (!table) {
+                       up_read(&current->mm->mmap_sem);
+                       return -EFAULT;
+               }
+               ret = CMPXCHG(&table[index], orig_pte, new_pte);
+               memunmap(table);
+               up_read(&current->mm->mmap_sem);
+       }
 
        return (ret != orig_pte);
 }
-- 
2.7.4