[PATCH v3 04/13] X86/KVM: Handle PFNs outside of kernel reach when touching GPTEs

KarimAllah Ahmed
2018-10-20T22:22:16Z

From: Filippo Sironi <sironi@amazon.de>

cmpxchg_gpte() calls get_user_pages_fast() to retrieve the number of
pages and the respective struct page to map 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 back 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 <sironi@amazon.de>
Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
---
arch/x86/kvm/paging_tmpl.h | 38 +++++++++++++++++++++++++++++---------
1 file changed, 29 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 14ffd97..8f7bc8f 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -141,15 +141,35 @@ 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