[PATCH] unpaged: anon in VM_UNPAGED

copy_one_pte needs to copy the anonymous COWed pages in a VM_UNPAGED area,
zap_pte_range needs to free them, do_wp_page needs to COW them: just like
ordinary pages, not like the unpaged.

But recognizing them is a little subtle: because PageReserved is no longer a
condition for remap_pfn_range, we can now mmap all of /dev/mem (whether the
distro permits, and whether it's advisable on this or that architecture, is
another matter).  So if we can see a PageAnon, it may not be ours to mess with
(or may be ours from elsewhere in the address space).  I suspect there's an
entertaining insoluble self-referential problem here, but the page_is_anon
function does a good practical job, and MAP_PRIVATE PROT_WRITE VM_UNPAGED will
always be an odd choice.

In updating the comment on page_address_in_vma, noticed a potential NULL
dereference, in a path we don't actually take, but fixed it.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 46 insertions, 24 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 107b619cfb16..3666a4c6dd22 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -350,6 +350,22 @@ void print_bad_pte(struct vm_area_struct *vma, pte_t pte, unsigned long vaddr)
 }
 
 /*
+ * page_is_anon applies strict checks for an anonymous page belonging to
+ * this vma at this address.  It is used on VM_UNPAGED vmas, which are
+ * usually populated with shared originals (which must not be counted),
+ * but occasionally contain private COWed copies (when !VM_SHARED, or
+ * perhaps via ptrace when VM_SHARED).  An mmap of /dev/mem might window
+ * free pages, pages from other processes, or from other parts of this:
+ * it's tricky, but try not to be deceived by foreign anonymous pages.
+ */
+static inline int page_is_anon(struct page *page,
+                        struct vm_area_struct *vma, unsigned long addr)
+{
+        return page && PageAnon(page) && page_mapped(page) &&
+                page_address_in_vma(page, vma) == addr;
+}
+
+/*
  * copy one vm_area from one task to the other. Assumes the page tables
  * already present in the new task to be cleared in the whole range
  * covered by this vma.
@@ -381,23 +397,22 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
                 goto out_set_pte;
         }
 
-        /* If the region is VM_UNPAGED, the mapping is not
-         * mapped via rmap - duplicate the pte as is.
-         */
-        if (vm_flags & VM_UNPAGED)
-                goto out_set_pte;
-
         pfn = pte_pfn(pte);
-        /* If the pte points outside of valid memory but
+        page = pfn_valid(pfn)? pfn_to_page(pfn): NULL;
+
+        if (unlikely(vm_flags & VM_UNPAGED))
+                if (!page_is_anon(page, vma, addr))
+                        goto out_set_pte;
+
+        /*
+         * If the pte points outside of valid memory but
          * the region is not VM_UNPAGED, we have a problem.
          */
-        if (unlikely(!pfn_valid(pfn))) {
+        if (unlikely(!page)) {
                 print_bad_pte(vma, pte, addr);
                 goto out_set_pte; /* try to do something sane */
         }
 
-        page = pfn_to_page(pfn);
-
         /*
          * If it's a COW mapping, write protect it both
          * in the parent and the child
@@ -568,17 +583,20 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
                         continue;
                 }
                 if (pte_present(ptent)) {
-                        struct page *page = NULL;
+                        struct page *page;
+                        unsigned long pfn;
 
                         (*zap_work) -= PAGE_SIZE;
 
-                        if (!(vma->vm_flags & VM_UNPAGED)) {
-                                unsigned long pfn = pte_pfn(ptent);
-                                if (unlikely(!pfn_valid(pfn)))
-                                        print_bad_pte(vma, ptent, addr);
-                                else
-                                        page = pfn_to_page(pfn);
-                        }
+                        pfn = pte_pfn(ptent);
+                        page = pfn_valid(pfn)? pfn_to_page(pfn): NULL;
+
+                        if (unlikely(vma->vm_flags & VM_UNPAGED)) {
+                                if (!page_is_anon(page, vma, addr))
+                                        page = NULL;
+                        } else if (unlikely(!page))
+                                print_bad_pte(vma, ptent, addr);
+
                         if (unlikely(details) && page) {
                                 /*
                                  * unmap_shared_mapping_pages() wants to
@@ -1295,10 +1313,11 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
         old_page = pfn_to_page(pfn);
         src_page = old_page;
 
-        if (unlikely(vma->vm_flags & VM_UNPAGED)) {
-                old_page = NULL;
-                goto gotten;
-        }
+        if (unlikely(vma->vm_flags & VM_UNPAGED))
+                if (!page_is_anon(old_page, vma, address)) {
+                        old_page = NULL;
+                        goto gotten;
+                }
 
         if (PageAnon(old_page) && !TestSetPageLocked(old_page)) {
                 int reuse = can_share_swap_page(old_page);
diff --git a/mm/rmap.c b/mm/rmap.c
index 377d417ec15f..2e034a0b89ab 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -225,7 +225,9 @@ vma_address(struct page *page, struct vm_area_struct *vma)
 
 /*
  * At what user virtual address is page expected in vma? checking that the
- * page matches the vma: currently only used by unuse_process, on anon pages.
+ * page matches the vma: currently only used on anon pages, by unuse_vma;
+ * and by extraordinary checks on anon pages in VM_UNPAGED vmas, taking
+ * care that an mmap of /dev/mem might window free and foreign pages.
  */
 unsigned long page_address_in_vma(struct page *page, struct vm_area_struct *vma)
 {
@@ -234,7 +236,8 @@ unsigned long page_address_in_vma(struct page *page, struct vm_area_struct *vma)
                     (void *)page->mapping - PAGE_MAPPING_ANON)
                         return -EFAULT;
         } else if (page->mapping && !(vma->vm_flags & VM_NONLINEAR)) {
-                if (vma->vm_file->f_mapping != page->mapping)
+                if (!vma->vm_file ||
+                    vma->vm_file->f_mapping != page->mapping)
                         return -EFAULT;
         } else
                 return -EFAULT;