[PATCH 08/11] unpaged: anon in VM_UNPAGED

Hugh Dickins
2005-11-17T19:39:23Z

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>
---

mm/memory.c | 63 +++++++++++++++++++++++++++++++++++++++---------------------
mm/rmap.c | 7 ++++--
2 files changed, 46 insertions(+), 24 deletions(-)

--- unpaged07/mm/memory.c 2005-11-17 15:11:30.000000000 +0000
+++ unpaged08/mm/memory.c 2005-11-17 15:11:43.000000000 +0000
@@ -350,6 +350,22 @@ void print_bad_pte(struct vm_area_struct
}

/*
+ * 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, s
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(struc
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 *
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);
--- unpaged07/mm/rmap.c 2005-11-17 15:11:16.000000000 +0000
+++ unpaged08/mm/rmap.c 2005-11-17 15:11:43.000000000 +0000
@@ -225,7 +225,9 @@ vma_address(struct page *page, struct vm

/*
* 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
(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;