vma_adjust: fix the copying of anon_vma chains

When we move the boundaries between two vma's due to things like
mprotect, we need to make sure that the anon_vma of the pages that got
moved from one vma to another gets properly copied around.  And that was
not always the case, in this rather hard-to-follow code sequence.

Clarify the code, and fix it so that it copies the anon_vma from the
right source.

Reviewed-by: Rik van Riel <riel@redhat.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Tested-by: Borislav Petkov <bp@alien8.de> [ "Yeah, not so much this one either" ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/mmap.c b/mm/mmap.c
index acb023e..f90ea92 100644 (file)
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -507,11 +507,12 @@ int vma_adjust(struct vm_area_struct *vma, unsigned long start,
        struct address_space *mapping = NULL;
        struct prio_tree_root *root = NULL;
        struct file *file = vma->vm_file;
-       struct anon_vma *anon_vma = NULL;
        long adjust_next = 0;
        int remove_next = 0;
 
        if (next && !insert) {
+               struct vm_area_struct *exporter = NULL;
+
                if (end >= next->vm_end) {
                        /*
                         * vma expands, overlapping all the next, and
@@ -519,7 +520,7 @@ int vma_adjust(struct vm_area_struct *vma, unsigned long start,
                         */
 again:                 remove_next = 1 + (end > next->vm_end);
                        end = next->vm_end;
-                       anon_vma = next->anon_vma;
+                       exporter = next;
                        importer = vma;
                } else if (end > next->vm_start) {
                        /*
@@ -527,7 +528,7 @@ again:                      remove_next = 1 + (end > next->vm_end);
                         * mprotect case 5 shifting the boundary up.
                         */
                        adjust_next = (end - next->vm_start) >> PAGE_SHIFT;
-                       anon_vma = next->anon_vma;
+                       exporter = next;
                        importer = vma;
                } else if (end < vma->vm_end) {
                        /*
@@ -536,28 +537,19 @@ again:                    remove_next = 1 + (end > next->vm_end);
                         * mprotect case 4 shifting the boundary down.
                         */
                        adjust_next = - ((vma->vm_end - end) >> PAGE_SHIFT);
-                       anon_vma = next->anon_vma;
+                       exporter = vma;
                        importer = next;
                }
-       }
 
-       /*
-        * When changing only vma->vm_end, we don't really need anon_vma lock.
-        */
-       if (vma->anon_vma && (insert || importer || start != vma->vm_start))
-               anon_vma = vma->anon_vma;
-       if (anon_vma) {
                /*
                 * Easily overlooked: when mprotect shifts the boundary,
                 * make sure the expanding vma has anon_vma set if the
                 * shrinking vma had, to cover any anon pages imported.
                 */
-               if (importer && !importer->anon_vma) {
-                       /* Block reverse map lookups until things are set up. */
-                       if (anon_vma_clone(importer, vma)) {
+               if (exporter && exporter->anon_vma && !importer->anon_vma) {
+                       if (anon_vma_clone(importer, exporter))
                                return -ENOMEM;
-                       }
-                       importer->anon_vma = anon_vma;
+                       importer->anon_vma = exporter->anon_vma;
                }
        }