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>

1 files changed, 8 insertions, 16 deletions

diff --git a/mm/mmap.c b/mm/mmap.c
index acb023e2d35..f90ea92f755 100644
--- 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;
                 }
         }