mm: fix corner case in anon_vma endless growing prevention

commit b800c91a0517071156e772d4fb329ad33590da62 upstream.

Fix for BUG_ON(anon_vma->degree) splashes in unlink_anon_vmas() ("kernel
BUG at mm/rmap.c:399!") caused by commit 7a3ef208e662 ("mm: prevent
endless growth of anon_vma hierarchy")

Anon_vma_clone() is usually called for a copy of source vma in
destination argument.  If source vma has anon_vma it should be already
in dst->anon_vma.  NULL in dst->anon_vma is used as a sign that it's
called from anon_vma_fork().  In this case anon_vma_clone() finds
anon_vma for reusing.

Vma_adjust() calls it differently and this breaks anon_vma reusing
logic: anon_vma_clone() links vma to old anon_vma and updates degree
counters but vma_adjust() overrides vma->anon_vma right after that.  As
a result final unlink_anon_vmas() decrements degree for wrong anon_vma.

This patch assigns ->anon_vma before calling anon_vma_clone().

Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>
Reported-and-tested-by: Chris Clayton <chris2553@googlemail.com>
Reported-and-tested-by: Oded Gabbay <oded.gabbay@amd.com>
Reported-and-tested-by: Chih-Wei Huang <cwhuang@android-x86.org>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Daniel Forrest <dan.forrest@ssec.wisc.edu>
Cc: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2: vma_adjust() didn't use a variable to propagate
 the error code from anon_vma_clone(); change that at the same time]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/mm/mmap.c b/mm/mmap.c
index a3446759fe8e106e93341c6f832fc1481f69dde6..13b5685db36dc45c65ebc56e073e9d2f73e677f4 100644 (file)
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -537,9 +537,14 @@ again:                     remove_next = 1 + (end > next->vm_end);
                 * shrinking vma had, to cover any anon pages imported.
                 */
                if (exporter && exporter->anon_vma && !importer->anon_vma) {
-                       if (anon_vma_clone(importer, exporter))
-                               return -ENOMEM;
+                       int error;
+
                        importer->anon_vma = exporter->anon_vma;
+                       error = anon_vma_clone(importer, exporter);
+                       if (error) {
+                               importer->anon_vma = NULL;
+                               return error;
+                       }
                }
        }