[PATCH 3.13.y-ckt 039/103] mm: fix corner case in anon_vma endless growing prevention

[Kamal Mostafa]

3.13.11-ckt16 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Konstantin Khlebnikov <koct9i at gmail.com>

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 at gmail.com>
Reported-and-tested-by: Chris Clayton <chris2553 at googlemail.com>
Reported-and-tested-by: Oded Gabbay <oded.gabbay at amd.com>
Reported-and-tested-by: Chih-Wei Huang <cwhuang at android-x86.org>
Acked-by: Rik van Riel <riel at redhat.com>
Acked-by: Vlastimil Babka <vbabka at suse.cz>
Cc: Daniel Forrest <dan.forrest at ssec.wisc.edu>
Cc: Michal Hocko <mhocko at suse.cz>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 mm/mmap.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index de25399..d3b8887 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -744,10 +744,12 @@ again:			remove_next = 1 + (end > next->vm_end);
 		if (exporter && exporter->anon_vma && !importer->anon_vma) {
 			int error;
 
+			importer->anon_vma = exporter->anon_vma;
 			error = anon_vma_clone(importer, exporter);
-			if (error)
+			if (error) {
+				importer->anon_vma = NULL;
 				return error;
-			importer->anon_vma = exporter->anon_vma;
+			}
 		}
 	}
 
-- 
1.9.1