[PATCH 3.2 104/152] mm: fix corner case in anon_vma endless growing prevention

From: Ben Hutchings
Date: Mon Feb 16 2015 - 21:32:56 EST

3.2.67-rc1 review patch. If anyone has any objections, please let me know.


From: Konstantin Khlebnikov <koct9i@xxxxxxxxx>

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@xxxxxxxxx>
Reported-and-tested-by: Chris Clayton <chris2553@xxxxxxxxxxxxxx>
Reported-and-tested-by: Oded Gabbay <oded.gabbay@xxxxxxx>
Reported-and-tested-by: Chih-Wei Huang <cwhuang@xxxxxxxxxxxxxxx>
Acked-by: Rik van Riel <riel@xxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Daniel Forrest <dan.forrest@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[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@xxxxxxxxxxxxxxx>
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -537,9 +537,14 @@ again: remove_next = 1 + (end > next->
* 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;
+ }

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