[PATCH 3.4 176/176] mm: fix anon_vma->degree underflow in anon_vma endless growing prevention

From: lizf
Date: Thu Apr 09 2015 - 04:59:14 EST

From: Leon Yu <chianglungyu@xxxxxxxxx>

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


commit 3fe89b3e2a7bbf3e97657104b9b33a9d81b950b3 upstream.

I have constantly stumbled upon "kernel BUG at mm/rmap.c:399!" after
upgrading to 3.19 and had no luck with 4.0-rc1 neither.

So, after looking into new logic introduced by commit 7a3ef208e662 ("mm:
prevent endless growth of anon_vma hierarchy"), I found chances are that
unlink_anon_vmas() is called without incrementing dst->anon_vma->degree
in anon_vma_clone() due to allocation failure. If dst->anon_vma is not
NULL in error path, its degree will be incorrectly decremented in
unlink_anon_vmas() and eventually underflow when exiting as a result of
another call to unlink_anon_vmas(). That's how "kernel BUG at
mm/rmap.c:399!" is triggered for me.

This patch fixes the underflow by dropping dst->anon_vma when allocation
fails. It's safe to do so regardless of original value of dst->anon_vma
because dst->anon_vma doesn't have valid meaning if anon_vma_clone()
fails. Besides, callers don't care dst->anon_vma in such case neither.

Also suggested by Michal Hocko, we can clean up vma_adjust() a bit as
anon_vma_clone() now does the work.

[akpm@xxxxxxxxxxxxxxxxxxxx: tweak comment]
Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
Signed-off-by: Leon Yu <chianglungyu@xxxxxxxxx>
Signed-off-by: Konstantin Khlebnikov <koct9i@xxxxxxxxx>
Reviewed-by: Michal Hocko <mhocko@xxxxxxx>
Acked-by: Rik van Riel <riel@xxxxxxxxxx>
Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Zefan Li <lizefan@xxxxxxxxxx>
mm/mmap.c | 4 +---
mm/rmap.c | 7 +++++++
2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index f880ca1..208e70f 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -571,10 +571,8 @@ again: remove_next = 1 + (end > next->vm_end);

importer->anon_vma = exporter->anon_vma;
error = anon_vma_clone(importer, exporter);
- if (error) {
- importer->anon_vma = NULL;
+ if (error)
return error;
- }

diff --git a/mm/rmap.c b/mm/rmap.c
index b7a64ec..57f503b 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -292,6 +292,13 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
return 0;

+ /*
+ * dst->anon_vma is dropped here otherwise its degree can be incorrectly
+ * decremented in unlink_anon_vmas().
+ * We can safely do this because callers of anon_vma_clone() don't care
+ * about dst->anon_vma if anon_vma_clone() failed.
+ */
+ dst->anon_vma = NULL;
return -ENOMEM;

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