[PATCH 3.2 156/221] mm: fix anon_vma->degree underflow in anon_vma endless growing prevention

[Ben Hutchings]

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

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

From: Leon Yu <chianglungyu@xxxxxxxxx>

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: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 mm/mmap.c | 4 +---
 mm/rmap.c | 7 +++++++
 2 files changed, 8 insertions(+), 3 deletions(-)

--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -541,10 +541,8 @@ again:			remove_next = 1 + (end > next->
 
 			importer->anon_vma = exporter->anon_vma;
 			error = anon_vma_clone(importer, exporter);
-			if (error) {
-				importer->anon_vma = NULL;
+			if (error)
 				return error;
-			}
 		}
 	}
 
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -295,6 +295,13 @@ int anon_vma_clone(struct vm_area_struct
 	return 0;
 
  enomem_failure:
+	/*
+	 * 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;
 	unlink_anon_vmas(dst);
 	return -ENOMEM;
 }

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