From: Leon Yu <chianglungyu@xxxxxxxxx>
Subject: mm: fix anon_vma->degree underflow in anon_vma endless growing prevention
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 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>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/mmap.c | 4 +---
mm/rmap.c | 7 +++++++
2 files changed, 8 insertions(+), 3 deletions(-)
diff -puN mm/mmap.c~mm-fix-anon_vma-degree-underflow-in-anon_vma-endless-growing-prevention mm/mmap.c
--- a/mm/mmap.c~mm-fix-anon_vma-degree-underflow-in-anon_vma-endless-growing-prevention
+++ a/mm/mmap.c
@@ -774,10 +774,8 @@ 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;
- }
}
}
diff -puN mm/rmap.c~mm-fix-anon_vma-degree-underflow-in-anon_vma-endless-growing-prevention mm/rmap.c
--- a/mm/rmap.c~mm-fix-anon_vma-degree-underflow-in-anon_vma-endless-growing-prevention
+++ a/mm/rmap.c
@@ -287,6 +287,13 @@ 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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