[3.13.y-ckt stable] Patch "mm: fix anon_vma->degree underflow in anon_vma endless growing prevention" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    mm: fix anon_vma->degree underflow in anon_vma endless growing prevention

to the linux-3.13.y-queue branch of the 3.13.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11-ckt20.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From ed3d98afe7bd93e259b1ff710de8543b4969b94b Mon Sep 17 00:00:00 2001
From: Leon Yu <chianglungyu at gmail.com>
Date: Wed, 25 Mar 2015 15:55:11 -0700
Subject: mm: fix anon_vma->degree underflow in anon_vma endless growing
 prevention

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 at linux-foundation.org: tweak comment]
Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
Signed-off-by: Leon Yu <chianglungyu at gmail.com>
Signed-off-by: Konstantin Khlebnikov <koct9i at gmail.com>
Reviewed-by: Michal Hocko <mhocko at suse.cz>
Acked-by: Rik van Riel <riel at redhat.com>
Acked-by: David Rientjes <rientjes at google.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 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 e359e9e..e3bd471 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -746,10 +746,8 @@ 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 ea81a86..f748513 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -287,6 +287,13 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
 	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;
 }
--
1.9.1