linux-kernel - [PATCH v2] mm: fix anon_vma->degree underflow in anon

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Date:	Wed,  4 Mar 2015 20:52:21 +0800
From:	Leon Yu <chianglungyu@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Konstantin Khlebnikov <koct9i@...il.com>,
	Rik van Riel <riel@...hat.com>, Michal Hocko <mhocko@...e.cz>
Cc:	Leon Yu <chianglungyu@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>,
	Daniel Forrest <dan.forrest@...c.wisc.edu>,
	Chris Clayton <chris2553@...glemail.com>,
	Oded Gabbay <oded.gabbay@....com>,
	Chih-Wei Huang <cwhuang@...roid-x86.org>,
	<stable@...r.kernel.org>
Subject: [PATCH v2] 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.

Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
Signed-off-by: Leon Yu <chianglungyu@...il.com>
Signed-off-by: Konstantin Khlebnikov <koct9i@...il.com>
Reviewed-by: Michal Hocko <mhocko@...e.cz>
Acked-by: David Rientjes <rientjes@...gle.com>
Cc: <stable@...r.kernel.org>
---
Changes in v2:
 - clean up vma_adjust() per Michal Hocko's suggestion
 - comment tweaked by Andrew Morton

 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 da9990a..9ec50a3 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -774,10 +774,8 @@ 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 5e3e090..bed3cf2 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;
 }
-- 
2.3.1

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/