From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pb0-f44.google.com (mail-pb0-f44.google.com [209.85.160.44]) by kanga.kvack.org (Postfix) with ESMTP id 765116B0069 for ; Fri, 6 Jun 2014 10:21:49 -0400 (EDT) Received: by mail-pb0-f44.google.com with SMTP id rq2so2530978pbb.31 for ; Fri, 06 Jun 2014 07:21:49 -0700 (PDT) Received: from mailout2.w1.samsung.com (mailout2.w1.samsung.com. [210.118.77.12]) by mx.google.com with ESMTPS id bz3si19765362pbd.157.2014.06.06.07.21.48 for (version=TLSv1 cipher=RC4-MD5 bits=128/128); Fri, 06 Jun 2014 07:21:48 -0700 (PDT) Received: from eucpsbgm2.samsung.com (unknown [203.254.199.245]) by mailout2.w1.samsung.com (Oracle Communications Messaging Server 7u4-24.01(7.0.4.24.0) 64bit (built Nov 17 2011)) with ESMTP id <0N6R002Z83VXKD00@mailout2.w1.samsung.com> for linux-mm@kvack.org; Fri, 06 Jun 2014 15:21:33 +0100 (BST) Message-id: <5391CD5A.20503@samsung.com> Date: Fri, 06 Jun 2014 18:16:58 +0400 From: Andrey Ryabinin MIME-version: 1.0 Subject: Re: [PATCH] mm: rmap: fix use-after-free in __put_anon_vma References: <1402054255-4930-1-git-send-email-a.ryabinin@samsung.com> <20140606115620.GS3213@twins.programming.kicks-ass.net> In-reply-to: <20140606115620.GS3213@twins.programming.kicks-ass.net> Content-type: text/plain; charset=UTF-8 Content-transfer-encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Peter Zijlstra Cc: Andrew Morton , Linus Torvalds , linux-mm@kvack.org, linux-kernel@vger.kernel.org, dvyukov@google.com, koct9i@gmail.com On 06/06/14 15:56, Peter Zijlstra wrote: > On Fri, Jun 06, 2014 at 03:30:55PM +0400, Andrey Ryabinin wrote: >> While working address sanitizer for kernel I've discovered use-after-free >> bug in __put_anon_vma. >> For the last anon_vma, anon_vma->root freed before child anon_vma. >> Later in anon_vma_free(anon_vma) we are referencing to already freed anon_vma->root >> to check rwsem. >> This patch puts freeing of child anon_vma before freeing of anon_vma->root. > > Yes, I think that is right indeed. > > Very hard to hit, but valid since not all callers hold rcu_read_lock(). > >> >> Cc: stable@vger.kernel.org # v3.0+ >> Signed-off-by: Andrey Ryabinin >> --- >> mm/rmap.c | 7 ++++--- >> 1 file changed, 4 insertions(+), 3 deletions(-) >> >> diff --git a/mm/rmap.c b/mm/rmap.c >> index 9c3e773..161bffc7 100644 >> --- a/mm/rmap.c >> +++ b/mm/rmap.c >> @@ -1564,10 +1564,11 @@ void __put_anon_vma(struct anon_vma *anon_vma) >> { >> struct anon_vma *root = anon_vma->root; >> >> - if (root != anon_vma && atomic_dec_and_test(&root->refcount)) >> + if (root != anon_vma && atomic_dec_and_test(&root->refcount)) { >> + anon_vma_free(anon_vma); >> anon_vma_free(root); >> - >> - anon_vma_free(anon_vma); >> + } else >> + anon_vma_free(anon_vma); >> } > > Why not simply move the freeing of anon_vma before the root, like: > > anon_vma_free(anon_vma); > if (root != anon_vma && atomic_dec_and_test(&root->refcount)) > anon_vma_free(root); > > ? > IMO It looks more logical to decrement root's refcounter before freeing child vma. In fact I wasn't completely sure that it is safe to do so. But after some digging, now it looks safe to me. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org