From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt0-f199.google.com (mail-qt0-f199.google.com [209.85.216.199]) by kanga.kvack.org (Postfix) with ESMTP id DD3876810B7 for ; Fri, 25 Aug 2017 10:40:41 -0400 (EDT) Received: by mail-qt0-f199.google.com with SMTP id q7so41358qta.2 for ; Fri, 25 Aug 2017 07:40:41 -0700 (PDT) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id o2si6082863qkc.93.2017.08.25.07.40.40 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 25 Aug 2017 07:40:40 -0700 (PDT) Date: Fri, 25 Aug 2017 16:40:36 +0200 From: Oleg Nesterov Subject: Re: [PATCH] fork: fix incorrect fput of ->exe_file causing use-after-free Message-ID: <20170825144036.GA26620@redhat.com> References: <20170823211408.31198-1-ebiggers3@gmail.com> <20170824132041.GA22882@redhat.com> <20170824165935.GA21624@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170824165935.GA21624@gmail.com> Sender: owner-linux-mm@kvack.org List-ID: To: Eric Biggers Cc: linux-mm@kvack.org, Andrew Morton , linux-kernel@vger.kernel.org, Dmitry Vyukov , Ingo Molnar , Konstantin Khlebnikov , Michal Hocko , Peter Zijlstra , Vlastimil Babka , stable@vger.kernel.org, Eric Biggers On 08/24, Eric Biggers wrote: > > On Thu, Aug 24, 2017 at 03:20:41PM +0200, Oleg Nesterov wrote: > > On 08/23, Eric Biggers wrote: > > > > > > From: Eric Biggers > > > > > > Commit 7c051267931a ("mm, fork: make dup_mmap wait for mmap_sem for > > > write killable") made it possible to kill a forking task while it is > > > waiting to acquire its ->mmap_sem for write, in dup_mmap(). However, it > > > was overlooked that this introduced an new error path before a reference > > > is taken on the mm_struct's ->exe_file. > > > > Hmm. Unless I am totally confused, the same problem with mm->exol_area? > > I'll recheck.... > > I'm not sure what you mean by ->exol_area. I meant mm->uprobes_state.xol_area, sorry > > > --- a/kernel/fork.c > > > +++ b/kernel/fork.c > > > @@ -806,6 +806,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p, > > > mm_init_cpumask(mm); > > > mm_init_aio(mm); > > > mm_init_owner(mm, p); > > > + RCU_INIT_POINTER(mm->exe_file, NULL); > > > > Can't we simply move > > > > RCU_INIT_POINTER(mm->exe_file, get_mm_exe_file(oldmm)); > > > > from dup_mmap() here? Afaics this doesn't need mmap_sem. > > > > Two problems, even assuming that get_mm_exe_file() doesn't require mmap_sem: > > - If mm_alloc_pgd() or init_new_context() in mm_init() fails, mm_init() doesn't > do the full mmput(), so the file reference would not be dropped. So it would > need to be changed to drop the file reference too. Ah yes, I forgot that mm_init() can fail after that, thanks for correcting me. Oleg. -- 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