From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Thu, 24 Aug 2017 09:59:35 -0700 From: Eric Biggers To: Oleg Nesterov 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 Subject: Re: [PATCH] fork: fix incorrect fput of ->exe_file causing use-after-free Message-ID: <20170824165935.GA21624@gmail.com> References: <20170823211408.31198-1-ebiggers3@gmail.com> <20170824132041.GA22882@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170824132041.GA22882@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: 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. > > > --- 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. - The file would also be set when called from mm_alloc() which is used when exec'ing a new task. *Maybe* it would be safe to do temporarily, but it's pointless because ->exe_file will be set later by flush_old_exec(). Eric -- 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