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[209.132.180.67]) by mx.google.com with ESMTP id l136si158350oib.277.2020.03.10.16.22.43; Tue, 10 Mar 2020 16:22:55 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; dkim=pass header.i=@google.com header.s=20161025 header.b=bXtZeMLw; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=google.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726604AbgCJXWT (ORCPT + 99 others); Tue, 10 Mar 2020 19:22:19 -0400 Received: from mail-oi1-f196.google.com ([209.85.167.196]:34730 "EHLO mail-oi1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727484AbgCJXWT (ORCPT ); Tue, 10 Mar 2020 19:22:19 -0400 Received: by mail-oi1-f196.google.com with SMTP id g6so31375oiy.1 for ; Tue, 10 Mar 2020 16:22:18 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=0SG9alWkJ/Jgp88pmVWrlnAeORjU1Bi5WlBRFbHa1ck=; b=bXtZeMLwoJSnRz7jYAT9Nxgt4EjIpibQndWXUYf0e25pPrU5HPPn1B9ylL4DXV6Z1j qeFDlH1hFWzJQ70cTl0cuy/Fbm/L0h0EWDQjgHIniSeYfa1Uk+XmSZLW1QgEJFMKAzjF VZiHdDIDdWe5BnM+5Noagr/GdY5lI37UwcLgkyMdVfsD/fe+MFTuOxrTeJi1nX4i8HOg VUkKfaAU3TrdPfbXYQqboyCHbc0eyYBaRmxuH52Xn5Dh/TCm1e02ASC5dirfvTF1YxPx lzPE++XTCgEvyiX6oaF3wYDbxIDXkcGIqFCjOGk8cU/ivzDnxBuuRsxii0nf2rzn+eB4 pQZg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=0SG9alWkJ/Jgp88pmVWrlnAeORjU1Bi5WlBRFbHa1ck=; b=iX1WGWS3JIJGLc7iaVOcC1ZDk6vF60b9OQPDgMUZw9H9q6Iz1qXgZ9N2F13XAWmulU yEyfnPuOTb9SBfKo1iSr591eCSEqTfvdrS4CFseUeiBOZgO/Torad9D23WDctG8Nezmt lkgRv+mgsnqYSpd/GY6aMDBE/icWruzAL0mJy6TgVBYOIuy3TcydyrOqIpalZkdphHZm 8+XbmtBZMWZYEue4Rpsx3HWtMT9pAKc/g5Ua1EeNhxKJsdrxDg2IPvwCHsaexkk5+BPL vXvFmCMpwT+HSDAOTFbHUfhhlmZrBhAPx5RrDarsuRBkg3+uKDTXC7vDpeDhbqRgkTL0 rU3w== X-Gm-Message-State: ANhLgQ3t9viz9sJ4F7t1+2RW+gyPHFKWL6XftWZ/cFoMgpOXIl2J+U6a ZKd3+SR0MO/hEsfT2qrxaPvjcJEfN+wB0V0LlT9pjA== X-Received: by 2002:aca:1913:: with SMTP id l19mr51096oii.47.1583882536478; Tue, 10 Mar 2020 16:22:16 -0700 (PDT) MIME-Version: 1.0 References: <87k142lpfz.fsf@x220.int.ebiederm.org> <875zfmloir.fsf@x220.int.ebiederm.org> <87v9nmjulm.fsf@x220.int.ebiederm.org> <202003021531.C77EF10@keescook> <20200303085802.eqn6jbhwxtmz4j2x@wittgenstein> <87v9nlii0b.fsf@x220.int.ebiederm.org> <87a74xi4kz.fsf@x220.int.ebiederm.org> <87r1y8dqqz.fsf@x220.int.ebiederm.org> <87tv32cxmf.fsf_-_@x220.int.ebiederm.org> <87v9ne5y4y.fsf_-_@x220.int.ebiederm.org> <87zhcq4jdj.fsf_-_@x220.int.ebiederm.org> <87wo7roq2c.fsf@x220.int.ebiederm.org> In-Reply-To: <87wo7roq2c.fsf@x220.int.ebiederm.org> From: Jann Horn Date: Wed, 11 Mar 2020 00:21:49 +0100 Message-ID: Subject: Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex To: "Eric W. Biederman" Cc: Bernd Edlinger , Christian Brauner , Kees Cook , Jonathan Corbet , Alexander Viro , Andrew Morton , Alexey Dobriyan , Thomas Gleixner , Oleg Nesterov , Frederic Weisbecker , Andrei Vagin , Ingo Molnar , "Peter Zijlstra (Intel)" , Yuyang Du , David Hildenbrand , Sebastian Andrzej Siewior , Anshuman Khandual , David Howells , James Morris , Greg Kroah-Hartman , Shakeel Butt , Jason Gunthorpe , Christian Kellner , Andrea Arcangeli , Aleksa Sarai , "Dmitry V. Levin" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-mm@kvack.org" , "stable@vger.kernel.org" , "linux-api@vger.kernel.org" Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 10, 2020 at 10:33 PM Eric W. Biederman wrote: > Jann Horn writes: > > On Sun, Mar 8, 2020 at 10:41 PM Eric W. Biederman wrote: > >> The cred_guard_mutex is problematic. The cred_guard_mutex is held > >> over the userspace accesses as the arguments from userspace are read. > >> The cred_guard_mutex is held of PTRACE_EVENT_EXIT as the the other > >> threads are killed. The cred_guard_mutex is held over > >> "put_user(0, tsk->clear_child_tid)" in exit_mm(). > >> > >> Any of those can result in deadlock, as the cred_guard_mutex is held > >> over a possible indefinite userspace waits for userspace. > >> > >> Add exec_update_mutex that is only held over exec updating process > >> with the new contents of exec, so that code that needs not to be > >> confused by exec changing the mm and the cred in ways that can not > >> happen during ordinary execution of a process. > >> > >> The plan is to switch the users of cred_guard_mutex to > >> exec_udpate_mutex one by one. This lets us move forward while still > >> being careful and not introducing any regressions. > > [...] > >> @@ -1034,6 +1035,11 @@ static int exec_mmap(struct mm_struct *mm) > >> return -EINTR; > >> } > >> } > >> + > >> + ret = mutex_lock_killable(&tsk->signal->exec_update_mutex); > >> + if (ret) > >> + return ret; > > > > We're already holding the old mmap_sem, and now nest the > > exec_update_mutex inside it; but then while still holding the > > exec_update_mutex, we do mmput(), which can e.g. end up in ksm_exit(), > > which can do down_write(&mm->mmap_sem) from __ksm_exit(). So I think > > at least lockdep will be unhappy, and I'm not sure whether it's an > > actual problem or not. > > Good point. I should double check the lock ordering here with mmap_sem. > It doesn't look like mmput takes mmap_sem You sure about that? mmput() -> __mmput() -> ksm_exit() -> __ksm_exit() -> down_write(&mm->mmap_sem) Or also: mmput() -> __mmput() -> khugepaged_exit() -> __khugepaged_exit() -> down_write(&mm->mmap_sem) Or is there a reason why those paths can't happen?