Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760544AbbLCQHj (ORCPT ); Thu, 3 Dec 2015 11:07:39 -0500 Received: from mail-ig0-f181.google.com ([209.85.213.181]:33596 "EHLO mail-ig0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752320AbbLCQHK (ORCPT ); Thu, 3 Dec 2015 11:07:10 -0500 MIME-Version: 1.0 In-Reply-To: <20151202161851.95d8fe811705c038e3fe2d33@linux-foundation.org> References: <20151203000342.GA30015@www.outflux.net> <20151202161851.95d8fe811705c038e3fe2d33@linux-foundation.org> Date: Thu, 3 Dec 2015 08:07:08 -0800 X-Google-Sender-Auth: eagwq2ih_Zj2NfjeT9SRC086avs Message-ID: Subject: Re: [PATCH v2] fs: clear file privilege bits when mmap writing From: Kees Cook To: Andrew Morton Cc: Jan Kara , Willy Tarreau , "Eric W. Biederman" , "Kirill A. Shutemov" , Oleg Nesterov , Rik van Riel , Chen Gang , Davidlohr Bueso , Andrea Arcangeli , Linux-MM , LKML Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2022 Lines: 49 On Wed, Dec 2, 2015 at 4:18 PM, Andrew Morton wrote: > On Wed, 2 Dec 2015 16:03:42 -0800 Kees Cook wrote: > >> Normally, when a user can modify a file that has setuid or setgid bits, >> those bits are cleared when they are not the file owner or a member >> of the group. This is enforced when using write and truncate but not >> when writing to a shared mmap on the file. This could allow the file >> writer to gain privileges by changing a binary without losing the >> setuid/setgid/caps bits. >> >> Changing the bits requires holding inode->i_mutex, so it cannot be done >> during the page fault (due to mmap_sem being held during the fault). >> Instead, clear the bits if PROT_WRITE is being used at mmap time. >> >> ... >> >> --- a/mm/mmap.c >> +++ b/mm/mmap.c >> @@ -1340,6 +1340,17 @@ unsigned long do_mmap(struct file *file, unsigned long addr, >> if (locks_verify_locked(file)) >> return -EAGAIN; >> >> + /* >> + * If we must remove privs, we do it here since >> + * doing it during page COW is expensive and >> + * cannot hold inode->i_mutex. >> + */ >> + if (prot & PROT_WRITE && !IS_NOSEC(inode)) { >> + mutex_lock(&inode->i_mutex); >> + file_remove_privs(file); >> + mutex_unlock(&inode->i_mutex); >> + } >> + > > Still ignoring the file_remove_privs() return value. If this is > deliberate then a description of the reasons should be included? Argh, yes, sorry. I will send a v3. -Kees -- Kees Cook Chrome OS & Brillo Security -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/