From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f42.google.com (mail-wm0-f42.google.com [74.125.82.42]) by kanga.kvack.org (Postfix) with ESMTP id C29336B0038 for ; Wed, 2 Dec 2015 19:18:54 -0500 (EST) Received: by wmec201 with SMTP id c201so3514733wme.0 for ; Wed, 02 Dec 2015 16:18:54 -0800 (PST) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTPS id m139si46770865wma.54.2015.12.02.16.18.53 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 02 Dec 2015 16:18:53 -0800 (PST) Date: Wed, 2 Dec 2015 16:18:51 -0800 From: Andrew Morton Subject: Re: [PATCH v2] fs: clear file privilege bits when mmap writing Message-Id: <20151202161851.95d8fe811705c038e3fe2d33@linux-foundation.org> In-Reply-To: <20151203000342.GA30015@www.outflux.net> References: <20151203000342.GA30015@www.outflux.net> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Kees Cook Cc: Jan Kara , Willy Tarreau , "Eric W. Biederman" , "Kirill A. Shutemov" , Oleg Nesterov , Rik van Riel , Chen Gang , Davidlohr Bueso , Andrea Arcangeli , linux-mm@kvack.org, linux-kernel@vger.kernel.org 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? -- 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