From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io0-f170.google.com (mail-io0-f170.google.com [209.85.223.170]) by kanga.kvack.org (Postfix) with ESMTP id 4D623828EB for ; Mon, 11 Jan 2016 14:38:34 -0500 (EST) Received: by mail-io0-f170.google.com with SMTP id 77so328697506ioc.2 for ; Mon, 11 Jan 2016 11:38:34 -0800 (PST) Received: from mail-io0-x22d.google.com (mail-io0-x22d.google.com. [2607:f8b0:4001:c06::22d]) by mx.google.com with ESMTPS id f186si29678947ioe.153.2016.01.11.11.38.33 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 11 Jan 2016 11:38:33 -0800 (PST) Received: by mail-io0-x22d.google.com with SMTP id q21so359796683iod.0 for ; Mon, 11 Jan 2016 11:38:33 -0800 (PST) MIME-Version: 1.0 In-Reply-To: References: <20160108232727.GA23490@www.outflux.net> Date: Mon, 11 Jan 2016 11:38:32 -0800 Message-ID: Subject: Re: [PATCH v6] fs: clear file privilege bits when mmap writing From: Kees Cook Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: To: Konstantin Khlebnikov Cc: Alexander Viro , Andy Lutomirski , Jan Kara , yalin wang , Willy Tarreau , Andrew Morton , linux-fsdevel , linux-arch , Linux API , "linux-mm@kvack.org" On Sun, Jan 10, 2016 at 7:48 AM, Konstantin Khlebnikov wrote: > On Sat, Jan 9, 2016 at 2:27 AM, 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). We >> could do this during vm_mmap_pgoff, but that would need coverage in >> mprotect as well, but to check for MAP_SHARED, we'd need to hold mmap_sem >> again. We could clear at open() time, but it's possible things are >> accidentally opening with O_RDWR and only reading. Better to clear on >> close and error failures (i.e. an improvement over now, which is not >> clearing at all). > > I think this should be done in mmap/mprotect. Code in sys_mmap is trivial. > > In sys_mprotect you can check file_needs_remove_privs() and VM_SHARED > under mmap_sem, then if needed grab reference to struct file from vma and > clear suid after unlocking mmap_sem. > > I haven't seen previous iterations, probably this approach has known flaws. mmap_sem is still needed in mprotect (to find and hold the vma), so it's not possible. I'd love to be proven wrong, but I didn't see a way. -Kees -- Kees Cook Chrome OS & Brillo Security -- 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