From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1163059AbbKFWbR (ORCPT ); Fri, 6 Nov 2015 17:31:17 -0500 Received: from mail-ob0-f172.google.com ([209.85.214.172]:35736 "EHLO mail-ob0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1033209AbbKFWbO (ORCPT ); Fri, 6 Nov 2015 17:31:14 -0500 MIME-Version: 1.0 In-Reply-To: References: <1446511187-9131-1-git-send-email-public@rsjtdrjgfuzkfg.com> <20151104002132.010ccd1d@rsjtdrjgfuzkfg.com> <20151104065820.GF21740@1wt.eu> From: Andy Lutomirski Date: Fri, 6 Nov 2015 14:30:54 -0800 Message-ID: Subject: Re: [RFC] namei: prevent sgid-hardlinks for unmapped gids To: Kees Cook Cc: Theodore Tso , Willy Tarreau , Dirk Steinmetz , Michael Kerrisk-manpages , Serge Hallyn , Seth Forshee , Alexander Viro , Linux FS Devel , LKML , "Eric W . Biederman" , Serge Hallyn , "security@kernel.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 6, 2015 at 1:59 PM, Kees Cook wrote: > Adding Ted, who might know how this all hooks together. (The context > is that a write() or truncate() on a setgid file clears the setgid, > but mmap writes don't.) > > On Wed, Nov 4, 2015 at 9:59 AM, Andy Lutomirski wrote: >> On Tue, Nov 3, 2015 at 10:58 PM, Willy Tarreau wrote: >>> On Tue, Nov 03, 2015 at 03:29:55PM -0800, Kees Cook wrote: >>>> Using "write" does kill the set-gid bit. I haven't looked at >>>> why. >>>> Al or anyone else, is there a meaningful distinction here? >>> >>> I remember this one, I got caught once while trying to put a shell into >>> a suid-writable file to get some privileges someone forgot to offer me :-) >>> >>> It's done by should_remove_suid() which is called upon write() and truncate(). > > file_remove_privs() seems to be the right entry point. > __generic_file_write_iter in mm/filemap.c calls it, though. Are these > callbacks not used for mmap writes? They're certainly not used early enough -- we need to remove suid when the page becomes writable via mmap (wp_page_shared), not when writeback happens, or at least not only when writeback happens. But IIRC mmaped writes go through a different path -- they go through the address_space ops with names like writepages. > >>> >>>> Should the >>>> mmap MAP_SHARED-write trigger the loss of the set-gid bit too? While >>>> holding the file open with either open or mmap, I get a Text-in-use >>>> error, so I would kind of expect the same behavior between either >>>> close() and munmap(). I wonder if this is a bug, and if so, then your >>>> link patch is indeed useful again. :) >>> >>> I don't see how this could be done with mmap(). Maybe we have a way to know >>> when the first write is performed via this path, I have no idea. >> >> do_wp_page might be a decent bet. > > Or wp_page_shared? Can we get back to a file from the mm at that point? vma->vm_file, presumably (after checking whether it's null). wp_page_shared AFAIK only happens from process context, and the vma and its file should be valid. We could also get to an inode via page->address_space->mapping, but I'm guessing that vma->vm_file would be more appropriate here. --Andy