Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753308AbbLII0s (ORCPT ); Wed, 9 Dec 2015 03:26:48 -0500 Received: from mx2.suse.de ([195.135.220.15]:55682 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751990AbbLII0r (ORCPT ); Wed, 9 Dec 2015 03:26:47 -0500 Date: Wed, 9 Dec 2015 09:26:38 +0100 From: Jan Kara To: Kees Cook Cc: yalin wang , Andrew Morton , Jan Kara , Willy Tarreau , "Eric W. Biederman" , "Kirill A. Shutemov" , Oleg Nesterov , Rik van Riel , Chen Gang , Davidlohr Bueso , Andrea Arcangeli , Linux-MM , LKML , Al Viro Subject: Re: [PATCH v2] clear file privilege bits when mmap writing Message-ID: <20151209082638.GA3137@quack.suse.cz> References: <20151203000342.GA30015@www.outflux.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2667 Lines: 62 On Mon 07-12-15 16:40:14, Kees Cook wrote: > On Mon, Dec 7, 2015 at 2:42 PM, Kees Cook wrote: > > On Thu, Dec 3, 2015 at 5:45 PM, yalin wang wrote: > >> > >>> On Dec 2, 2015, at 16:03, 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. > >>> > >>> Signed-off-by: Kees Cook > >>> Cc: stable@vger.kernel.org > >>> — > >> > >> is this means mprotect() sys call also need add this check? > >> mprotect() can change to PROT_WRITE, then it can write to a > >> read only map again , also a secure hole here . > > > > Yes, good point. This needs to be added. I will send a new patch. Thanks! > > This continues to look worse and worse. > > So... to check this at mprotect time, I have to know it's MAP_SHARED, > but that's in the vma_flags, which I can only see after holding > mmap_sem. > > The best I can think of now is to strip the bits at munmap time, since > you can't execute an mmapped file until it closes. > > Jan, thoughts on this? Umm, so we actually refuse to execute a file while someone has it open for writing (deny_write_access() in do_open_execat()). So dropping the suid / sgid bits when closing file for writing could be plausible. Grabbing i_mutex from __fput() context is safe (it gets called from task_work context when returning to userspace). That way we could actually remove the checks done for each write. To avoid unexpected removal of suid/sgid bits when someone just opens & closes the file, we could mark the file as needing suid/sgid treatment by a flag in inode->i_flags when file gets written to or mmaped and then check for this in __fput(). I've added Al Viro to CC just in case he is aware of some issues with this... Honza -- Jan Kara SUSE Labs, CR -- 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/