From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754206AbbLIWwy (ORCPT ); Wed, 9 Dec 2015 17:52:54 -0500 Received: from mail-ig0-f172.google.com ([209.85.213.172]:33354 "EHLO mail-ig0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752596AbbLIWwT convert rfc822-to-8bit (ORCPT ); Wed, 9 Dec 2015 17:52:19 -0500 MIME-Version: 1.0 In-Reply-To: <20151209082638.GA3137@quack.suse.cz> References: <20151203000342.GA30015@www.outflux.net> <20151209082638.GA3137@quack.suse.cz> Date: Wed, 9 Dec 2015 14:52:19 -0800 X-Google-Sender-Auth: eGX_59m0At9nwlUGSHfQjQhLyZo Message-ID: Subject: Re: [PATCH v2] clear file privilege bits when mmap writing From: Kees Cook To: Jan Kara Cc: yalin wang , Andrew Morton , Willy Tarreau , "Eric W. Biederman" , "Kirill A. Shutemov" , Oleg Nesterov , Rik van Riel , Chen Gang , Davidlohr Bueso , Andrea Arcangeli , Linux-MM , LKML , Al Viro Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 9, 2015 at 12:26 AM, Jan Kara wrote: > 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(). Yeah, this is ultimately where I ended up for the v4 (and fixed up in v5). I added the flag to file, though, not inode. Sending v5 now... -Kees > > I've added Al Viro to CC just in case he is aware of some issues with > this... > > Honza > -- > Jan Kara > SUSE Labs, CR -- Kees Cook Chrome OS & Brillo Security From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ig0-f178.google.com (mail-ig0-f178.google.com [209.85.213.178]) by kanga.kvack.org (Postfix) with ESMTP id E44D46B0255 for ; Wed, 9 Dec 2015 17:52:19 -0500 (EST) Received: by mail-ig0-f178.google.com with SMTP id g19so3753825igv.1 for ; Wed, 09 Dec 2015 14:52:19 -0800 (PST) Received: from mail-ig0-x22e.google.com (mail-ig0-x22e.google.com. [2607:f8b0:4001:c05::22e]) by mx.google.com with ESMTPS id n10si17460925ige.30.2015.12.09.14.52.19 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 09 Dec 2015 14:52:19 -0800 (PST) Received: by mail-ig0-x22e.google.com with SMTP id mv3so3788696igc.0 for ; Wed, 09 Dec 2015 14:52:19 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <20151209082638.GA3137@quack.suse.cz> References: <20151203000342.GA30015@www.outflux.net> <20151209082638.GA3137@quack.suse.cz> Date: Wed, 9 Dec 2015 14:52:19 -0800 Message-ID: Subject: Re: [PATCH v2] clear file privilege bits when mmap writing From: Kees Cook Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: To: Jan Kara Cc: yalin wang , Andrew Morton , Willy Tarreau , "Eric W. Biederman" , "Kirill A. Shutemov" , Oleg Nesterov , Rik van Riel , Chen Gang , Davidlohr Bueso , Andrea Arcangeli , Linux-MM , LKML , Al Viro On Wed, Dec 9, 2015 at 12:26 AM, Jan Kara wrote: > 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 bi= ts, >> >>> 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 d= one >> >>> 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 >> >>> =E2=80=94 >> >> >> >> 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. Than= ks! >> >> 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 fo= r > 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 avoi= d > 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 thi= s > in __fput(). Yeah, this is ultimately where I ended up for the v4 (and fixed up in v5). I added the flag to file, though, not inode. Sending v5 now... -Kees > > I've added Al Viro to CC just in case he is aware of some issues with > this... > > Honza > -- > Jan Kara > SUSE Labs, CR --=20 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