From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f197.google.com (mail-pf1-f197.google.com [209.85.210.197]) by kanga.kvack.org (Postfix) with ESMTP id ED67E6B079F for ; Sat, 10 Nov 2018 14:55:14 -0500 (EST) Received: by mail-pf1-f197.google.com with SMTP id g76-v6so4243446pfe.13 for ; Sat, 10 Nov 2018 11:55:14 -0800 (PST) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id w85-v6sor2695353pfk.48.2018.11.10.11.55.13 for (Google Transport Security); Sat, 10 Nov 2018 11:55:13 -0800 (PST) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (1.0) Subject: Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd From: Andy Lutomirski In-Reply-To: Date: Sat, 10 Nov 2018 11:55:10 -0800 Content-Transfer-Encoding: quoted-printable Message-Id: References: <20181108041537.39694-1-joel@joelfernandes.org> <20181110032005.GA22238@google.com> <69CE06CC-E47C-4992-848A-66EB23EE6C74@amacapital.net> <20181110182405.GB242356@google.com> Sender: owner-linux-mm@kvack.org List-ID: To: Daniel Colascione Cc: Joel Fernandes , Jann Horn , kernel list , John Reck , John Stultz , Todd Kjos , Greg Kroah-Hartman , Christoph Hellwig , Al Viro , Andrew Morton , Bruce Fields , Jeff Layton , Khalid Aziz , Lei.Yang@windriver.com, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org, Linux-MM , marcandre.lureau@redhat.com, Mike Kravetz , Minchan Kim , Shuah Khan , Valdis Kletnieks , Hugh Dickins , Linux API > On Nov 10, 2018, at 11:11 AM, Daniel Colascione wrote:= >=20 >> On Sat, Nov 10, 2018 at 10:45 AM, Daniel Colascione w= rote: >>> On Sat, Nov 10, 2018 at 10:24 AM, Joel Fernandes wrote: >>> Thanks Andy for your thoughts, my comments below: > [snip] >>> I don't see it as warty, different seals will work differently. It works= >>> quite well for our usecase, and since Linux is all about solving real >>> problems in the real work, it would be useful to have it. >>>=20 >>>> - causes a probably-observable effect in the file mode in F_GETFL. >>>=20 >>> Wouldn't that be the right thing to observe anyway? >>>=20 >>>> - causes reopen to fail. >>>=20 >>> So this concern isn't true anymore if we make reopen fail only for WRITE= >>> opens as Daniel suggested. I will make this change so that the security f= ix >>> is a clean one. >>>=20 >>>> - does *not* affect other struct files that may already exist on the sa= me inode. >>>=20 >>> TBH if you really want to block all writes to the file, then you want >>> F_SEAL_WRITE, not this seal. The usecase we have is the fd is sent over I= PC >>> to another process and we want to prevent any new writes in the receiver= >>> side. There is no way this other receiving process can have an existing f= d >>> unless it was already sent one without the seal applied. The proposed s= eal >>> could be renamed to F_SEAL_FD_WRITE if that is preferred. >>>=20 >>>> - mysteriously malfunctions if you try to set it again on another struc= t >>>> file that already exists >>>>=20 >>>=20 >>> I didn't follow this, could you explain more? >>>=20 >>>> - probably is insecure when used on hugetlbfs. >>>=20 >>> The usecase is not expected to prevent all writes, indeed the usecase >>> requires existing mmaps to continue to be able to write into the memory m= ap. >>> So would you call that a security issue too? The use of the seal wants t= o >>> allow existing mmap regions to be continue to be written into (I mention= ed >>> more details in the cover letter). >>>=20 >>>> I see two reasonable solutions: >>>>=20 >>>> 1. Don=E2=80=99t fiddle with the struct file at all. Instead make the i= node flag >>>> work by itself. >>>=20 >>> Currently, the various VFS paths check only the struct file's f_mode to d= eny >>> writes of already opened files. This would mean more checking in all tho= se >>> paths (and modification of all those paths). >>>=20 >>> Anyway going with that idea, we could >>> 1. call deny_write_access(file) from the memfd's seal path which decreme= nts >>> the inode::i_writecount. >>> 2. call get_write_access(inode) in the various VFS paths in addition to >>> checking for FMODE_*WRITE and deny the write (incase i_writecount is neg= ative) >>>=20 >>> That will prevent both reopens, and writes from succeeding. However I wo= rry a >>> bit about 2 not being too familiar with VFS internals, about what the >>> consequences of doing that may be. >>=20 >> IMHO, modifying both the inode and the struct file separately is fine, >> since they mean different things. In regular filesystems, it's fine to >> have a read-write open file description for a file whose inode grants >> write permission to nobody. Speaking of which: is fchmod enough to >> prevent this attack? >=20 > Well, yes and no. fchmod does prevent reopening the file RW, but > anyone with permissions (owner, CAP_FOWNER) can just fchmod it back. A > seal is supposed to be irrevocable, so fchmod-as-inode-seal probably > isn't sufficient by itself. While it might be good enough for Android > (in the sense that it'll prevent RW-reopens from other security > contexts to which we send an open memfd file), it's still conceptually > ugly, IMHO. Let's go with the original approach of just tweaking the > inode so that open-for-write is permanently blocked. This should be straightforward. Just add a new seal type and wire it up. It s= hould be considerably simpler than SEAL_WRITE.=