From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg1-f200.google.com (mail-pg1-f200.google.com [209.85.215.200]) by kanga.kvack.org (Postfix) with ESMTP id 035D36B074B for ; Sat, 10 Nov 2018 17:18:30 -0500 (EST) Received: by mail-pg1-f200.google.com with SMTP id f9so3581169pgs.13 for ; Sat, 10 Nov 2018 14:18:29 -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 m61-v6sor14111784plb.2.2018.11.10.14.18.28 for (Google Transport Security); Sat, 10 Nov 2018 14:18:28 -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: <20181110220933.GB96924@google.com> Date: Sat, 10 Nov 2018 14:18:23 -0800 Content-Transfer-Encoding: quoted-printable Message-Id: <907D942E-E321-4BD7-BED7-ACD1D96A3643@amacapital.net> References: <20181108041537.39694-1-joel@joelfernandes.org> <20181110032005.GA22238@google.com> <69CE06CC-E47C-4992-848A-66EB23EE6C74@amacapital.net> <20181110182405.GB242356@google.com> <20181110220933.GB96924@google.com> Sender: owner-linux-mm@kvack.org List-ID: To: Joel Fernandes Cc: Daniel Colascione , 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 2:09 PM, Joel Fernandes wrote= : >=20 >> On Sat, Nov 10, 2018 at 11:11:27AM -0800, Daniel Colascione wrote: >>> 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 work= s >>>> 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 WRIT= E >>>> opens as Daniel suggested. I will make this change so that the security= fix >>>> is a clean one. >>>>=20 >>>>> - does *not* affect other struct files that may already exist on the s= ame 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= IPC >>>> to another process and we want to prevent any new writes in the receive= r >>>> side. There is no way this other receiving process can have an existing= fd >>>> 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 stru= ct >>>>> 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= map. >>>> 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 mentio= ned >>>> 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= deny >>>> writes of already opened files. This would mean more checking in all th= ose >>>> 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 decrem= ents >>>> 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 ne= gative) >>>>=20 >>>> That will prevent both reopens, and writes from succeeding. However I w= orry 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. >=20 > Agreed with the idea of modifying both file and inode flags. I was thinkin= g > modifying i_mode may do the trick but as you pointed it probably could be > reverted by chmod or some other attribute setting calls. >=20 > OTOH, I don't think deny_write_access(file) can be reverted from any > user-facing path so we could do that from the seal to prevent the future > opens in write mode. I'll double check and test that out tomorrow. >=20 >=20 This seems considerably more complicated and more fragile than needed. Just a= dd a new F_SEAL_WRITE_FUTURE. Grep for F_SEAL_WRITE and make the _FUTURE va= riant work exactly like it with two exceptions: - shmem_mmap and maybe its hugetlbfs equivalent should check for it and act a= ccordingly. - add_seals won=E2=80=99t need the wait_for_pins and mapping_deny_write logi= c. That really should be all that=E2=80=99s needed.=