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([2600:1010:b026:ccca:748f:8cd2:cbf4:9038]) by smtp.gmail.com with ESMTPSA id f6-v6sm15382058pfg.128.2018.11.10.19.40.11 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 10 Nov 2018 19:40:11 -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 X-Mailer: iPhone Mail (16A404) In-Reply-To: <20181111023808.GA174670@google.com> Date: Sat, 10 Nov 2018 19:40:10 -0800 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 Content-Transfer-Encoding: quoted-printable Message-Id: <543A5181-3A16-438E-B372-97BEC48A74F8@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> <907D942E-E321-4BD7-BED7-ACD1D96A3643@amacapital.net> <20181111023808.GA174670@google.com> To: Joel Fernandes Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > On Nov 10, 2018, at 6:38 PM, Joel Fernandes wrote= : >=20 >> On Sat, Nov 10, 2018 at 02:18:23PM -0800, Andy Lutomirski wrote: >>=20 >>>> On Nov 10, 2018, at 2:09 PM, Joel Fernandes wr= ote: >>>>=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 wrote: >>>>>> 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 wo= rks >>>>>> 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 WR= ITE >>>>>> opens as Daniel suggested. I will make this change so that the securi= ty fix >>>>>> is a clean one. >>>>>>=20 >>>>>>> - does *not* affect other struct files that may already exist on the= same 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 ov= er IPC >>>>>> to another process and we want to prevent any new writes in the recei= ver >>>>>> side. There is no way this other receiving process can have an existi= ng fd >>>>>> unless it was already sent one without the seal applied. The propose= d seal >>>>>> could be renamed to F_SEAL_FD_WRITE if that is preferred. >>>>>>=20 >>>>>>> - mysteriously malfunctions if you try to set it again on another st= ruct >>>>>>> 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 memo= ry map. >>>>>> So would you call that a security issue too? The use of the seal want= s to >>>>>> allow existing mmap regions to be continue to be written into (I ment= ioned >>>>>> 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 th= e inode flag >>>>>>> work by itself. >>>>>>=20 >>>>>> Currently, the various VFS paths check only the struct file's f_mode t= o deny >>>>>> writes of already opened files. This would mean more checking in all t= hose >>>>>> 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 decr= ements >>>>>> the inode::i_writecount. >>>>>> 2. call get_write_access(inode) in the various VFS paths in addition t= o >>>>>> checking for FMODE_*WRITE and deny the write (incase i_writecount is n= egative) >>>>>>=20 >>>>>> That will prevent both reopens, and writes from succeeding. However I= worry 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 think= ing >>> modifying i_mode may do the trick but as you pointed it probably could b= e >>> 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 >>=20 >> This seems considerably more complicated and more fragile than needed. Ju= st >> add a new F_SEAL_WRITE_FUTURE. Grep for F_SEAL_WRITE and make the _FUTUR= E >> variant work exactly like it with two exceptions: >>=20 >> - shmem_mmap and maybe its hugetlbfs equivalent should check for it and a= ct >> accordingly. >=20 > There's more to it than that, we also need to block future writes through > write syscall, so we have to hook into the write path too once the seal is= > set, not just the mmap. That means we have to add code in mm/shmem.c to do= > that in all those handlers, to check for the seal (and hope we didn't miss= a > file_operations handler). Is that what you are proposing? The existing code already does this. That=E2=80=99s why I suggested grepping= :) >=20 > Also, it means we have to keep CONFIG_TMPFS enabled so that the > shmem_file_operations write handlers like write_iter are hooked up. Curren= tly > memfd works even with !CONFIG_TMPFS. If so, that sounds like it may already be a bug. >=20 >> - add_seals won=E2=80=99t need the wait_for_pins and mapping_deny_write l= ogic. >>=20 >> That really should be all that=E2=80=99s needed. >=20 > It seems a fair idea what you're saying. But I don't see how its less > complex.. IMO its far more simple to have VFS do the denial of the operati= ons > based on the flags of its datastructures.. and if it works (which I will t= est > to be sure it will), then we should be good. I agree it=E2=80=99s complicated, but the code is already written. You shou= ld just need to adjust some masks. >=20 > Btw by any chance, are you also coming by LPC conference next week? >=20 No. I=E2=80=99d like to, but I can=E2=80=99t make the trip this year. > thanks! >=20 > - Joel >=20