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Subject: Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20181111023808.GA174670@google.com>
Date:   Sat, 10 Nov 2018 19:40:10 -0800
Cc:     Daniel Colascione <dancol@google.com>,
        Jann Horn <jannh@google.com>,
        kernel list <linux-kernel@vger.kernel.org>,
        John Reck <jreck@google.com>,
        John Stultz <john.stultz@linaro.org>,
        Todd Kjos <tkjos@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Christoph Hellwig <hch@infradead.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Bruce Fields <bfields@fieldses.org>,
        Jeff Layton <jlayton@kernel.org>,
        Khalid Aziz <khalid.aziz@oracle.com>, Lei.Yang@windriver.com,
        linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        Linux-MM <linux-mm@kvack.org>, marcandre.lureau@redhat.com,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Minchan Kim <minchan@kernel.org>,
        Shuah Khan <shuah@kernel.org>,
        Valdis Kletnieks <valdis.kletnieks@vt.edu>,
        Hugh Dickins <hughd@google.com>,
        Linux API <linux-api@vger.kernel.org>
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References: <20181108041537.39694-1-joel@joelfernandes.org> <CAG48ez1h=v-JYnDw81HaYJzOfrNhwYksxmc2r=cJvdQVgYM+NA@mail.gmail.com> <CAG48ez0kQ4d566bXTFOYANDgii-stL-Qj-oyaBzvfxdV=PU-7g@mail.gmail.com> <20181110032005.GA22238@google.com> <69CE06CC-E47C-4992-848A-66EB23EE6C74@amacapital.net> <20181110182405.GB242356@google.com> <CAKOZuesQXRtthJTEr86LByH3gPpAdT-PQM0d1jqr131=zZNRKw@mail.gmail.com> <CAKOZueum8MtNvJ5P=W7_pRw62TdQdCgyjCwwbG1wezNboC1cxQ@mail.gmail.com> <20181110220933.GB96924@google.com> <907D942E-E321-4BD7-BED7-ACD1D96A3643@amacapital.net> <20181111023808.GA174670@google.com>
To:     Joel Fernandes <joel@joelfernandes.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Nov 10, 2018, at 6:38 PM, Joel Fernandes <joel@joelfernandes.org> 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 <joel@joelfernandes.org> 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 <dancol@google.co=
m> wrote:
>>>>>> On Sat, Nov 10, 2018 at 10:24 AM, Joel Fernandes <joel@joelfernandes.=
org> 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