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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: <CAKOZueum8MtNvJ5P=W7_pRw62TdQdCgyjCwwbG1wezNboC1cxQ@mail.gmail.com>
Date:   Sat, 10 Nov 2018 11:55:10 -0800
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        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>
To:     Daniel Colascione <dancol@google.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Nov 10, 2018, at 11:11 AM, Daniel Colascione <dancol@google.com> wrote:=

>=20
>> On Sat, Nov 10, 2018 at 10:45 AM, Daniel Colascione <dancol@google.com> w=
rote:
>>> 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 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.=