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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: <20181110220933.GB96924@google.com>
Date:   Sat, 10 Nov 2018 14:18:23 -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>
To:     Joel Fernandes <joel@joelfernandes.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Nov 10, 2018, at 2:09 PM, Joel Fernandes <joel@joelfernandes.org> 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 <dancol@google.com> w=
rote:
>>>> On Sat, Nov 10, 2018 at 10:24 AM, Joel Fernandes <joel@joelfernandes.or=
g> 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.=