From: Andy Lutomirski <luto@amacapital.net>
To: Joel Fernandes <joel@joelfernandes.org>
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>
Subject: Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd
Date: Sat, 10 Nov 2018 14:18:23 -0800 [thread overview]
Message-ID: <907D942E-E321-4BD7-BED7-ACD1D96A3643@amacapital.net> (raw)
In-Reply-To: <20181110220933.GB96924@google.com>
> On Nov 10, 2018, at 2:09 PM, Joel Fernandes <joel@joelfernandes.org> wrote:
>
>> 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> 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 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.
>>>>
>>>>> - causes a probably-observable effect in the file mode in F_GETFL.
>>>>
>>>> Wouldn't that be the right thing to observe anyway?
>>>>
>>>>> - causes reopen to fail.
>>>>
>>>> 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 fix
>>>> is a clean one.
>>>>
>>>>> - does *not* affect other struct files that may already exist on the same inode.
>>>>
>>>> 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 receiver
>>>> 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 seal
>>>> could be renamed to F_SEAL_FD_WRITE if that is preferred.
>>>>
>>>>> - mysteriously malfunctions if you try to set it again on another struct
>>>>> file that already exists
>>>>>
>>>>
>>>> I didn't follow this, could you explain more?
>>>>
>>>>> - probably is insecure when used on hugetlbfs.
>>>>
>>>> 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 to
>>>> allow existing mmap regions to be continue to be written into (I mentioned
>>>> more details in the cover letter).
>>>>
>>>>> I see two reasonable solutions:
>>>>>
>>>>> 1. Don’t fiddle with the struct file at all. Instead make the inode flag
>>>>> work by itself.
>>>>
>>>> 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 those
>>>> paths (and modification of all those paths).
>>>>
>>>> Anyway going with that idea, we could
>>>> 1. call deny_write_access(file) from the memfd's seal path which decrements
>>>> 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 negative)
>>>>
>>>> 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.
>>>
>>> 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?
>>
>> 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.
>
> Agreed with the idea of modifying both file and inode flags. I was thinking
> modifying i_mode may do the trick but as you pointed it probably could be
> reverted by chmod or some other attribute setting calls.
>
> 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.
>
>
This seems considerably more complicated and more fragile than needed. Just add a new F_SEAL_WRITE_FUTURE. Grep for F_SEAL_WRITE and make the _FUTURE variant work exactly like it with two exceptions:
- shmem_mmap and maybe its hugetlbfs equivalent should check for it and act accordingly.
- add_seals won’t need the wait_for_pins and mapping_deny_write logic.
That really should be all that’s needed.
next prev parent reply other threads:[~2018-11-10 22:18 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-08 4:15 [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd Joel Fernandes (Google)
2018-11-08 4:15 ` [PATCH v3 resend 2/2] selftests/memfd: Add tests for F_SEAL_FUTURE_WRITE seal Joel Fernandes (Google)
2018-11-09 8:49 ` [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd Joel Fernandes
2018-11-09 20:36 ` Andrew Morton
2018-11-10 3:54 ` Joel Fernandes
2018-11-09 21:06 ` Jann Horn
2018-11-09 21:19 ` Jann Horn
2018-11-10 3:20 ` Joel Fernandes
2018-11-10 6:05 ` Andy Lutomirski
2018-11-10 18:24 ` Joel Fernandes
2018-11-10 18:45 ` Daniel Colascione
2018-11-10 19:11 ` Daniel Colascione
2018-11-10 19:55 ` Andy Lutomirski
2018-11-10 22:09 ` Joel Fernandes
2018-11-10 22:18 ` Andy Lutomirski [this message]
2018-11-11 2:38 ` Joel Fernandes
2018-11-11 3:40 ` Andy Lutomirski
2018-11-11 4:01 ` Joel Fernandes
2018-11-11 8:09 ` Joel Fernandes
2018-11-11 8:30 ` Daniel Colascione
2018-11-11 15:14 ` Andy Lutomirski
2018-11-11 17:36 ` Joel Fernandes
[not found] ` <CAKOZuethC7+YrRyyGciUCfhSSa9cCcAFJ8g_qEw9uh3TBbyOcg@mail.gmail.com>
2018-11-10 17:10 ` Joel Fernandes
2018-11-09 21:40 ` Andy Lutomirski
2018-11-09 20:02 ` Michael Tirado
2018-11-10 1:49 ` Joel Fernandes
2018-11-09 22:20 ` Daniel Colascione
2018-11-09 22:37 ` Andy Lutomirski
2018-11-09 22:42 ` Daniel Colascione
2018-11-09 23:14 ` Andy Lutomirski
2018-11-10 1:36 ` Joel Fernandes
2018-11-09 23:46 ` Joel Fernandes
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