Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd

[Daniel Colascione <dancol@google.com>]

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.