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

[Andy Lutomirski <luto@amacapital.net>]

> On Nov 10, 2018, at 11:11 AM, Daniel Colascione <dancol@google.com> 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.

This should be straightforward. Just add a new seal type and wire it up. It should be considerably simpler than SEAL_WRITE.