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

[Andy Lutomirski]

> On Nov 10, 2018, at 6:38 PM, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> 
>> On Sat, Nov 10, 2018 at 02:18:23PM -0800, Andy Lutomirski wrote:
>> 
>>>> On Nov 10, 2018, at 2:09 PM, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx> wrote:
>>>>>> On Sat, Nov 10, 2018 at 10:24 AM, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> 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. Donât 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.
> 
> There's more to it than that, we also need to block future writes through
> write syscall, so we have to hook into the write path too once the seal is
> set, not just the mmap. That means we have to add code in mm/shmem.c to do
> that in all those handlers, to check for the seal (and hope we didn't miss a
> file_operations handler). Is that what you are proposing?

The existing code already does this. Thatâs why I suggested grepping :)

> 
> Also, it means we have to keep CONFIG_TMPFS enabled so that the
> shmem_file_operations write handlers like write_iter are hooked up. Currently
> memfd works even with !CONFIG_TMPFS.

If so, that sounds like it may already be a bug.

> 
>> - add_seals wonât need the wait_for_pins and mapping_deny_write logic.
>> 
>> That really should be all thatâs needed.
> 
> It seems a fair idea what you're saying. But I don't see how its less
> complex.. IMO its far more simple to have VFS do the denial of the operations
> based on the flags of its datastructures.. and if it works (which I will test
> to be sure it will), then we should be good.

I agree itâs complicated, but the code is already written.  You should just need to adjust some masks.

> 
> Btw by any chance, are you also coming by LPC conference next week?
> 

No.  Iâd like to, but I canât make the trip this year.

> thanks!
> 
> - Joel
>