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

[Andy Lutomirski]

> 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.

- add_seals wonât need the wait_for_pins and mapping_deny_write logic.

That really should be all thatâs needed.