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

[Joel Fernandes]

On Sat, Nov 10, 2018 at 07:40:10PM -0800, Andy Lutomirski wrote:
> 
> 
> > 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 :)

Ahh sorry I see your point now. Ok let me try this approach. Thank you!
Probably we can make this work this way and it is sufficient.

> > 
> > 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.

Actually, its not a bug. If CONFIG_TMPFS is disabled, then IIRC write syscall
will be prevented anyway and then the mmap is the only way. I'll double check
that once I work on this idea.

> > 
> >> - 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.
> 

Right.

> > 
> > 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.

Ok, no worries.

thanks,

- Joel