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

[Joel Fernandes <joel@joelfernandes.org>]

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@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.

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.

thanks,

 - Joel