Re: [RFC PATCH 2/3] fs: add RWF_ENCODED for writing compressed data

[Omar Sandoval <osandov@osandov.com>]

On Wed, Sep 25, 2019 at 05:11:29PM +1000, Dave Chinner wrote:
> On Tue, Sep 24, 2019 at 10:01:41PM +0200, Jann Horn wrote:
> > On Tue, Sep 24, 2019 at 9:35 PM Omar Sandoval <osandov@osandov.com> wrote:
> > > On Tue, Sep 24, 2019 at 10:15:13AM -0700, Omar Sandoval wrote:
> > > > On Thu, Sep 19, 2019 at 05:44:12PM +0200, Jann Horn wrote:
> > > > > On Thu, Sep 19, 2019 at 8:54 AM Omar Sandoval <osandov@osandov.com> wrote:
> > > > > > Btrfs can transparently compress data written by the user. However, we'd
> > > > > > like to add an interface to write pre-compressed data directly to the
> > > > > > filesystem. This adds support for so-called "encoded writes" via
> > > > > > pwritev2().
> > > > > >
> > > > > > A new RWF_ENCODED flags indicates that a write is "encoded". If this
> > > > > > flag is set, iov[0].iov_base points to a struct encoded_iov which
> > > > > > contains metadata about the write: namely, the compression algorithm and
> > > > > > the unencoded (i.e., decompressed) length of the extent. iov[0].iov_len
> > > > > > must be set to sizeof(struct encoded_iov), which can be used to extend
> > > > > > the interface in the future. The remaining iovecs contain the encoded
> > > > > > extent.
> > > > > >
> > > > > > A similar interface for reading encoded data can be added to preadv2()
> > > > > > in the future.
> > > > > >
> > > > > > Filesystems must indicate that they support encoded writes by setting
> > > > > > FMODE_ENCODED_IO in ->file_open().
> > > > > [...]
> > > > > > +int import_encoded_write(struct kiocb *iocb, struct encoded_iov *encoded,
> > > > > > +                        struct iov_iter *from)
> > > > > > +{
> > > > > > +       if (iov_iter_single_seg_count(from) != sizeof(*encoded))
> > > > > > +               return -EINVAL;
> > > > > > +       if (copy_from_iter(encoded, sizeof(*encoded), from) != sizeof(*encoded))
> > > > > > +               return -EFAULT;
> > > > > > +       if (encoded->compression == ENCODED_IOV_COMPRESSION_NONE &&
> > > > > > +           encoded->encryption == ENCODED_IOV_ENCRYPTION_NONE) {
> > > > > > +               iocb->ki_flags &= ~IOCB_ENCODED;
> > > > > > +               return 0;
> > > > > > +       }
> > > > > > +       if (encoded->compression > ENCODED_IOV_COMPRESSION_TYPES ||
> > > > > > +           encoded->encryption > ENCODED_IOV_ENCRYPTION_TYPES)
> > > > > > +               return -EINVAL;
> > > > > > +       if (!capable(CAP_SYS_ADMIN))
> > > > > > +               return -EPERM;
> > > > >
> > > > > How does this capable() check interact with io_uring? Without having
> > > > > looked at this in detail, I suspect that when an encoded write is
> > > > > requested through io_uring, the capable() check might be executed on
> > > > > something like a workqueue worker thread, which is probably running
> > > > > with a full capability set.
> > > >
> > > > I discussed this more with Jens. You're right, per-IO permission checks
> > > > aren't going to work. In fully-polled mode, we never get an opportunity
> > > > to check capabilities in right context. So, this will probably require a
> > > > new open flag.
> > >
> > > Actually, file_ns_capable() accomplishes the same thing without a new
> > > open flag. Changing the capable() check to file_ns_capable() in
> > > init_user_ns should be enough.
> > 
> > +Aleksa for openat2() and open() space
> > 
> > Mmh... but if the file descriptor has been passed through a privilege
> > boundary, it isn't really clear whether the original opener of the
> > file intended for this to be possible. For example, if (as a
> > hypothetical example) the init process opens a service's logfile with
> > root privileges, then passes the file descriptor to that logfile to
> > the service on execve(), that doesn't mean that the service should be
> > able to perform compressed writes into that file, I think.
> 
> Where's the privilege boundary that is being crossed?
> 
> We're talking about user data read/write access here, not some
> special security capability. Access to the data has already been
> permission checked, so why should the format that the data is
> supplied to the kernel in suddenly require new privilege checks?
> 
> i.e. writing encoded data to a file requires exactly the same
> access permissions as writing cleartext data to the file. The only
> extra information here is whether the _filesystem_ supports encoded
> data, and that doesn't change regardless of what the open file gets
> passed to. Hence the capability is either there or it isn't, it
> doesn't transform not matter what privilege boundary the file is
> passed across. Similarly, we have permission to access the data
> or we don't through the struct file, it doesn't transform either.
> 
> Hence I don't see why CAP_SYS_ADMIN or any special permissions are
> needed for an application with access permissions to file data to
> use these RWF_ENCODED IO interfaces. I am inclined to think the
> permission check here is wrong and should be dropped, and then all
> these issues go away.
> 
> Yes, the app that is going to use this needs root perms because it
> accesses all data in the fs (it's a backup app!), but that doesn't
> mean you can only use RWF_ENCODED if you have root perms.

For RWF_ENCODED writes, the risk here is that we'd be adding a way for
an unprivileged process to feed arbitrary data to zlib/lzo/zstd in the
kernel. From what I could find, this is a new attack surface for
unprivileged processes, and based on the search results for
"$compression_algorithm CVE", there are real bugs here.

For RWF_ENCODED reads, there's another potential issue that occurred to
me. There are a few operations for which we may need to chop up a
compressed extent: hole punch, truncate, reflink, and dedupe. Rather
than recompressing the data, Btrfs keeps the whole extent on disk and
updates the file metadata to refer to a piece of the extent. If we want
to support RWF_ENCODED reads for such extents (and I think we do), then
we need to return the entire original extent along with that metadata.
For an unprivileged reader, there's a security issue that we may be
returning data that the reader wasn't supposed to see. (A privileged
reader can go and read the block device anyways.)

So, in my opinion, both reads and writes should require privilege just
to be on the safe side.