Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode

Al Viro
2017-05-10T02:45:24Z

On Wed, May 10, 2017 at 03:11:18AM +0100, Al Viro wrote:
> On Tue, May 09, 2017 at 09:03:22AM -0700, Christoph Hellwig wrote:
> > On Tue, May 09, 2017 at 06:02:50AM -0700, Christoph Hellwig wrote:
> > > On Tue, May 09, 2017 at 06:00:01AM -0700, Andy Lutomirski wrote:
> > > > fs/splice.c has some, ahem, interesting uses that have been the source
> > > > of nasty exploits in the past. Converting them to use iov_iter
> > > > properly would be really, really nice. Christoph, I don't suppose
> > > > you'd like to do that?
> > >
> > > I can take care of all the fs code including this one.
>
> Oh?
>
> > I spent the afternoon hacking up where I'd like this to head. It's
> > completely untested as of now:
> >
> > http://git.infradead.org/users/hch/vfs.git/shortlog/refs/heads/setfs-elimination
>
> And just what happens to driver that has no ->read_iter()? Unless I'm
> seriously misreading that, NAK with extreme prejudice.

FWIW, some parts of that queue are obviously sane; it's the conversions of
kernel_write() and friends to ->read_iter/->write_iter() that are non-starters.
That stuff is used in too many situations; we can't guarantee that all of
them will be for files that have those.

As for default_file_splice_read(), I seriously suspect that with your change
we could as well just make it return -EINVAL and be done with that; places
that have ->read_iter() tend to have explicit ->splice_read() and it looks
like the ones that do not should simply use generic_file_read_iter().
I hadn't checked that, but there's not a lot of those:

arch/s390/hypfs/inode.c:437: .read_iter = hypfs_read_iter,
drivers/char/mem.c:798: .read_iter = read_iter_null,
drivers/char/mem.c:813: .read_iter = read_iter_zero,
drivers/char/mem.c:824: .read_iter = read_iter_zero,
drivers/char/raw.c:286: .read_iter = blkdev_read_iter,
drivers/net/tap.c:1134: .read_iter = tap_read_iter,
drivers/net/tun.c:2423: .read_iter = tun_chr_read_iter,
drivers/usb/gadget/function/f_fs.c:1255: .read_iter = ffs_epfile_read_iter,
drivers/usb/gadget/legacy/inode.c:703: .read_iter = ep_read_iter,
drivers/vhost/net.c:1252: .read_iter = vhost_net_chr_read_iter,
fs/9p/vfs_file.c:641: .read_iter = generic_file_read_iter,
fs/9p/vfs_file.c:652: .read_iter = generic_file_read_iter,
fs/9p/vfs_file.c:664: .read_iter = v9fs_file_read_iter,
fs/9p/vfs_file.c:675: .read_iter = v9fs_file_read_iter,
fs/9p/vfs_file.c:687: .read_iter = v9fs_mmap_file_read_iter,
fs/9p/vfs_file.c:698: .read_iter = v9fs_mmap_file_read_iter,
fs/fuse/cuse.c:180: .read_iter = cuse_read_iter,
fs/fuse/file.c:3015: .read_iter = fuse_direct_read_iter,
fs/hugetlbfs/inode.c:980: .read_iter = hugetlbfs_read_iter,
fs/ncpfs/file.c:248: .read_iter = ncp_file_read_iter,
fs/orangefs/file.c:742: .read_iter = orangefs_file_read_iter,
fs/pipe.c:1011: .read_iter = pipe_read,
sound/core/pcm_native.c:3696: .read_iter = snd_pcm_readv,

is the full list and I'm fairly certain that most of them will work with
generic_file_splice_read() just fine. drivers/char definitely will, so
will ncpfs/orangefs/hugetlbfs/most of 9p ones (two of the latter might
need some care in p9_client_read(), but that should be doable easily enough).
pipe is irrelevant (->splice_read() won't be called for those). fuse ones
should be doable, but that might take a bit more infrastructure work in
lib/iov_iter.c. vhost, gadgetfs, tun/tap - no idea at the moment.