On Wed, Feb 03, 2016 at 11:05:32AM -0800, Nikhilesh Reddy wrote:
> On 02/01/2016 11:45 AM, Jann Horn wrote:
> >On Mon, Feb 01, 2016 at 11:28:51AM -0800, Nikhilesh Reddy wrote:
> >>On Mon 01 Feb 2016 11:15:56 AM PST, Jann Horn wrote:
> >>>On Mon, Feb 01, 2016 at 10:56:27AM -0800, Nikhilesh Reddy wrote:
> >>>>diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c
> >>>[...]
> >>>>+static ssize_t fuse_passthrough_read_write_iter(struct kiocb *iocb,
> >>>>+ struct iov_iter *iter, int do_write)
> >>>>+{
> >>>>+ ssize_t ret_val;
> >>>>+ struct fuse_file *ff;
> >>>>+ struct file *fuse_file, *passthrough_filp;
> >>>>+ struct inode *fuse_inode, *passthrough_inode;
> >>>>+
> >>>>+ ff = iocb->ki_filp->private_data;
> >>>>+ fuse_file = iocb->ki_filp;
> >>>>+ passthrough_filp = ff->passthrough_filp;
> >>>>+
> >>>>+ /* lock passthrough file to prevent it from being released */
> >>>>+ get_file(passthrough_filp);
> >>>>+ iocb->ki_filp = passthrough_filp;
> >>>>+ fuse_inode = fuse_file->f_path.dentry->d_inode;
> >>>>+ passthrough_inode = file_inode(passthrough_filp);
> >>>>+
> >>>>+ if (do_write) {
> >>>>+ if (!passthrough_filp->f_op->write_iter)
> >>>>+ return -EIO;
> >>>>+ ret_val = passthrough_filp->f_op->write_iter(iocb, iter);
> >>>>+
> >>>>+ if (ret_val >= 0 || ret_val == -EIOCBQUEUED) {
> >>>>+ fsstack_copy_inode_size(fuse_inode, passthrough_inode);
> >>>>+ fsstack_copy_attr_times(fuse_inode, passthrough_inode);
> >>>>+ }
> >>>>+ } else {
> >>>>+ if (!passthrough_filp->f_op->read_iter)
> >>>>+ return -EIO;
> >>>>+ ret_val = passthrough_filp->f_op->read_iter(iocb, iter);
> >>>>+ if (ret_val >= 0 || ret_val == -EIOCBQUEUED)
> >>>>+ fsstack_copy_attr_atime(fuse_inode, passthrough_inode);
> >>>>+ }
> >>>>+
> >>>>+ iocb->ki_filp = fuse_file;
> >>>>+
> >>>>+ /* unlock passthrough file */
> >>>>+ fput(passthrough_filp);
> >>>
> >>>Why the get_file() and fput() in this method? This doesn't look right. There
> >>>is no lock you're releasing between get_file() and fput(). What are they
> >>>intended for?
> >>
> >>Hi
> >>
> >>Thanks for reviewing the code.
> >>
> >>The passthrough file could be released under our feet say if the userspace
> >>fuse daemon crashed or was killed ( while we are processing the read or the
> >>write) causing bad things to happen.
> >>The calls here are to increase the count temporarily and then decrease it
> >>so that we dont release in the middle of a write and everything is
> >>gracefully handled...
> >>
> >>I have a comment right before the get_file call above saying the same thing.
> >>Please let me know if you have any more questions.
> >
> >If that is the case, why can't the passthrough file be released before the
> >get_file() call, e.g. while the core processing the filesystem read request
> >is entering fuse_passthrough_read_write_iter()?
> >
> >As far as I can tell, you can drop the get_file() and fput() calls.
> >fuse_setup_passthrough() already took a reference to the file for you, that
> >reference can only be dropped in fuse_passthrough_release(), and the VFS
> >ensures that no release call happens while a read or write is pending.
> >
> I just feel uncomfortable with dropping them. I thought they could be
> released ( i/o ) takes longer than the actual execution... but if i can be
> sure of it then maybe..
These get_file() and fput() calls aren't useful.
And I think they can lead to a reference count leak, which would lead to a
use-after-free vulnerability on 32bit kernels, because you forgot to fput()
in the error cases where you return -EIO.
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