linux-kernel - Re: [PATCH v5] fuse: Add support for passthrough read/write

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Date:	Wed, 3 Feb 2016 20:56:44 +0100
From:	Jann Horn <jann@...jh.net>
To:	Nikhilesh Reddy <reddyn@...eaurora.org>
Cc:	torvalds@...ux-foundation.org, Miklos Szeredi <miklos@...redi.hu>,
	fuse-devel <fuse-devel@...ts.sourceforge.net>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	gregkh@...uxfoundation.org, linux-fsdevel@...r.kernel.org,
	viro@...iv.linux.org.uk, Richard Weinberger <richard@....at>,
	Theodore Ts'o <tytso@....edu>, jack@...e.cz,
	Antonio SJ Musumeci <trapexit@...wn.link>, sven.utcke@....de,
	Nikolaus Rath <nikolaus@...h.org>,
	Jann Horn <jannhorn@...glemail.com>,
	Mike Shal <marfey@...il.com>
Subject: Re: [PATCH v5] fuse: Add support for passthrough read/write

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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