On Mon, Feb 01, 2016 at 10:56:27AM -0800, Nikhilesh Reddy wrote:
> Add support for filesystem passthrough read/write of files
> when enabled in userspace through the option FUSE_PASSTHROUGH.
[...]
> 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);
Uh... how do you know at this point that the file is actually writable?
Normally, e.g. vfs_write() will ensure that the file is writable, and
e.g. generic_file_write_iter() won't check for writability as far as I
can tell. This might allow someone to use the passthrough mechanism to
overwrite a file he is only allowed to read, but not write, like
/etc/passwd.
Also, I think this might bypass mandatory locks, the
security_file_permission hook (which seems like a bad idea anyway
though), inotify/fsnotify and sb_start_write.
> +
> + 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);
> +
> + return ret_val;
> +}
> +
> +ssize_t fuse_passthrough_read_iter(struct kiocb *iocb, struct iov_iter *to)
> +{
> + return fuse_passthrough_read_write_iter(iocb, to, 0);
> +}
> +
> +ssize_t fuse_passthrough_write_iter(struct kiocb *iocb, struct iov_iter
> *from)
> +{
> + return fuse_passthrough_read_write_iter(iocb, from, 1);
> +}
> +
> +void fuse_passthrough_release(struct fuse_file *ff)
> +{
> + if (!(ff->passthrough_filp))
> + return;
> +
> + /* Release the passthrough file. */
> + fput(ff->passthrough_filp);
> + ff->passthrough_filp = NULL;
> +}
> diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
> index c9aca04..a08933a 100644
> --- a/include/uapi/linux/fuse.h
> +++ b/include/uapi/linux/fuse.h
> @@ -250,6 +250,7 @@ struct fuse_file_lock {
> #define FUSE_ASYNC_DIO (1 << 15)
> #define FUSE_WRITEBACK_CACHE (1 << 16)
> #define FUSE_NO_OPEN_SUPPORT (1 << 17)
> +#define FUSE_PASSTHROUGH (1 << 18)
>
> /**
> * CUSE INIT request/reply flags
> @@ -480,7 +481,7 @@ struct fuse_create_in {
> struct fuse_open_out {
> uint64_t fh;
> uint32_t open_flags;
> - uint32_t padding;
> + int32_t passthrough_fd;
> };
>
> struct fuse_release_in {
> --
> 1.8.2.1
>
>
> --
> Thanks
> Nikhilesh Reddy
>
> Qualcomm Innovation Center, Inc.
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> a Linux Foundation Collaborative Project.
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