Re: slab-out-of-bounds in iov_iter_revert()

[Qian Cai]

On Thu, 2020-09-17 at 19:45 +0100, Al Viro wrote:
> On Thu, Sep 17, 2020 at 01:42:44PM -0400, Qian Cai wrote:
> > > 	How much IO does it take to trigger that on your reproducer?
> > 
> > That is something I don't know for sure because it is always reproducible by
> > running the trinity fuzzer for a few seconds (32 threads). I did another run
> > below (still with your patch applied) and then tried to capture some logs
> > here:
> > 
> > http://people.redhat.com/qcai/iov_iter_revert/
> 
> FWIW, there were several bugs in that patch:
> 	* 'shortened' possibly left uninitialized
> 	* possible error returns with reexpand not done
> 
> Could you try this instead?

This works fine. Thanks for taking care of this, Al.

> 
> Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> ---
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 6611ef3269a8..43c165e796da 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -3091,11 +3091,10 @@ fuse_direct_IO(struct kiocb *iocb, struct iov_iter
> *iter)
>  	ssize_t ret = 0;
>  	struct file *file = iocb->ki_filp;
>  	struct fuse_file *ff = file->private_data;
> -	bool async_dio = ff->fc->async_dio;
>  	loff_t pos = 0;
>  	struct inode *inode;
>  	loff_t i_size;
> -	size_t count = iov_iter_count(iter);
> +	size_t count = iov_iter_count(iter), shortened = 0;
>  	loff_t offset = iocb->ki_pos;
>  	struct fuse_io_priv *io;
>  
> @@ -3103,17 +3102,9 @@ fuse_direct_IO(struct kiocb *iocb, struct iov_iter
> *iter)
>  	inode = file->f_mapping->host;
>  	i_size = i_size_read(inode);
>  
> -	if ((iov_iter_rw(iter) == READ) && (offset > i_size))
> +	if ((iov_iter_rw(iter) == READ) && (offset >= i_size))
>  		return 0;
>  
> -	/* optimization for short read */
> -	if (async_dio && iov_iter_rw(iter) != WRITE && offset + count > i_size)
> {
> -		if (offset >= i_size)
> -			return 0;
> -		iov_iter_truncate(iter, fuse_round_up(ff->fc, i_size - offset));
> -		count = iov_iter_count(iter);
> -	}
> -
>  	io = kmalloc(sizeof(struct fuse_io_priv), GFP_KERNEL);
>  	if (!io)
>  		return -ENOMEM;
> @@ -3129,15 +3120,22 @@ fuse_direct_IO(struct kiocb *iocb, struct iov_iter
> *iter)
>  	 * By default, we want to optimize all I/Os with async request
>  	 * submission to the client filesystem if supported.
>  	 */
> -	io->async = async_dio;
> +	io->async = ff->fc->async_dio;
>  	io->iocb = iocb;
>  	io->blocking = is_sync_kiocb(iocb);
>  
> +	/* optimization for short read */
> +	if (io->async && !io->write && offset + count > i_size) {
> +		iov_iter_truncate(iter, fuse_round_up(ff->fc, i_size - offset));
> +		shortened = count - iov_iter_count(iter);
> +		count -= shortened;
> +	}
> +
>  	/*
>  	 * We cannot asynchronously extend the size of a file.
>  	 * In such case the aio will behave exactly like sync io.
>  	 */
> -	if ((offset + count > i_size) && iov_iter_rw(iter) == WRITE)
> +	if ((offset + count > i_size) && io->write)
>  		io->blocking = true;
>  
>  	if (io->async && io->blocking) {
> @@ -3155,6 +3153,7 @@ fuse_direct_IO(struct kiocb *iocb, struct iov_iter
> *iter)
>  	} else {
>  		ret = __fuse_direct_read(io, iter, &pos);
>  	}
> +	iov_iter_reexpand(iter, iov_iter_count(iter) + shortened);
>  
>  	if (io->async) {
>  		bool blocking = io->blocking;
>