Code: static ssize_t fuse_dev_splice_write(struct pipe_inode_info *pipe,
				     struct file *out, loff_t *ppos,
				     size_t len, unsigned int flags)
{
	unsigned nbuf;
	unsigned idx;
	struct pipe_buffer *bufs;
	struct fuse_copy_state cs;
	struct fuse_dev *fud;
	size_t rem;
	ssize_t ret;

	fud = fuse_get_dev(out);
	if (!fud)
		return -EPERM;

	pipe_lock(pipe);

	bufs = kvmalloc_array(pipe->nrbufs, sizeof(struct pipe_buffer),
			      GFP_KERNEL);
	if (!bufs) {
		pipe_unlock(pipe);
		return -ENOMEM;
	}

	nbuf = 0;
	rem = 0;
	for (idx = 0; idx < pipe->nrbufs && rem < len; idx++)
 		rem += pipe->bufs[(pipe->curbuf + idx) & (pipe->buffers - 1)].len;
 
 	ret = -EINVAL;
	if (rem < len) {
		pipe_unlock(pipe);
		goto out;
	}
 
 	rem = len;
 	while (rem) {
		struct pipe_buffer *ibuf;
		struct pipe_buffer *obuf;

		BUG_ON(nbuf >= pipe->buffers);
		BUG_ON(!pipe->nrbufs);
		ibuf = &pipe->bufs[pipe->curbuf];
		obuf = &bufs[nbuf];

		if (rem >= ibuf->len) {
			*obuf = *ibuf;
			ibuf->ops = NULL;
 			pipe->curbuf = (pipe->curbuf + 1) & (pipe->buffers - 1);
 			pipe->nrbufs--;
 		} else {
			pipe_buf_get(pipe, ibuf);
 			*obuf = *ibuf;
 			obuf->flags &= ~PIPE_BUF_FLAG_GIFT;
 			obuf->len = rem;
			ibuf->offset += obuf->len;
			ibuf->len -= obuf->len;
		}
		nbuf++;
		rem -= obuf->len;
	}
	pipe_unlock(pipe);

	fuse_copy_init(&cs, 0, NULL);
	cs.pipebufs = bufs;
	cs.nr_segs = nbuf;
	cs.pipe = pipe;

	if (flags & SPLICE_F_MOVE)
		cs.move_pages = 1;

 	ret = fuse_dev_do_write(fud, &cs, len);
 
 	pipe_lock(pipe);
 	for (idx = 0; idx < nbuf; idx++)
 		pipe_buf_release(pipe, &bufs[idx]);
 	pipe_unlock(pipe);
 
out:
 	kvfree(bufs);
 	return ret;
 }

Vulnerability Type: Overflow 
CWE ID: CWE-416
Summary: The Linux kernel before 5.1-rc5 allows page->_refcount reference count overflow, with resultant use-after-free issues, if about 140 GiB of RAM exists. This is related to fs/fuse/dev.c, fs/pipe.c, fs/splice.c, include/linux/mm.h, include/linux/pipe_fs_i.h, kernel/trace/trace.c, mm/gup.c, and mm/hugetlb.c. It can occur with FUSE requests.
Commit Message: Merge branch 'page-refs' (page ref overflow)

Merge page ref overflow branch.

Jann Horn reported that he can overflow the page ref count with
sufficient memory (and a filesystem that is intentionally extremely
slow).

Admittedly it's not exactly easy.  To have more than four billion
references to a page requires a minimum of 32GB of kernel memory just
for the pointers to the pages, much less any metadata to keep track of
those pointers.  Jann needed a total of 140GB of memory and a specially
crafted filesystem that leaves all reads pending (in order to not ever
free the page references and just keep adding more).

Still, we have a fairly straightforward way to limit the two obvious
user-controllable sources of page references: direct-IO like page
references gotten through get_user_pages(), and the splice pipe page
duplication.  So let's just do that.

* branch page-refs:
  fs: prevent page refcount overflow in pipe_buf_get
  mm: prevent get_user_pages() from overflowing page refcount
  mm: add 'try_get_page()' helper function
  mm: make page ref count overflow check tighter and more explicit