Code: struct bio *bio_map_user_iov(struct request_queue *q,
			     const struct iov_iter *iter,
			     gfp_t gfp_mask)
{
	int j;
	int nr_pages = 0;
	struct page **pages;
	struct bio *bio;
	int cur_page = 0;
	int ret, offset;
	struct iov_iter i;
	struct iovec iov;

	iov_for_each(iov, i, *iter) {
		unsigned long uaddr = (unsigned long) iov.iov_base;
		unsigned long len = iov.iov_len;
		unsigned long end = (uaddr + len + PAGE_SIZE - 1) >> PAGE_SHIFT;
		unsigned long start = uaddr >> PAGE_SHIFT;

		/*
		 * Overflow, abort
		 */
		if (end < start)
			return ERR_PTR(-EINVAL);

		nr_pages += end - start;
		/*
		 * buffer must be aligned to at least logical block size for now
		 */
		if (uaddr & queue_dma_alignment(q))
			return ERR_PTR(-EINVAL);
	}

	if (!nr_pages)
		return ERR_PTR(-EINVAL);

	bio = bio_kmalloc(gfp_mask, nr_pages);
	if (!bio)
		return ERR_PTR(-ENOMEM);

	ret = -ENOMEM;
	pages = kcalloc(nr_pages, sizeof(struct page *), gfp_mask);
	if (!pages)
		goto out;

	iov_for_each(iov, i, *iter) {
		unsigned long uaddr = (unsigned long) iov.iov_base;
		unsigned long len = iov.iov_len;
		unsigned long end = (uaddr + len + PAGE_SIZE - 1) >> PAGE_SHIFT;
		unsigned long start = uaddr >> PAGE_SHIFT;
		const int local_nr_pages = end - start;
		const int page_limit = cur_page + local_nr_pages;

		ret = get_user_pages_fast(uaddr, local_nr_pages,
				(iter->type & WRITE) != WRITE,
				&pages[cur_page]);
		if (ret < local_nr_pages) {
			ret = -EFAULT;
			goto out_unmap;
		}

 		offset = offset_in_page(uaddr);
 		for (j = cur_page; j < page_limit; j++) {
 			unsigned int bytes = PAGE_SIZE - offset;
 
 			if (len <= 0)
 				break;
			
			if (bytes > len)
				bytes = len;

			/*
			 * sorry...
			 */
			if (bio_add_pc_page(q, bio, pages[j], bytes, offset) <
 					    bytes)
 				break;
 
 			len -= bytes;
 			offset = 0;
 		}

		cur_page = j;
		/*
		 * release the pages we didn't map into the bio, if any
		 */
		while (j < page_limit)
			put_page(pages[j++]);
	}

	kfree(pages);

	bio_set_flag(bio, BIO_USER_MAPPED);

	/*
	 * subtle -- if bio_map_user_iov() ended up bouncing a bio,
	 * it would normally disappear when its bi_end_io is run.
	 * however, we need it for the unmap, so grab an extra
	 * reference to it
	 */
	bio_get(bio);
	return bio;

 out_unmap:
	for (j = 0; j < nr_pages; j++) {
		if (!pages[j])
			break;
		put_page(pages[j]);
	}
 out:
	kfree(pages);
	bio_put(bio);
	return ERR_PTR(ret);
}

Vulnerability Type: 
CWE ID: CWE-772
Summary: The bio_map_user_iov and bio_unmap_user functions in block/bio.c in the Linux kernel before 4.13.8 do unbalanced refcounting when a SCSI I/O vector has small consecutive buffers belonging to the same page. The bio_add_pc_page function merges them into one, but the page reference is never dropped. This causes a memory leak and possible system lockup (exploitable against the host OS by a guest OS user, if a SCSI disk is passed through to a virtual machine) due to an out-of-memory condition.
Commit Message: fix unbalanced page refcounting in bio_map_user_iov

bio_map_user_iov and bio_unmap_user do unbalanced pages refcounting if
IO vector has small consecutive buffers belonging to the same page.
bio_add_pc_page merges them into one, but the page reference is never
dropped.

Cc: stable@vger.kernel.org
Signed-off-by: Vitaly Mayatskikh <v.mayatskih@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>