Commit message: sg_start_req(): make sure that there's not too many elements in iovec

unfortunately, allowing an arbitrary 16bit value means a possibility of
overflow in the calculation of total number of pages in bio_map_user_iov() -
we rely on there being no more than PAGE_SIZE members of sum in the
first loop there.  If that sum wraps around, we end up allocating
too small array of pointers to pages and it's easy to overflow it in
the second loop.

X-Coverup: TINC (and there's no lumber cartel either)
Cc: stable@vger.kernel.org # way, way back
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
CVE: CVE-2015-5707
CWE: CWE-189
Vulnerability description: Integer overflow in the sg_start_req function in drivers/scsi/sg.c in the Linux kernel 2.6.x through 4.x before 4.1 allows local users to cause a denial of service or possibly have unspecified other impact via a large iov_count value in a write request.

Project: linux
File: sg.c

sg_start_req(Sg_request *srp, unsigned char *cmd)
{
	int res;
	struct request *rq;
	Sg_fd *sfp = srp->parentfp;
	sg_io_hdr_t *hp = &srp->header;
	int dxfer_len = (int) hp->dxfer_len;
	int dxfer_dir = hp->dxfer_direction;
	unsigned int iov_count = hp->iovec_count;
	Sg_scatter_hold *req_schp = &srp->data;
	Sg_scatter_hold *rsv_schp = &sfp->reserve;
	struct request_queue *q = sfp->parentdp->device->request_queue;
	struct rq_map_data *md, map_data;
	int rw = hp->dxfer_direction == SG_DXFER_TO_DEV ? WRITE : READ;
	unsigned char *long_cmdp = NULL;

	SCSI_LOG_TIMEOUT(4, sg_printk(KERN_INFO, sfp->parentdp,
				      "sg_start_req: dxfer_len=%d\n",
				      dxfer_len));

	if (hp->cmd_len > BLK_MAX_CDB) {
		long_cmdp = kzalloc(hp->cmd_len, GFP_KERNEL);
		if (!long_cmdp)
			return -ENOMEM;
	}

	/*
	 * NOTE
	 *
	 * With scsi-mq enabled, there are a fixed number of preallocated
	 * requests equal in number to shost->can_queue.  If all of the
	 * preallocated requests are already in use, then using GFP_ATOMIC with
	 * blk_get_request() will return -EWOULDBLOCK, whereas using GFP_KERNEL
	 * will cause blk_get_request() to sleep until an active command
	 * completes, freeing up a request.  Neither option is ideal, but
	 * GFP_KERNEL is the better choice to prevent userspace from getting an
	 * unexpected EWOULDBLOCK.
	 *
	 * With scsi-mq disabled, blk_get_request() with GFP_KERNEL usually
	 * does not sleep except under memory pressure.
	 */
	rq = blk_get_request(q, rw, GFP_KERNEL);
	if (IS_ERR(rq)) {
		kfree(long_cmdp);
		return PTR_ERR(rq);
	}

	blk_rq_set_block_pc(rq);

	if (hp->cmd_len > BLK_MAX_CDB)
		rq->cmd = long_cmdp;
	memcpy(rq->cmd, cmd, hp->cmd_len);
	rq->cmd_len = hp->cmd_len;

	srp->rq = rq;
	rq->end_io_data = srp;
	rq->sense = srp->sense_b;
	rq->retries = SG_DEFAULT_RETRIES;

	if ((dxfer_len <= 0) || (dxfer_dir == SG_DXFER_NONE))
		return 0;

	if (sg_allow_dio && hp->flags & SG_FLAG_DIRECT_IO &&
	    dxfer_dir != SG_DXFER_UNKNOWN && !iov_count &&
	    !sfp->parentdp->device->host->unchecked_isa_dma &&
	    blk_rq_aligned(q, (unsigned long)hp->dxferp, dxfer_len))
		md = NULL;
	else
		md = &map_data;

	if (md) {
		if (!sg_res_in_use(sfp) && dxfer_len <= rsv_schp->bufflen)
			sg_link_reserve(sfp, srp, dxfer_len);
		else {
			res = sg_build_indirect(req_schp, sfp, dxfer_len);
			if (res)
				return res;
		}

		md->pages = req_schp->pages;
		md->page_order = req_schp->page_order;
		md->nr_entries = req_schp->k_use_sg;
		md->offset = 0;
		md->null_mapped = hp->dxferp ? 0 : 1;
		if (dxfer_dir == SG_DXFER_TO_FROM_DEV)
			md->from_user = 1;
		else
 			md->from_user = 0;
 	}
 
 	if (iov_count) {
 		int size = sizeof(struct iovec) * iov_count;
 		struct iovec *iov;
		struct iov_iter i;

		iov = memdup_user(hp->dxferp, size);
		if (IS_ERR(iov))
			return PTR_ERR(iov);

		iov_iter_init(&i, rw, iov, iov_count,
			      min_t(size_t, hp->dxfer_len,
				    iov_length(iov, iov_count)));

		res = blk_rq_map_user_iov(q, rq, md, &i, GFP_ATOMIC);
		kfree(iov);
	} else
		res = blk_rq_map_user(q, rq, md, hp->dxferp,
				      hp->dxfer_len, GFP_ATOMIC);

	if (!res) {
		srp->bio = rq->bio;

		if (!md) {
			req_schp->dio_in_use = 1;
			hp->info |= SG_INFO_DIRECT_IO;
		}
	}
	return res;
}