Commit message: cifs: ensure that uncached writes handle unmapped areas correctly

It's possible for userland to pass down an iovec via writev() that has a
bogus user pointer in it. If that happens and we're doing an uncached
write, then we can end up getting less bytes than we expect from the
call to iov_iter_copy_from_user. This is CVE-2014-0069

cifs_iovec_write isn't set up to handle that situation however. It'll
blindly keep chugging through the page array and not filling those pages
with anything useful. Worse yet, we'll later end up with a negative
number in wdata->tailsz, which will confuse the sending routines and
cause an oops at the very least.

Fix this by having the copy phase of cifs_iovec_write stop copying data
in this situation and send the last write as a short one. At the same
time, we want to avoid sending a zero-length write to the server, so
break out of the loop and set rc to -EFAULT if that happens. This also
allows us to handle the case where no address in the iovec is valid.

[Note: Marking this for stable on v3.4+ kernels, but kernels as old as
       v2.6.38 may have a similar problem and may need similar fix]

Cc: <stable@vger.kernel.org> # v3.4+
Reviewed-by: Pavel Shilovsky <piastry@etersoft.ru>
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>
CVE: CVE-2014-0069
CWE: CWE-119
Vulnerability description: The cifs_iovec_write function in fs/cifs/file.c in the Linux kernel through 3.13.5 does not properly handle uncached write operations that copy fewer than the requested number of bytes, which allows local users to obtain sensitive information from kernel memory, cause a denial of service (memory corruption and system crash), or possibly gain privileges via a writev system call with a crafted pointer.

Project: linux
File: file.c

cifs_iovec_write(struct file *file, const struct iovec *iov,
 		 unsigned long nr_segs, loff_t *poffset)
 {
 	unsigned long nr_pages, i;
	size_t bytes, copied, len, cur_len;
 	ssize_t total_written = 0;
 	loff_t offset;
 	struct iov_iter it;
	struct cifsFileInfo *open_file;
	struct cifs_tcon *tcon;
	struct cifs_sb_info *cifs_sb;
	struct cifs_writedata *wdata, *tmp;
	struct list_head wdata_list;
	int rc;
	pid_t pid;

	len = iov_length(iov, nr_segs);
	if (!len)
		return 0;

	rc = generic_write_checks(file, poffset, &len, 0);
	if (rc)
		return rc;

	INIT_LIST_HEAD(&wdata_list);
	cifs_sb = CIFS_SB(file->f_path.dentry->d_sb);
	open_file = file->private_data;
	tcon = tlink_tcon(open_file->tlink);

	if (!tcon->ses->server->ops->async_writev)
		return -ENOSYS;

	offset = *poffset;

	if (cifs_sb->mnt_cifs_flags & CIFS_MOUNT_RWPIDFORWARD)
		pid = open_file->pid;
	else
		pid = current->tgid;

	iov_iter_init(&it, iov, nr_segs, len, 0);
	do {
		size_t save_len;

		nr_pages = get_numpages(cifs_sb->wsize, len, &cur_len);
		wdata = cifs_writedata_alloc(nr_pages,
					     cifs_uncached_writev_complete);
		if (!wdata) {
			rc = -ENOMEM;
			break;
		}

		rc = cifs_write_allocate_pages(wdata->pages, nr_pages);
		if (rc) {
			kfree(wdata);
			break;
		}
 
 		save_len = cur_len;
 		for (i = 0; i < nr_pages; i++) {
			bytes = min_t(const size_t, cur_len, PAGE_SIZE);
 			copied = iov_iter_copy_from_user(wdata->pages[i], &it,
							 0, bytes);
 			cur_len -= copied;
 			iov_iter_advance(&it, copied);
			/*
			 * If we didn't copy as much as we expected, then that
			 * may mean we trod into an unmapped area. Stop copying
			 * at that point. On the next pass through the big
			 * loop, we'll likely end up getting a zero-length
			 * write and bailing out of it.
			 */
			if (copied < bytes)
				break;
 		}
 		cur_len = save_len - cur_len;
 
		/*
		 * If we have no data to send, then that probably means that
		 * the copy above failed altogether. That's most likely because
		 * the address in the iovec was bogus. Set the rc to -EFAULT,
		 * free anything we allocated and bail out.
		 */
		if (!cur_len) {
			for (i = 0; i < nr_pages; i++)
				put_page(wdata->pages[i]);
			kfree(wdata);
			rc = -EFAULT;
			break;
		}

		/*
		 * i + 1 now represents the number of pages we actually used in
		 * the copy phase above. Bring nr_pages down to that, and free
		 * any pages that we didn't use.
		 */
		for ( ; nr_pages > i + 1; nr_pages--)
			put_page(wdata->pages[nr_pages - 1]);

 		wdata->sync_mode = WB_SYNC_ALL;
 		wdata->nr_pages = nr_pages;
 		wdata->offset = (__u64)offset;
		wdata->cfile = cifsFileInfo_get(open_file);
		wdata->pid = pid;
		wdata->bytes = cur_len;
		wdata->pagesz = PAGE_SIZE;
		wdata->tailsz = cur_len - ((nr_pages - 1) * PAGE_SIZE);
		rc = cifs_uncached_retry_writev(wdata);
		if (rc) {
			kref_put(&wdata->refcount,
				 cifs_uncached_writedata_release);
			break;
		}

		list_add_tail(&wdata->list, &wdata_list);
		offset += cur_len;
		len -= cur_len;
	} while (len > 0);

	/*
	 * If at least one write was successfully sent, then discard any rc
	 * value from the later writes. If the other write succeeds, then
	 * we'll end up returning whatever was written. If it fails, then
	 * we'll get a new rc value from that.
	 */
	if (!list_empty(&wdata_list))
		rc = 0;

	/*
	 * Wait for and collect replies for any successful sends in order of
	 * increasing offset. Once an error is hit or we get a fatal signal
	 * while waiting, then return without waiting for any more replies.
	 */
restart_loop:
	list_for_each_entry_safe(wdata, tmp, &wdata_list, list) {
		if (!rc) {
			/* FIXME: freezable too? */
			rc = wait_for_completion_killable(&wdata->done);
			if (rc)
				rc = -EINTR;
			else if (wdata->result)
				rc = wdata->result;
			else
				total_written += wdata->bytes;

			/* resend call if it's a retryable error */
			if (rc == -EAGAIN) {
				rc = cifs_uncached_retry_writev(wdata);
				goto restart_loop;
			}
		}
		list_del_init(&wdata->list);
		kref_put(&wdata->refcount, cifs_uncached_writedata_release);
	}

	if (total_written > 0)
		*poffset += total_written;

	cifs_stats_bytes_written(tcon, total_written);
	return total_written ? total_written : (ssize_t)rc;
}