libceph: fall back to sendmsg for slab pages

skb_can_coalesce() allows coalescing neighboring slab objects into
a single frag:

  return page == skb_frag_page(frag) &&
         off == frag->page_offset + skb_frag_size(frag);

ceph_tcp_sendpage() can be handed slab pages.  One example of this is
XFS: it passes down sector sized slab objects for its metadata I/O.  If
the kernel client is co-located on the OSD node, the skb may go through
loopback and pop on the receive side with the exact same set of frags.
When tcp_recvmsg() attempts to copy out such a frag, hardened usercopy
complains because the size exceeds the object's allocated size:

  usercopy: kernel memory exposure attempt detected from ffff9ba917f20a00 (kmalloc-512) (1024 bytes)

Although skb_can_coalesce() could be taught to return false if the
resulting frag would cross a slab object boundary, we already have
a fallback for non-refcounted pages.  Utilize it for slab pages too.

Cc: stable@vger.kernel.org # 4.8+
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

net/ceph/messenger.c

@@ -580,9 +580,15 @@ static int ceph_tcp_sendpage(struct socket *sock, struct page *page,
 	struct bio_vec bvec;
 	int ret;
 
-	/* sendpage cannot properly handle pages with page_count == 0,
-	 * we need to fallback to sendmsg if that's the case */
-	if (page_count(page) >= 1)
+	/*
+	 * sendpage cannot properly handle pages with page_count == 0,
+	 * we need to fall back to sendmsg if that's the case.
+	 *
+	 * Same goes for slab pages: skb_can_coalesce() allows
+	 * coalescing neighboring slab objects into a single frag which
+	 * triggers one of hardened usercopy checks.
+	 */
+	if (page_count(page) >= 1 && !PageSlab(page))
 		return __ceph_tcp_sendpage(sock, page, offset, size, more);
 
 	bvec.bv_page = page;