[RFC PATCH v2 09/48] tcp: Make sendmsg(MSG_SPLICE_PAGES) copy unspliceable data

[David Howells <dhowells@redhat.com>]

If sendmsg() with MSG_SPLICE_PAGES encounters a page that shouldn't be
spliced - a slab page, for instance, or one with a zero count - make
tcp_sendmsg() copy it.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Eric Dumazet <edumazet@google.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
cc: netdev@vger.kernel.org
---
 net/ipv4/tcp.c | 28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 910b327c236e..6ef0518eb706 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1417,10 +1417,10 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 				goto do_error;
 			copy = err;
 		} else if (zc == 2) {
-			/* Splice in data. */
+			/* Splice in data if we can; copy if we can't. */
 			struct page *page = NULL, **pages = &page;
 			size_t off = 0, part;
-			bool can_coalesce;
+			bool can_coalesce, put = false;
 			int i = skb_shinfo(skb)->nr_frags;
 
 			copy = iov_iter_extract_pages(&msg->msg_iter, &pages,
@@ -1447,12 +1447,34 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 				goto wait_for_space;
 			copy = part;
 
+			if (!sendpage_ok(page)) {
+				const void *p = kmap_local_page(page);
+				void *q;
+
+				q = page_frag_memdup(NULL, p + off, copy,
+						     sk->sk_allocation, ULONG_MAX);
+				kunmap_local(p);
+				if (!q) {
+					iov_iter_revert(&msg->msg_iter, copy);
+					err = copy ?: -ENOMEM;
+					goto do_error;
+				}
+				page = virt_to_page(q);
+				off = offset_in_page(q);
+				put = true;
+				can_coalesce = false;
+			}
+
 			if (can_coalesce) {
 				skb_frag_size_add(&skb_shinfo(skb)->frags[i - 1], copy);
 			} else {
-				get_page(page);
+				if (!put)
+					get_page(page);
+				put = false;
 				skb_fill_page_desc_noacc(skb, i, page, off, copy);
 			}
+			if (put)
+				put_page(page);
 			page = NULL;
 
 			if (!(flags & MSG_NO_SHARED_FRAGS))