[PATCH net-next v4 17/20] ip, udp: 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
__ip_append_data() copy it.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
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/ip_output.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 13d19867ffd3..e34c86b1b59a 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -967,13 +967,32 @@ static int __ip_splice_pages(struct sock *sk, struct sk_buff *skb,
 	struct page *page = NULL, **pages = &page;
 	ssize_t copy = *pcopy;
 	size_t off;
+	bool put = false;
 	int err;
 
 	copy = iov_iter_extract_pages(&msg->msg_iter, &pages, copy, 1, 0, &off);
 	if (copy <= 0)
 		return copy ?: -EIO;
 
+	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);
+			return -ENOMEM;
+		}
+		page = virt_to_page(q);
+		off = offset_in_page(q);
+		put = true;
+	}
+
 	err = skb_append_pagefrags(skb, page, off, copy);
+	if (put)
+		put_page(page);
 	if (err < 0) {
 		iov_iter_revert(&msg->msg_iter, copy);
 		return err;