crypto: af_alg: Pin pages rather than ref'ing if appropriate

Convert AF_ALG to use iov_iter_extract_pages() instead of
iov_iter_get_pages().  This will pin pages or leave them unaltered rather
than getting a ref on them as appropriate to the iterator.

The pages need to be pinned for DIO-read rather than having refs taken on
them to prevent VM copy-on-write from malfunctioning during a concurrent
fork() (the result of the I/O would otherwise end up only visible to the
child process and not the parent).

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Herbert Xu <herbert@gondor.apana.org.au>
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: linux-crypto@vger.kernel.org
cc: netdev@vger.kernel.org
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

crypto/af_alg.c

@@ -533,14 +533,17 @@ static const struct net_proto_family alg_family = {
 
 int af_alg_make_sg(struct af_alg_sgl *sgl, struct iov_iter *iter, int len)
 {
+	struct page **pages = sgl->pages;
 	size_t off;
 	ssize_t n;
 	int npages, i;
 
-	n = iov_iter_get_pages2(iter, sgl->pages, len, ALG_MAX_PAGES, &off);
+	n = iov_iter_extract_pages(iter, &pages, len, ALG_MAX_PAGES, 0, &off);
 	if (n < 0)
 		return n;
 
+	sgl->need_unpin = iov_iter_extract_will_pin(iter);
+
 	npages = DIV_ROUND_UP(off + n, PAGE_SIZE);
 	if (WARN_ON(npages == 0))
 		return -EINVAL;
@@ -573,8 +576,9 @@ void af_alg_free_sg(struct af_alg_sgl *sgl)
 {
 	int i;
 
-	for (i = 0; i < sgl->npages; i++)
-		put_page(sgl->pages[i]);
+	if (sgl->need_unpin)
+		for (i = 0; i < sgl->npages; i++)
+			unpin_user_page(sgl->pages[i]);
 }
 EXPORT_SYMBOL_GPL(af_alg_free_sg);
 

include/crypto/if_alg.h

@@ -59,6 +59,7 @@ struct af_alg_sgl {
 	struct scatterlist sg[ALG_MAX_PAGES + 1];
 	struct page *pages[ALG_MAX_PAGES];
 	unsigned int npages;
+	bool need_unpin;
 };
 
 /* TX SGL entry */