[RFC PATCH v2 21/48] crypto: af_alg: Pin pages rather than ref'ing if appropriate

[David Howells <dhowells@redhat.com>]

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: linux-crypto@vger.kernel.org
---
 crypto/af_alg.c         | 10 +++++++---
 include/crypto/if_alg.h |  1 +
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/crypto/af_alg.c b/crypto/af_alg.c
index 5f7252a5b7b4..7caff10df643 100644
--- a/crypto/af_alg.c
+++ b/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);
 
diff --git a/include/crypto/if_alg.h b/include/crypto/if_alg.h
index 7e76623f9ec3..46494b33f5bc 100644
--- a/include/crypto/if_alg.h
+++ b/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 */