tls: cap the output scatter list to something reasonable

TLS recvmsg() passes user pages as destination for decrypt.
The decrypt operation is repeated record by record, each
record being 16kB, max. TLS allocates an sg_table and uses
iov_iter_get_pages() to populate it with enough pages to
fit the decrypted record.

Even though we decrypt a single message at a time we size
the sg_table based on the entire length of the iovec.
This leads to unnecessarily large allocations, risking
triggering OOM conditions.

Use iov_iter_truncate() / iov_iter_reexpand() to construct
a "capped" version of iov_iter_npages(). Alternatively we
could parametrize iov_iter_npages() to take the size as
arg instead of using i->count, or do something else..
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/uio.h

@@ -273,6 +273,23 @@ static inline void iov_iter_reexpand(struct iov_iter *i, size_t count)
 	i->count = count;
 }
 
+static inline int
+iov_iter_npages_cap(struct iov_iter *i, int maxpages, size_t max_bytes)
+{
+	size_t shorted = 0;
+	int npages;
+
+	if (iov_iter_count(i) > max_bytes) {
+		shorted = iov_iter_count(i) - max_bytes;
+		iov_iter_truncate(i, max_bytes);
+	}
+	npages = iov_iter_npages(i, INT_MAX);
+	if (shorted)
+		iov_iter_reexpand(i, iov_iter_count(i) + shorted);
+
+	return npages;
+}
+
 struct csum_state {
 	__wsum csum;
 	size_t off;

net/tls/tls_sw.c

@@ -1433,7 +1433,8 @@ static int decrypt_internal(struct sock *sk, struct sk_buff *skb,
 
 	if (*zc && (out_iov || out_sg)) {
 		if (out_iov)
-			n_sgout = iov_iter_npages(out_iov, INT_MAX) + 1;
+			n_sgout = 1 +
+				iov_iter_npages_cap(out_iov, INT_MAX, data_len);
 		else
 			n_sgout = sg_nents(out_sg);
 		n_sgin = skb_nsg(skb, rxm->offset + prot->prepend_size,