diff --git a/include/linux/igmp.h b/include/linux/igmp.h
index a0cf0398519f..e075611344ef 100644
--- a/include/linux/igmp.h
+++ b/include/linux/igmp.h
@@ -14,6 +14,7 @@
 #include <linux/timer.h>
 #include <linux/in.h>
 #include <linux/ip.h>
+#include <linux/net.h>
 #include <linux/refcount.h>
 #include <linux/sockptr.h>
 #include <uapi/linux/igmp.h>
@@ -273,7 +274,7 @@ extern int ip_mc_source(int add, int omode, struct sock *sk,
 		struct ip_mreq_source *mreqs, int ifindex);
 extern int ip_mc_msfilter(struct sock *sk, struct ip_msfilter *msf,int ifindex);
 extern int ip_mc_msfget(struct sock *sk, struct ip_msfilter *msf,
-			sockptr_t optval, sockptr_t optlen);
+			sockopt_t *opt);
 extern int ip_mc_gsfget(struct sock *sk, struct group_filter *gsf,
 			sockptr_t optval, size_t offset);
 extern int ip_mc_sf_allow(const struct sock *sk, __be32 local, __be32 rmt,
diff --git a/include/linux/net.h b/include/linux/net.h
index 470100ae7107..e2a866fbcfa5 100644
--- a/include/linux/net.h
+++ b/include/linux/net.h
@@ -47,6 +47,9 @@ typedef struct sockopt {
 	int optlen;
 } sockopt_t;
 
+int sockptr_to_sockopt(sockopt_t *opt, sockptr_t optval, sockptr_t optlen,
+		       struct kvec *kvec);
+
 /*
  * Initialize a user-backed sockopt_t from the (optval, optlen) __user pair of
  * a getsockopt() callback. Used by transitional __user getsockopt wrappers
@@ -70,6 +73,35 @@ static inline int sockopt_init_user(sockopt_t *opt, char __user *optval,
 	return 0;
 }
 
+/*
+ * Grow optval to @size, for the options whose reply is sized by a count the
+ * caller left in optval rather than by optlen. Those write past optlen today
+ * and userspace relies on it.
+ *
+ * Call it before writing through opt->iter_out: it re-anchors the iterator at
+ * the head of optval. Only a user buffer can be longer than the optlen the
+ * caller declared, so a kernel-backed optval is refused with -EINVAL.
+ */
+static inline int sockopt_expand_out(sockopt_t *opt, size_t size)
+{
+	if (size <= (size_t)opt->optlen)
+		return 0;
+
+	if (size > INT_MAX)
+		return -EINVAL;
+
+	/* Re-anchoring reads iter_out.ubuf, so the iterator has to be a user
+	 * buffer that nothing has written through yet.
+	 */
+	if (WARN_ON_ONCE(!iter_is_ubuf(&opt->iter_out) ||
+			 iov_iter_count(&opt->iter_out) != (size_t)opt->optlen))
+		return -EINVAL;
+
+	iov_iter_ubuf(&opt->iter_out, ITER_DEST, opt->iter_out.ubuf, size);
+
+	return 0;
+}
+
 struct poll_table_struct;
 struct pipe_inode_info;
 struct inode;
diff --git a/net/ipv4/igmp.c b/net/ipv4/igmp.c
index d56355aca797..144fca158adc 100644
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c
@@ -2709,8 +2709,8 @@ int ip_mc_msfilter(struct sock *sk, struct ip_msfilter *msf, int ifindex)
 		err = ip_mc_leave_group(sk, &imr);
 	return err;
 }
-int ip_mc_msfget(struct sock *sk, struct ip_msfilter *msf,
-		 sockptr_t optval, sockptr_t optlen)
+
+int ip_mc_msfget(struct sock *sk, struct ip_msfilter *msf, sockopt_t *opt)
 {
 	int err, len, count, copycount, msf_size;
 	struct ip_mreqn	imr;
@@ -2755,14 +2755,19 @@ int ip_mc_msfget(struct sock *sk, struct ip_msfilter *msf,
 	len = flex_array_size(psl, sl_addr, copycount);
 	msf->imsf_numsrc = count;
 	msf_size = IP_MSFILTER_SIZE(copycount);
-	if (copy_to_sockptr(optlen, &msf_size, sizeof(int)) ||
-	    copy_to_sockptr(optval, msf, IP_MSFILTER_SIZE(0))) {
+
+	/* The source list is sized by the imsf_numsrc the caller left in
+	 * optval, not by optlen, which only has to cover the fixed part.
+	 */
+	err = sockopt_expand_out(opt, msf_size);
+	if (err)
+		return err;
+
+	opt->optlen = msf_size;
+	if (copy_to_iter(msf, IP_MSFILTER_SIZE(0), &opt->iter_out) !=
+	    IP_MSFILTER_SIZE(0))
 		return -EFAULT;
-	}
-	if (len &&
-	    copy_to_sockptr_offset(optval,
-				   offsetof(struct ip_msfilter, imsf_slist_flex),
-				   psl->sl_addr, len))
+	if (len && copy_to_iter(psl->sl_addr, len, &opt->iter_out) != len)
 		return -EFAULT;
 	return 0;
 done:
diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index a55ef327ec93..e06c1f48ecad 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -1706,6 +1706,8 @@ int do_ip_getsockopt(struct sock *sk, int level, int optname,
 	case IP_MSFILTER:
 	{
 		struct ip_msfilter msf;
+		struct kvec kvec;
+		sockopt_t opt;
 
 		if (len < IP_MSFILTER_SIZE(0)) {
 			err = -EINVAL;
@@ -1715,7 +1717,13 @@ int do_ip_getsockopt(struct sock *sk, int level, int optname,
 			err = -EFAULT;
 			goto out;
 		}
-		err = ip_mc_msfget(sk, &msf, optval, optlen);
+		err = sockptr_to_sockopt(&opt, optval, optlen, &kvec);
+		if (err)
+			goto out;
+
+		err = ip_mc_msfget(sk, &msf, &opt);
+		if (!err && copy_to_sockptr(optlen, &opt.optlen, sizeof(int)))
+			err = -EFAULT;
 		goto out;
 	}
 	case MCAST_MSFILTER:
diff --git a/net/socket.c b/net/socket.c
index 4cb8662869f0..bb0e1e118ef6 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -2436,8 +2436,8 @@ INDIRECT_CALLABLE_DECLARE(bool tcp_bpf_bypass_getsockopt(int level,
  * It is important to remember that both iov points to the same data, but,
  * .iter_in is read-only and .iter_out is write-only by the protocol callbacks
  */
-static int sockptr_to_sockopt(sockopt_t *opt, sockptr_t optval,
-			      sockptr_t optlen, struct kvec *kvec)
+int sockptr_to_sockopt(sockopt_t *opt, sockptr_t optval,
+		       sockptr_t optlen, struct kvec *kvec)
 {
 	int koptlen;