[PATCH v2 5/6] xfrm/compat: Translate 32-bit user_policy from sockptr

[Dmitry Safonov <dima@arista.com>]

XFRM is disabled for compatible users because of the UABI difference.
The difference is in structures paddings and in the result the size
of netlink messages differ.

Possibility for compatible application to manage xfrm tunnels was
disabled by: the commmit 19d7df69fdb2 ("xfrm: Refuse to insert 32 bit
userspace socket policies on 64 bit systems") and the commit 74005991b78a
("xfrm: Do not parse 32bits compiled xfrm netlink msg on 64bits host").

This is my second attempt to resolve the xfrm/compat problem by adding
the 64=>32 and 32=>64 bit translators those non-visibly to a user
provide translation between compatible user and kernel.
Previous attempt was to interrupt the message ABI according to a syscall
by xfrm_user, which resulted in over-complicated code [1].

Florian Westphal provided the idea of translator and some draft patches
in the discussion. In these patches, his idea is reused and some of his
initial code is also present.

Provide compat_xfrm_userpolicy_info translation for xfrm setsocketopt().
Reallocate buffer and put the missing padding for 64-bit message.

[1]: https://lkml.kernel.org/r/20180726023144.31066-1-dima@arista.com
Signed-off-by: Dmitry Safonov <dima@arista.com>
---
 include/net/xfrm.h     |  5 +++++
 net/xfrm/xfrm_compat.c | 25 +++++++++++++++++++++++++
 net/xfrm/xfrm_state.c  | 11 ++++++++---
 3 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/include/net/xfrm.h b/include/net/xfrm.h
index 242e690674c6..633c210bd2dd 100644
--- a/include/net/xfrm.h
+++ b/include/net/xfrm.h
@@ -2009,6 +2009,7 @@ extern const int xfrm_msg_min[XFRM_NR_MSGTYPES];
 extern struct nlmsghdr *xfrm_user_rcv_msg_compat(const struct nlmsghdr *nlh,
 			int maxtype, const struct nla_policy *policy,
 			struct netlink_ext_ack *extack);
+extern int xfrm_user_policy_compat(u8 **pdata32, int optlen);
 #else
 static inline int xfrm_alloc_compat(struct sk_buff *skb)
 {
@@ -2025,6 +2026,10 @@ static inline struct nlmsghdr *xfrm_user_rcv_msg_compat(const struct nlmsghdr *n
 {
 	return ERR_PTR(-EOPNOTSUPP);
 }
+static inline int xfrm_user_policy_compat(u8 **pdata32, int optlen)
+{
+	return -EOPNOTSUPP;
+}
 #endif
 
 #if IS_ENABLED(CONFIG_IPV6)
diff --git a/net/xfrm/xfrm_compat.c b/net/xfrm/xfrm_compat.c
index 79daa7f47d5a..990eecfc4c0e 100644
--- a/net/xfrm/xfrm_compat.c
+++ b/net/xfrm/xfrm_compat.c
@@ -582,3 +582,28 @@ struct nlmsghdr *xfrm_user_rcv_msg_compat(const struct nlmsghdr *h32,
 
 	return h64;
 }
+
+int xfrm_user_policy_compat(u8 **pdata32, int optlen)
+{
+	struct compat_xfrm_userpolicy_info *p = (void *)*pdata32;
+	u8 *src_templates, *dst_templates;
+	u8 *data64;
+
+	if (optlen < sizeof(*p))
+		return -EINVAL;
+
+	data64 = kmalloc_track_caller(optlen + 4, GFP_USER | __GFP_NOWARN);
+	if (!data64)
+		return -ENOMEM;
+
+	memcpy(data64, *pdata32, sizeof(*p));
+	memset(data64 + sizeof(*p), 0, 4);
+
+	src_templates = *pdata32 + sizeof(*p);
+	dst_templates = data64 + sizeof(*p) + 4;
+	memcpy(dst_templates, src_templates, optlen - sizeof(*p));
+
+	kfree(*pdata32);
+	*pdata32 = data64;
+	return 0;
+}
diff --git a/net/xfrm/xfrm_state.c b/net/xfrm/xfrm_state.c
index 69520ad3d83b..053e6fe6ea7a 100644
--- a/net/xfrm/xfrm_state.c
+++ b/net/xfrm/xfrm_state.c
@@ -2271,9 +2271,6 @@ int xfrm_user_policy(struct sock *sk, int optname, sockptr_t optval, int optlen)
 	struct xfrm_mgr *km;
 	struct xfrm_policy *pol = NULL;
 
-	if (in_compat_syscall())
-		return -EOPNOTSUPP;
-
 	if (sockptr_is_null(optval) && !optlen) {
 		xfrm_sk_policy_insert(sk, XFRM_POLICY_IN, NULL);
 		xfrm_sk_policy_insert(sk, XFRM_POLICY_OUT, NULL);
@@ -2288,6 +2285,14 @@ int xfrm_user_policy(struct sock *sk, int optname, sockptr_t optval, int optlen)
 	if (IS_ERR(data))
 		return PTR_ERR(data);
 
+	if (in_compat_syscall()) {
+		err = xfrm_user_policy_compat(&data, optlen);
+		if (err) {
+			kfree(data);
+			return err;
+		}
+	}
+
 	err = -EINVAL;
 	rcu_read_lock();
 	list_for_each_entry_rcu(km, &xfrm_km_list, list) {
-- 
2.27.0