[PATCH v2] sock: Use unsafe_memcpy() for sock_copy()

[Kees Cook]

While testing for places where zero-sized destinations were still showing
up in the kernel, sock_copy() and inet_reqsk_clone() were found, which
are using very specific memcpy() offsets for both avoiding a portion of
struct sock, and copying beyond the end of it (since struct sock is really
just a common header before the protocol-specific allocation). Instead
of trying to unravel this historical lack of container_of(), just switch
to unsafe_memcpy(), since that's effectively what was happening already
(memcpy() wasn't checking 0-sized destinations while the code base was
being converted away from fake flexible arrays).

Avoid the following false positive warning with future changes to
CONFIG_FORTIFY_SOURCE:

  memcpy: detected field-spanning write (size 3068) of destination "&nsk->__sk_common.skc_dontcopy_end" at net/core/sock.c:2057 (size 0)

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx

v2- add inet_reqsk_clone() instance too
v1- https://lore.kernel.org/lkml/20240216204423.work.066-kees@xxxxxxxxxx/
---
 net/core/sock.c                 | 5 +++--
 net/ipv4/inet_connection_sock.c | 5 +++--
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/net/core/sock.c b/net/core/sock.c
index 0a7f46c37f0c..b7ea358eb18f 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -2053,8 +2053,9 @@ static void sock_copy(struct sock *nsk, const struct sock *osk)
 
 	memcpy(nsk, osk, offsetof(struct sock, sk_dontcopy_begin));
 
-	memcpy(&nsk->sk_dontcopy_end, &osk->sk_dontcopy_end,
-	       prot->obj_size - offsetof(struct sock, sk_dontcopy_end));
+	unsafe_memcpy(&nsk->sk_dontcopy_end, &osk->sk_dontcopy_end,
+		      prot->obj_size - offsetof(struct sock, sk_dontcopy_end),
+		      /* alloc is larger than struct, see sk_prot_alloc() */);
 
 #ifdef CONFIG_SECURITY_NETWORK
 	nsk->sk_security = sptr;
diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
index 459af1f89739..4a1d96ba3ad1 100644
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -906,8 +906,9 @@ static struct request_sock *inet_reqsk_clone(struct request_sock *req,
 
 	memcpy(nreq_sk, req_sk,
 	       offsetof(struct sock, sk_dontcopy_begin));
-	memcpy(&nreq_sk->sk_dontcopy_end, &req_sk->sk_dontcopy_end,
-	       req->rsk_ops->obj_size - offsetof(struct sock, sk_dontcopy_end));
+	unsafe_memcpy(&nreq_sk->sk_dontcopy_end, &req_sk->sk_dontcopy_end,
+		      req->rsk_ops->obj_size - offsetof(struct sock, sk_dontcopy_end),
+		      /* alloc is larger than struct, see sk_prot_alloc() */);
 
 	sk_node_init(&nreq_sk->sk_node);
 	nreq_sk->sk_tx_queue_mapping = req_sk->sk_tx_queue_mapping;
-- 
2.34.1