mptcp: Fix data stream corruption in the address announcement

commit 2c1f97a52cb827a5f2768e67a9dddffae1ed47ab upstream.

Because of the size restriction in the TCP options space, the MPTCP
ADD_ADDR option is exclusive and cannot be sent with other MPTCP ones.
For this reason, in the linked mptcp_out_options structure, group of
fields linked to different options are part of the same union.

There is a case where the mptcp_pm_add_addr_signal() function can modify
opts->addr, but not ended up sending an ADD_ADDR. Later on, back in
mptcp_established_options, other options will be sent, but with
unexpected data written in other fields due to the union, e.g. in
opts->ext_copy. This could lead to a data stream corruption in the next
packet.

Using an intermediate variable, prevents from corrupting previously
established DSS option. The assignment of the ADD_ADDR option
parameters is now done once we are sure this ADD_ADDR option can be set
in the packet, e.g. after having dropped other suboptions.

Fixes: 1bff1e4 ("mptcp: optimize out option generation")
Cc: [email protected]
Suggested-by: Paolo Abeni <[email protected]>
Signed-off-by: Arthur Mongodin <[email protected]>
Reviewed-by: Matthieu Baerts (NGI0) <[email protected]>
[ Matt: the commit message has been updated: long lines splits and some
  clarifications. ]
Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>
Reviewed-by: Simon Horman <[email protected]>
Link: https://patch.msgid.link/20250314-net-mptcp-fix-data-stream-corr-sockopt-v1-1-122dbb249db3@kernel.org
Signed-off-by: Paolo Abeni <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Arthur Mongodin

net/mptcp/options.c

@@ -649,6 +649,7 @@ static bool mptcp_established_options_add_addr(struct sock *sk, struct sk_buff *
 	struct mptcp_sock *msk = mptcp_sk(subflow->conn);
 	bool drop_other_suboptions = false;
 	unsigned int opt_size = *size;
+	struct mptcp_addr_info addr;
 	bool echo;
 	int len;
 
@@ -657,7 +658,7 @@ static bool mptcp_established_options_add_addr(struct sock *sk, struct sk_buff *
 	 */
 	if (!mptcp_pm_should_add_signal(msk) ||
 	    (opts->suboptions & (OPTION_MPTCP_MPJ_ACK | OPTION_MPTCP_MPC_ACK)) ||
-	    !mptcp_pm_add_addr_signal(msk, skb, opt_size, remaining, &opts->addr,
+	    !mptcp_pm_add_addr_signal(msk, skb, opt_size, remaining, &addr,
 		    &echo, &drop_other_suboptions))
 		return false;
 
@@ -670,7 +671,7 @@ static bool mptcp_established_options_add_addr(struct sock *sk, struct sk_buff *
 	else if (opts->suboptions & OPTION_MPTCP_DSS)
 		return false;
 
-	len = mptcp_add_addr_len(opts->addr.family, echo, !!opts->addr.port);
+	len = mptcp_add_addr_len(addr.family, echo, !!addr.port);
 	if (remaining < len)
 		return false;
 
@@ -687,6 +688,7 @@ static bool mptcp_established_options_add_addr(struct sock *sk, struct sk_buff *
 		opts->ahmac = 0;
 		*size -= opt_size;
 	}
+	opts->addr = addr;
 	opts->suboptions |= OPTION_MPTCP_ADD_ADDR;
 	if (!echo) {
 		opts->ahmac = add_addr_generate_hmac(msk->local_key,