CVE-2026-98432

In the Linux kernel, the following vulnerability has been resolved:

ipv4: free inet_opt and ireq_opt after an RCU grace period

tcp_v4_syn_recv_sock() transfers ownership of ireq->ireq_opt to the

child socket (newinet->inet_opt) without copying it.

Another cpu can concurrently process a retransmitted SYN for the same

request socket, and send a SYNACK from tcp_check_req().

tcp_v4_send_synack() and inet_csk_route_req() read ireq->ireq_opt

under rcu_read_lock() only, and ip_build_and_send_pkt() and

ip_options_build() then read opt->optlen twice.

Note that the SYNACK timer itself is not an issue: it holds a

reference on its request socket, and inet_csk_reqsk_queue_drop()

calls timer_delete_sync() before the child can be freed.

Since commit 079096f103fa ("tcp/dccp: install syn_recv requests

into ehash table"), request sockets are processed without holding

the listener lock, so nothing prevents the child socket from being

freed while the SYNACK is still being built. TCP child sockets do

not have SOCK_RCU_FREE, and inet_sock_destruct() frees inet_opt

with a plain kfree(), leading to a use-after-free in

ip_options_build().

A similar issue exists with request socket migration

(net.ipv4.tcp_migrate_req=1, or a BPF_SK_REUSEPORT_SELECT_OR_MIGRATE

program). reqsk_timer_handler() clones the request socket with

inet_reqsk_clone(), so that the old request socket and its clone

share the same ireq_opt, then reqsk_migrate_reset() clears the

pointer in the old one. Another cpu holding a reference on the old

request socket can still be using these options (sending a SYNACK,

or creating a child in tcp_v4_syn_recv_sock()) when the clone is

freed, and tcp_v4_reqsk_destructor() also uses a plain kfree().

Readers already use RCU, and other paths replacing inet_opt

(do_ip_setsockopt(), cipso_v4_sock_setattr()...) already use

kfree_rcu(). Use kfree_rcu() in inet_sock_destruct() and

tcp_v4_reqsk_destructor() as well.

IPv6 is not affected by the first issue, because tcp_v6_syn_recv_sock()

duplicates the options. tcp_v6_reqsk_destructor() has the same

migration issue with ipv6_opt, which is only set by CALIPSO. This

will be addressed in a separate patch.