CVE-2026-98276

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

net: lock the socket in sock_gettstamp()

sk->sk_flags must only be changed while holding the socket lock, because sock_set_flag() and sock_reset_flag() use non atomic operations (__set_bit() and __clear_bit()).

sock_gettstamp() is one of the last places where a bit of sk->sk_flags is changed from a syscall without owning the socket lock, through sock_enable_timestamp(sk, SOCK_TIMESTAMP).

sk_set_memalloc() and sk_clear_memalloc() also change sk->sk_flags without the socket lock, but their callers (nbd, iscsi_tcp, nvme-tcp, sunrpc, wireguard) need a careful audit, this will be addressed in a separate patch.

Jungwoo Lee and Wongi Lee reported an UDP socket use-after-free caused by this bug: a SIOCGSTAMPNS_NEW ioctl racing with bind() can cancel the SOCK_RCU_FREE bit that udp_lib_get_port() just set, because both threads perform a read-modify-write on the same word.

CPU 0 (bind) CPU 1 (SIOCGSTAMPNS_NEW)

read sk_flags = F read sk_flags = F compute F | BIT(SOCK_RCU_FREE) compute F | BIT(SOCK_TIMESTAMP) store F | BIT(SOCK_RCU_FREE) sk_add_node_rcu(sk, …) store F | BIT(SOCK_TIMESTAMP)

After the lost update, SOCK_RCU_FREE is clear while the socket is visible to lockless UDP receive lookups. sk_destruct() then frees the socket immediately instead of waiting for a RCU grace period, while the receive path still holds a reference-less pointer to it:

BUG: KASAN: slab-use-after-free in ipv4_pktinfo_prepare+0x30/0x410 Read of size 8 at addr ffff888008806610 by task exploit/207 CPU: 0 UID: 1000 PID: 207 Comm: exploit Not tainted 6.12.95+ #1 ipv4_pktinfo_prepare+0x30/0x410 udp_queue_rcv_one_skb+0x51c/0x1180 udp_unicast_rcv_skb+0x109/0x350 ip_protocol_deliver_rcu+0x14b/0x310 ip_local_deliver_finish+0x29d/0x390 ip_local_deliver+0x24d/0x2a0

Only grab the socket lock when SOCK_TIMESTAMP has to be set, to keep the common case lockless.

Affected Software

References

• https://git.kernel.org/stable/c/17b2a1eb97fdb2a2cbaeab3b146b26797ea9311f
• https://git.kernel.org/stable/c/18899e2e4023369a8f7739c2255a59a9748d8d17
• https://git.kernel.org/stable/c/1f73253add8365d0dad0a4f421acaa8c21d20cef
• https://git.kernel.org/stable/c/3b12d3967e96f1b7b977d9fc352ae29a1b299a82
• https://git.kernel.org/stable/c/88c804847dd87dc613b771b392ccecdb94725032
• https://git.kernel.org/stable/c/899650bbf985b7bfd2a7b808357df9b16e6d6959
• https://git.kernel.org/stable/c/9ed55f3dbef4f4adfe65eb03b0c35c53229a8490
• https://git.kernel.org/stable/c/d9f96bc2d822501f84d1caa6275a2c6b316ca2c4