net/smc: fix UAF in smc_cdc_rx_handler() by pinning the socket

Overview

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

net/smc: fix UAF in smc_cdc_rx_handler() by pinning the socket

smc_cdc_rx_handler() looks up the connection by token under the link group's conns_lock, drops the lock, and then dereferences conn and the smc_sock derived from it, ending in sock_hold(&smc->sk) inside smc_cdc_msg_recv(). No reference is held across the lock release.

The only reference pinning the socket while the connection is discoverable in the link group is taken in smc_lgr_register_conn() (sock_hold) and dropped in __smc_lgr_unregister_conn() (sock_put), both under conns_lock. Once the handler drops conns_lock, a concurrent close() -> smc_release() -> smc_conn_free() -> smc_lgr_unregister_conn() can drop that reference and free the smc_sock, so the handler's later sock_hold() runs on freed memory:

WARNING: lib/refcount.c:25 at refcount_warn_saturate Workqueue: rxe_wq do_work refcount_warn_saturate (lib/refcount.c:25) smc_cdc_msg_recv (net/smc/smc_cdc.c:430) smc_cdc_rx_handler (net/smc/smc_cdc.c:502) smc_wr_rx_tasklet_fn (net/smc/smc_wr.c:445) tasklet_action_common (kernel/softirq.c:938) handle_softirqs (kernel/softirq.c:622) Kernel panic - not syncing: panic_on_warn set

Only SMC-R is affected. The SMC-D receive tasklet is stopped by tasklet_kill(&conn->rx_tsklet) in smc_conn_free() before the connection is unregistered, so it cannot run concurrently with the free.

Take the socket reference while still holding conns_lock, so the registration reference can no longer be the last one, and drop it once the handler is done.

Affected

• Linux >= d7b0e37c1ac152905b18a5b9506179091a35b0b6 < 8de4f665d0febfb92803dece377791a563fc7041
• Linux >= d7b0e37c1ac152905b18a5b9506179091a35b0b6 < 8145b432136285e01091815b48ceb2dae261f262
• Linux >= d7b0e37c1ac152905b18a5b9506179091a35b0b6 < 1951bffbc6493ec34cff3956b29d4bc6606904a6
• Linux >= d7b0e37c1ac152905b18a5b9506179091a35b0b6 < 647b19e5cc145a2f1f685ae8ff3805a17356888c
• Linux >= d7b0e37c1ac152905b18a5b9506179091a35b0b6 < 472e9d7c0d5b03be3ff91ff941f57da822b031bc
• Linux >= d7b0e37c1ac152905b18a5b9506179091a35b0b6 < 3bfb96d9bc6a7ed0b99c7db329cc2e22a28d84bb
• Linux >= d7b0e37c1ac152905b18a5b9506179091a35b0b6 < ce5aa8084329351086894aa34d77e40301d5bd3d
• Linux >= d7b0e37c1ac152905b18a5b9506179091a35b0b6 < 9d160b35cc34a2ba8229d07651468a7848325135
• Linux 4.18

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.