From: Martin KaFai Lau <martin.lau@xxxxxxxxx> Date: Thu, 3 Oct 2024 18:14:09 -0700 > Hi, > > We are seeing a use-after-free from a bpf prog attached to > trace_tcp_retransmit_synack. The program passes the req->sk to the > bpf_sk_storage_get_tracing kernel helper which does check for null before using it. > > fastopen is not used. > > We got a kfence report on use-after-free (pasted at the end). It is running with > an older 6.4 kernel and we hardly hit this in production. > > From the upstream code, del_timer_sync() should have been done by > inet_csk_reqsk_queue_drop() before "req->sk = child;" is assigned in > inet_csk_reqsk_queue_add(). My understanding is the req->rsk_timer should have > been stopped before the "req->sk = child;" assignment. There seems to be a small race window in reqsk_queue_unlink(). expire_timers() first calls detach_timer(, true), which marks the timer as not pending, and then calls reqsk_timer_handler(). If reqsk_queue_unlink() calls timer_pending() just before expire_timers() calls reqsk_timer_handler(), reqsk_queue_unlink() could miss del_timer_sync() ? ---8<--- diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c index 2c5632d4fddb..4ba47ee6c9da 100644 --- a/net/ipv4/inet_connection_sock.c +++ b/net/ipv4/inet_connection_sock.c @@ -1045,7 +1045,7 @@ static bool reqsk_queue_unlink(struct request_sock *req) found = __sk_nulls_del_node_init_rcu(sk); spin_unlock(lock); } - if (timer_pending(&req->rsk_timer) && del_timer_sync(&req->rsk_timer)) + if (del_timer_sync(&req->rsk_timer)) reqsk_put(req); return found; } ---8<--- > > or there are cases that req->sk is not NULL in the reqsk_timer_handler()? > > BUG: KFENCE: use-after-free read in bpf_sk_storage_get_tracing+0x2e/0x1b0 > > Use-after-free read at 0x00000000a891fb3a (in kfence-#1): > bpf_sk_storage_get_tracing+0x2e/0x1b0 > bpf_prog_5ea3e95db6da0438_tcp_retransmit_synack+0x1d20/0x1dda > bpf_trace_run2+0x4c/0xc0 > tcp_rtx_synack+0xf9/0x100 > reqsk_timer_handler+0xda/0x3d0 > run_timer_softirq+0x292/0x8a0 > irq_exit_rcu+0xf5/0x320 > sysvec_apic_timer_interrupt+0x6d/0x80 > asm_sysvec_apic_timer_interrupt+0x16/0x20 > intel_idle_irq+0x5a/0xa0 > cpuidle_enter_state+0x94/0x273 > cpu_startup_entry+0x15e/0x260 > start_secondary+0x8a/0x90 > secondary_startup_64_no_verify+0xfa/0xfb > > kfence-#1: 0x00000000a72cc7b6-0x00000000d97616d9, size=2376, cache=TCPv6 > > allocated by task 0 on cpu 9 at 260507.901592s: > sk_prot_alloc+0x35/0x140 > sk_clone_lock+0x1f/0x3f0 > inet_csk_clone_lock+0x15/0x160 > tcp_create_openreq_child+0x1f/0x410 > tcp_v6_syn_recv_sock+0x1da/0x700 > tcp_check_req+0x1fb/0x510 > tcp_v6_rcv+0x98b/0x1420 > ipv6_list_rcv+0x2258/0x26e0 > napi_complete_done+0x5b1/0x2990 > mlx5e_napi_poll+0x2ae/0x8d0 > net_rx_action+0x13e/0x590 > irq_exit_rcu+0xf5/0x320 > common_interrupt+0x80/0x90 > asm_common_interrupt+0x22/0x40 > cpuidle_enter_state+0xfb/0x273 > cpu_startup_entry+0x15e/0x260 > start_secondary+0x8a/0x90 > secondary_startup_64_no_verify+0xfa/0xfb > > freed by task 0 on cpu 9 at 260507.927527s: > rcu_core_si+0x4ff/0xf10 > irq_exit_rcu+0xf5/0x320 > sysvec_apic_timer_interrupt+0x6d/0x80 > asm_sysvec_apic_timer_interrupt+0x16/0x20 > cpuidle_enter_state+0xfb/0x273 > cpu_startup_entry+0x15e/0x260 > start_secondary+0x8a/0x90 > secondary_startup_64_no_verify+0xfa/0xfb > > Thanks, > Martin
