CVE-2024-50154

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

tcp/dccp: Don't use timer_pending() in reqsk_queue_unlink().

Martin KaFai Lau reported use-after-free [0] in reqsk_timer_handler().

"""

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.

"""

The commit 83fccfc3940c ("inet: fix potential deadlock in

reqsk_queue_unlink()") added timer_pending() in reqsk_queue_unlink() not

to call del_timer_sync() from reqsk_timer_handler(), but it introduced a

small race window.

Before the timer is called, expire_timers() calls detach_timer(timer, true)

to clear timer->entry.pprev and marks it as not pending.

If reqsk_queue_unlink() checks timer_pending() just after expire_timers()

calls detach_timer(), TCP will miss del_timer_sync(); the reqsk timer will

continue running and send multiple SYN+ACKs until it expires.

The reported UAF could happen if req->sk is close()d earlier than the timer

expiration, which is 63s by default.

The scenario would be

1. inet_csk_complete_hashdance() calls inet_csk_reqsk_queue_drop(),

but del_timer_sync() is missed

2. reqsk timer is executed and scheduled again

3. req->sk is accept()ed and reqsk_put() decrements rsk_refcnt, but

reqsk timer still has another one, and inet_csk_accept() does not

clear req->sk for non-TFO sockets

4. sk is close()d

5. reqsk timer is executed again, and BPF touches req->sk

Let's not use timer_pending() by passing the caller context to

__inet_csk_reqsk_queue_drop().

Note that reqsk timer is pinned, so the issue does not happen in most

use cases. [1]

[0]

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