CVE-2024-57996

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

net_sched: sch_sfq: don't allow 1 packet limit

The current implementation does not work correctly with a limit of

1. iproute2 actually checks for this and this patch adds the check in

kernel as well.

This fixes the following syzkaller reported crash:

UBSAN: array-index-out-of-bounds in net/sched/sch_sfq.c:210:6

index 65535 is out of range for type 'struct sfq_head[128]'

CPU: 0 PID: 2569 Comm: syz-executor101 Not tainted 5.10.0-smp-DEV #1

Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024

Call Trace:

__dump_stack lib/dump_stack.c:79 [inline]

dump_stack+0x125/0x19f lib/dump_stack.c:120

ubsan_epilogue lib/ubsan.c:148 [inline]

__ubsan_handle_out_of_bounds+0xed/0x120 lib/ubsan.c:347

sfq_link net/sched/sch_sfq.c:210 [inline]

sfq_dec+0x528/0x600 net/sched/sch_sfq.c:238

sfq_dequeue+0x39b/0x9d0 net/sched/sch_sfq.c:500

sfq_reset+0x13/0x50 net/sched/sch_sfq.c:525

qdisc_reset+0xfe/0x510 net/sched/sch_generic.c:1026

tbf_reset+0x3d/0x100 net/sched/sch_tbf.c:319

qdisc_reset+0xfe/0x510 net/sched/sch_generic.c:1026

dev_reset_queue+0x8c/0x140 net/sched/sch_generic.c:1296

netdev_for_each_tx_queue include/linux/netdevice.h:2350 [inline]

dev_deactivate_many+0x6dc/0xc20 net/sched/sch_generic.c:1362

__dev_close_many+0x214/0x350 net/core/dev.c:1468

dev_close_many+0x207/0x510 net/core/dev.c:1506

unregister_netdevice_many+0x40f/0x16b0 net/core/dev.c:10738

unregister_netdevice_queue+0x2be/0x310 net/core/dev.c:10695

unregister_netdevice include/linux/netdevice.h:2893 [inline]

__tun_detach+0x6b6/0x1600 drivers/net/tun.c:689

tun_detach drivers/net/tun.c:705 [inline]

tun_chr_close+0x104/0x1b0 drivers/net/tun.c:3640

__fput+0x203/0x840 fs/file_table.c:280

task_work_run+0x129/0x1b0 kernel/task_work.c:185

exit_task_work include/linux/task_work.h:33 [inline]

do_exit+0x5ce/0x2200 kernel/exit.c:931

do_group_exit+0x144/0x310 kernel/exit.c:1046

__do_sys_exit_group kernel/exit.c:1057 [inline]

__se_sys_exit_group kernel/exit.c:1055 [inline]

__x64_sys_exit_group+0x3b/0x40 kernel/exit.c:1055

do_syscall_64+0x6c/0xd0

entry_SYSCALL_64_after_hwframe+0x61/0xcb

RIP: 0033:0x7fe5e7b52479

Code: Unable to access opcode bytes at RIP 0x7fe5e7b5244f.

RSP: 002b:00007ffd3c800398 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7

RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fe5e7b52479

RDX: 000000000000003c RSI: 00000000000000e7 RDI: 0000000000000000

RBP: 00007fe5e7bcd2d0 R08: ffffffffffffffb8 R09: 0000000000000014

R10: 0000000000000000 R11: 0000000000000246 R12: 00007fe5e7bcd2d0

R13: 0000000000000000 R14: 00007fe5e7bcdd20 R15: 00007fe5e7b24270

The crash can be also be reproduced with the following (with a tc

recompiled to allow for sfq limits of 1):

tc qdisc add dev dummy0 handle 1: root tbf rate 1Kbit burst 100b lat 1s

../iproute2-6.9.0/tc/tc qdisc add dev dummy0 handle 2: parent 1:10 sfq limit 1

ifconfig dummy0 up

ping -I dummy0 -f -c2 -W0.1 8.8.8.8

sleep 1

Scenario that triggers the crash:

* the first packet is sent and queued in TBF and SFQ; qdisc qlen is 1

* TBF dequeues: it peeks from SFQ which moves the packet to the

gso_skb list and keeps qdisc qlen set to 1. TBF is out of tokens so

it schedules itself for later.

* the second packet is sent and TBF tries to queues it to SFQ. qdisc

qlen is now 2 and because the SFQ limit is 1 the packet is dropped

by SFQ. At this point qlen is 1, and all of the SFQ slots are empty,

however q->tail is not NULL.

At this point, assuming no more packets are queued, when sch_dequeue

runs again it will decrement the qlen for the current empty slot

causing an underflow and the subsequent out of bounds access.