CVE-2021-46936

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

net: fix use-after-free in tw_timer_handler

A real world panic issue was found as follow in Linux 5.4.

BUG: unable to handle page fault for address: ffffde49a863de28

PGD 7e6fe62067 P4D 7e6fe62067 PUD 7e6fe63067 PMD f51e064067 PTE 0

RIP: 0010:tw_timer_handler+0x20/0x40

Call Trace:

<IRQ>

call_timer_fn+0x2b/0x120

run_timer_softirq+0x1ef/0x450

__do_softirq+0x10d/0x2b8

irq_exit+0xc7/0xd0

smp_apic_timer_interrupt+0x68/0x120

apic_timer_interrupt+0xf/0x20

This issue was also reported since 2017 in the thread [1],

unfortunately, the issue was still can be reproduced after fixing

DCCP.

The ipv4_mib_exit_net is called before tcp_sk_exit_batch when a net

namespace is destroyed since tcp_sk_ops is registered befrore

ipv4_mib_ops, which means tcp_sk_ops is in the front of ipv4_mib_ops

in the list of pernet_list. There will be a use-after-free on

net->mib.net_statistics in tw_timer_handler after ipv4_mib_exit_net

if there are some inflight time-wait timers.

This bug is not introduced by commit f2bf415cfed7 ("mib: add net to

NET_ADD_STATS_BH") since the net_statistics is a global variable

instead of dynamic allocation and freeing. Actually, commit

61a7e26028b9 ("mib: put net statistics on struct net") introduces

the bug since it put net statistics on struct net and free it when

net namespace is destroyed.

Moving init_ipv4_mibs() to the front of tcp_init() to fix this bug

and replace pr_crit() with panic() since continuing is meaningless

when init_ipv4_mibs() fails.

[1] https://groups.google.com/g/syzkaller/c/p1tn-_Kc6l4/m/smuL_FMAAgAJ?pli=1