Use After Free Affecting kernel-tools-devel package, versions <0:4.14.262-200.489.amzn2

Note: Versions mentioned in the description apply only to the upstream kernel-tools-devel package and not the kernel-tools-devel package as distributed by Amazon-Linux. See How to fix? for Amazon-Linux:2 relevant fixed versions and status.

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

• http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-46936
• https://git.kernel.org/stable/c/08eacbd141e2495d2fcdde84358a06c4f95cbb13
• https://git.kernel.org/stable/c/15579e1301f856ad9385d720c9267c11032a5022
• https://git.kernel.org/stable/c/2386e81a1d277f540e1285565c9d41d531bb69d4
• https://git.kernel.org/stable/c/5c2fe20ad37ff56070ae0acb34152333976929b4
• https://git.kernel.org/stable/c/a8e1944b44f94f5c5f530e434c5eaee787254566
• https://git.kernel.org/stable/c/e22e45fc9e41bf9fcc1e92cfb78eb92786728ef0
• https://git.kernel.org/stable/c/e73164e89d1be561228a4534e1091369ee4ba41a
• https://git.kernel.org/stable/c/fe5838c22b986c1190f1dce9aa09bf6a491c1a69