CVE ID:	CVE-2021-46936
Description:	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: 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
Test IDs:	None available
Cross References:	Common Vulnerability Exposure (CVE) ID: CVE-2021-46936 https://git.kernel.org/stable/c/08eacbd141e2495d2fcdde84358a06c4f95cbb13 https://git.kernel.org/stable/c/08eacbd141e2495d2fcdde84358a06c4f95cbb13 https://git.kernel.org/stable/c/15579e1301f856ad9385d720c9267c11032a5022 https://git.kernel.org/stable/c/15579e1301f856ad9385d720c9267c11032a5022 https://git.kernel.org/stable/c/2386e81a1d277f540e1285565c9d41d531bb69d4 https://git.kernel.org/stable/c/2386e81a1d277f540e1285565c9d41d531bb69d4 https://git.kernel.org/stable/c/5c2fe20ad37ff56070ae0acb34152333976929b4 https://git.kernel.org/stable/c/5c2fe20ad37ff56070ae0acb34152333976929b4 https://git.kernel.org/stable/c/a8e1944b44f94f5c5f530e434c5eaee787254566 https://git.kernel.org/stable/c/a8e1944b44f94f5c5f530e434c5eaee787254566 https://git.kernel.org/stable/c/e22e45fc9e41bf9fcc1e92cfb78eb92786728ef0 https://git.kernel.org/stable/c/e22e45fc9e41bf9fcc1e92cfb78eb92786728ef0 https://git.kernel.org/stable/c/e73164e89d1be561228a4534e1091369ee4ba41a https://git.kernel.org/stable/c/e73164e89d1be561228a4534e1091369ee4ba41a https://git.kernel.org/stable/c/fe5838c22b986c1190f1dce9aa09bf6a491c1a69 https://git.kernel.org/stable/c/fe5838c22b986c1190f1dce9aa09bf6a491c1a69