rds: tcp: Fix use-after-free of net in reqsk_timer_handler().

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

rds: tcp: Fix use-after-free of net in reqsk_timer_handler().

syzkaller reported a warning of netns tracker [0] followed by KASAN
splat [1] and another ref tracker warning [1].

syzkaller could not find a repro, but in the log, the only suspicious
sequence was as follows:

18:26:22 executing program 1:
r0 = socket$inet6_mptcp(0xa, 0x1, 0x106)
...
connect$inet6(r0, &(0x7f0000000080)={0xa, 0x4001, 0x0, @loopback}, 0x1c) (async)

The notable thing here is 0x4001 in connect(), which is RDS_TCP_PORT.

So, the scenario would be:

1. unshare(CLONE_NEWNET) creates a per netns tcp listener in
rds_tcp_listen_init().
2. syz-executor connect()s to it and creates a reqsk.
3. syz-executor exit()s immediately.
4. netns is dismantled. [0]
5. reqsk timer is fired, and UAF happens while freeing reqsk. [1]
6. listener is freed after RCU grace period. [2]

Basically, reqsk assumes that the listener guarantees netns safety
until all reqsk timers are expired by holding the listener's refcount.
However, this was not the case for kernel sockets.

Commit 740ea3c4a0b2 ("tcp: Clean up kernel listener's reqsk in
inet_twsk_purge()") fixed this issue only for per-netns ehash.

Let's apply the same fix for the global ehash.

[0]:
ref_tracker: net notrefcnt@0000000065449cc3 has 1/1 users at
sk_alloc (./include/net/net_namespace.h:337 net/core/sock.c:2146)
inet6_create (net/ipv6/af_inet6.c:192 net/ipv6/af_inet6.c:119)
__sock_create (net/socket.c:1572)
rds_tcp_listen_init (net/rds/tcp_listen.c:279)
rds_tcp_init_net (net/rds/tcp.c:577)
ops_init (net/core/net_namespace.c:137)
setup_net (net/core/net_namespace.c:340)
copy_net_ns (net/core/net_namespace.c:497)
create_new_namespaces (kernel/nsproxy.c:110)
unshare_nsproxy_namespaces (kernel/nsproxy.c:228 (discriminator 4))
ksys_unshare (kernel/fork.c:3429)
__x64_sys_unshare (kernel/fork.c:3496)
do_syscall_64 (arch/x86/entry/common.c:52 arch/x86/entry/common.c:83)
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:129)
...
WARNING: CPU: 0 PID: 27 at lib/ref_tracker.c:179 ref_tracker_dir_exit (lib/ref_tracker.c:179)

[1]:
BUG: KASAN: slab-use-after-free in inet_csk_reqsk_queue_drop (./include/net/inet_hashtables.h:180 net/ipv4/inet_connection_sock.c:952 net/ipv4/inet_connection_sock.c:966)
Read of size 8 at addr ffff88801b370400 by task swapper/0/0
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
Call Trace:
<IRQ>
dump_stack_lvl (lib/dump_stack.c:107 (discriminator 1))
print_report (mm/kasan/report.c:378 mm/kasan/report.c:488)
kasan_report (mm/kasan/report.c:603)
inet_csk_reqsk_queue_drop (./include/net/inet_hashtables.h:180 net/ipv4/inet_connection_sock.c:952 net/ipv4/inet_connection_sock.c:966)
reqsk_timer_handler (net/ipv4/inet_connection_sock.c:979 net/ipv4/inet_connection_sock.c:1092)
call_timer_fn (./arch/x86/include/asm/jump_label.h:27 ./include/linux/jump_label.h:207 ./include/trace/events/timer.h:127 kernel/time/timer.c:1701)
__run_timers.part.0 (kernel/time/timer.c:1752 kernel/time/timer.c:2038)
run_timer_softirq (kernel/time/timer.c:2053)
__do_softirq (./arch/x86/include/asm/jump_label.h:27 ./include/linux/jump_label.h:207 ./include/trace/events/irq.h:142 kernel/softirq.c:554)
irq_exit_rcu (kernel/softirq.c:427 kernel/softirq.c:632 kernel/softirq.c:644)
sysvec_apic_timer_interrupt (arch/x86/kernel/apic/apic.c:1076 (discriminator 14))
</IRQ>

Allocated by task 258 on cpu 0 at 83.612050s:
kasan_save_stack (mm/kasan/common.c:48)
kasan_save_track (mm/kasan/common.c:68)
__kasan_slab_alloc (mm/kasan/common.c:343)
kmem_cache_alloc (mm/slub.c:3813 mm/slub.c:3860 mm/slub.c:3867)
copy_net_ns (./include/linux/slab.h:701 net/core/net_namespace.c:421 net/core/net_namespace.c:480)
create_new_namespaces (kernel/nsproxy.c:110)
unshare_nsproxy_name
---truncated---