fs/proc/proc_sysctl.c: Fix a NULL pointer dereference

Syzkaller report this:

  sysctl could not get directory: /net//bridge -12
  kasan: CONFIG_KASAN_INLINE enabled
  kasan: GPF could be caused by NULL-ptr deref or user memory access
  general protection fault: 0000 [#1] SMP KASAN PTI
  CPU: 1 PID: 7027 Comm: syz-executor.0 Tainted: G         C        5.1.0-rc3+ #8
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
  RIP: 0010:__write_once_size include/linux/compiler.h:220 [inline]
  RIP: 0010:__rb_change_child include/linux/rbtree_augmented.h:144 [inline]
  RIP: 0010:__rb_erase_augmented include/linux/rbtree_augmented.h:186 [inline]
  RIP: 0010:rb_erase+0x5f4/0x19f0 lib/rbtree.c:459
  Code: 00 0f 85 60 13 00 00 48 89 1a 48 83 c4 18 5b 5d 41 5c 41 5d 41 5e 41 5f c3 48 89 f2 48 b8 00 00 00 00 00 fc ff df 48 c1 ea 03 <80> 3c 02 00 0f 85 75 0c 00 00 4d 85 ed 4c 89 2e 74 ce 4c 89 ea 48
  RSP: 0018:ffff8881bb507778 EFLAGS: 00010206
  RAX: dffffc0000000000 RBX: ffff8881f224b5b8 RCX: ffffffff818f3f6a
  RDX: 000000000000000a RSI: 0000000000000050 RDI: ffff8881f224b568
  RBP: 0000000000000000 R08: ffffed10376a0ef4 R09: ffffed10376a0ef4
  R10: 0000000000000001 R11: ffffed10376a0ef4 R12: ffff8881f224b558
  R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
  FS:  00007f3e7ce13700(0000) GS:ffff8881f7300000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 00007fd60fbe9398 CR3: 00000001cb55c001 CR4: 00000000007606e0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  PKRU: 55555554
  Call Trace:
   erase_entry fs/proc/proc_sysctl.c:178 [inline]
   erase_header+0xe3/0x160 fs/proc/proc_sysctl.c:207
   start_unregistering fs/proc/proc_sysctl.c:331 [inline]
   drop_sysctl_table+0x558/0x880 fs/proc/proc_sysctl.c:1631
   get_subdir fs/proc/proc_sysctl.c:1022 [inline]
   __register_sysctl_table+0xd65/0x1090 fs/proc/proc_sysctl.c:1335
   br_netfilter_init+0x68/0x1000 [br_netfilter]
   do_one_initcall+0xbc/0x47d init/main.c:901
   do_init_module+0x1b5/0x547 kernel/module.c:3456
   load_module+0x6405/0x8c10 kernel/module.c:3804
   __do_sys_finit_module+0x162/0x190 kernel/module.c:3898
   do_syscall_64+0x9f/0x450 arch/x86/entry/common.c:290
   entry_SYSCALL_64_after_hwframe+0x49/0xbe
  Modules linked in: br_netfilter(+) backlight comedi(C) hid_sensor_hub max3100 ti_ads8688 udc_core fddi snd_mona leds_gpio rc_streamzap mtd pata_netcell nf_log_common rc_winfast udp_tunnel snd_usbmidi_lib snd_usb_toneport snd_usb_line6 snd_rawmidi snd_seq_device snd_hwdep videobuf2_v4l2 videobuf2_common videodev media videobuf2_vmalloc videobuf2_memops rc_gadmei_rm008z 8250_of smm665 hid_tmff hid_saitek hwmon_vid rc_ati_tv_wonder_hd_600 rc_core pata_pdc202xx_old dn_rtmsg as3722 ad714x_i2c ad714x snd_soc_cs4265 hid_kensington panel_ilitek_ili9322 drm drm_panel_orientation_quirks ipack cdc_phonet usbcore phonet hid_jabra hid extcon_arizona can_dev industrialio_triggered_buffer kfifo_buf industrialio adm1031 i2c_mux_ltc4306 i2c_mux ipmi_msghandler mlxsw_core snd_soc_cs35l34 snd_soc_core snd_pcm_dmaengine snd_pcm snd_timer ac97_bus snd_compress snd soundcore gpio_da9055 uio ecdh_generic mdio_thunder of_mdio fixed_phy libphy mdio_cavium iptable_security iptable_raw iptable_mangle
   iptable_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 iptable_filter bpfilter ip6_vti ip_vti ip_gre ipip sit tunnel4 ip_tunnel hsr veth netdevsim vxcan batman_adv cfg80211 rfkill chnl_net caif nlmon dummy team bonding vcan bridge stp llc ip6_gre gre ip6_tunnel tunnel6 tun joydev mousedev ppdev tpm kvm_intel kvm irqbypass crct10dif_pclmul crc32_pclmul crc32c_intel ghash_clmulni_intel aesni_intel ide_pci_generic piix aes_x86_64 crypto_simd cryptd ide_core glue_helper input_leds psmouse intel_agp intel_gtt serio_raw ata_generic i2c_piix4 agpgart pata_acpi parport_pc parport floppy rtc_cmos sch_fq_codel ip_tables x_tables sha1_ssse3 sha1_generic ipv6 [last unloaded: br_netfilter]
  Dumping ftrace buffer:
     (ftrace buffer empty)
  ---[ end trace 68741688d5fbfe85 ]---

commit 23da9588 ("fs/proc/proc_sysctl.c: fix NULL pointer
dereference in put_links") forgot to handle start_unregistering() case,
while header->parent is NULL, it calls erase_header() and as seen in the
above syzkaller call trace, accessing &header->parent->root will trigger
a NULL pointer dereference.

As that commit explained, there is also no need to call
start_unregistering() if header->parent is NULL.

Link: http://lkml.kernel.org/r/20190409153622.28112-1-yuehaibing@huawei.com
Fixes: 23da9588 ("fs/proc/proc_sysctl.c: fix NULL pointer dereference in put_links")
Fixes: 0e47c99d ("sysctl: Replace root_list with links between sysctl_table_sets")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/proc_sysctl.c

@@ -1626,9 +1626,11 @@ static void drop_sysctl_table(struct ctl_table_header *header)
 	if (--header->nreg)
 		return;
 
-	if (parent)
+	if (parent) {
 		put_links(header);
-	start_unregistering(header);
+		start_unregistering(header);
+	}
+
 	if (!--header->count)
 		kfree_rcu(header, rcu);