eventfs: Use list_del_rcu() for SRCU protected list variable

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

eventfs: Use list_del_rcu() for SRCU protected list variable

Chi Zhiling reported:

We found a null pointer accessing in tracefs[1], the reason is that the
variable 'ei_child' is set to LIST_POISON1, that means the list was
removed in eventfs_remove_rec. so when access the ei_child->is_freed, the
panic triggered.

by the way, the following script can reproduce this panic

loop1 (){
while true
do
echo "p:kp submit_bio" > /sys/kernel/debug/tracing/kprobe_events
echo "" > /sys/kernel/debug/tracing/kprobe_events
done
}
loop2 (){
while true
do
tree /sys/kernel/debug/tracing/events/kprobes/
done
}
loop1 &
loop2

[1]:
[ 1147.959632][T17331] Unable to handle kernel paging request at virtual address dead000000000150
[ 1147.968239][T17331] Mem abort info:
[ 1147.971739][T17331] ESR = 0x0000000096000004
[ 1147.976172][T17331] EC = 0x25: DABT (current EL), IL = 32 bits
[ 1147.982171][T17331] SET = 0, FnV = 0
[ 1147.985906][T17331] EA = 0, S1PTW = 0
[ 1147.989734][T17331] FSC = 0x04: level 0 translation fault
[ 1147.995292][T17331] Data abort info:
[ 1147.998858][T17331] ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000
[ 1148.005023][T17331] CM = 0, WnR = 0, TnD = 0, TagAccess = 0
[ 1148.010759][T17331] GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
[ 1148.016752][T17331] [dead000000000150] address between user and kernel address ranges
[ 1148.024571][T17331] Internal error: Oops: 0000000096000004 [#1] SMP
[ 1148.030825][T17331] Modules linked in: team_mode_loadbalance team nlmon act_gact cls_flower sch_ingress bonding tls macvlan dummy ib_core bridge stp llc veth amdgpu amdxcp mfd_core gpu_sched drm_exec drm_buddy radeon crct10dif_ce video drm_suballoc_helper ghash_ce drm_ttm_helper sha2_ce ttm sha256_arm64 i2c_algo_bit sha1_ce sbsa_gwdt cp210x drm_display_helper cec sr_mod cdrom drm_kms_helper binfmt_misc sg loop fuse drm dm_mod nfnetlink ip_tables autofs4 [last unloaded: tls]
[ 1148.072808][T17331] CPU: 3 PID: 17331 Comm: ls Tainted: G W ------- ---- 6.6.43 #2
[ 1148.081751][T17331] Source Version: 21b3b386e948bedd29369af66f3e98ab01b1c650
[ 1148.088783][T17331] Hardware name: Greatwall GW-001M1A-FTF/GW-001M1A-FTF, BIOS KunLun BIOS V4.0 07/16/2020
[ 1148.098419][T17331] pstate: 20000005 (nzCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 1148.106060][T17331] pc : eventfs_iterate+0x2c0/0x398
[ 1148.111017][T17331] lr : eventfs_iterate+0x2fc/0x398
[ 1148.115969][T17331] sp : ffff80008d56bbd0
[ 1148.119964][T17331] x29: ffff80008d56bbf0 x28: ffff001ff5be2600 x27: 0000000000000000
[ 1148.127781][T17331] x26: ffff001ff52ca4e0 x25: 0000000000009977 x24: dead000000000100
[ 1148.135598][T17331] x23: 0000000000000000 x22: 000000000000000b x21: ffff800082645f10
[ 1148.143415][T17331] x20: ffff001fddf87c70 x19: ffff80008d56bc90 x18: 0000000000000000
[ 1148.151231][T17331] x17: 0000000000000000 x16: 0000000000000000 x15: ffff001ff52ca4e0
[ 1148.159048][T17331] x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000000000
[ 1148.166864][T17331] x11: 0000000000000000 x10: 0000000000000000 x9 : ffff8000804391d0
[ 1148.174680][T17331] x8 : 0000000180000000 x7 : 0000000000000018 x6 : 0000aaab04b92862
[ 1148.182498][T17331] x5 : 0000aaab04b92862 x4 : 0000000080000000 x3 : 0000000000000068
[ 1148.190314][T17331] x2 : 000000000000000f x1 : 0000000000007ea8 x0 : 0000000000000001
[ 1148.198131][T17331] Call trace:
[ 1148.201259][T17331] eventfs_iterate+0x2c0/0x398
[ 1148.205864][T17331] iterate_dir+0x98/0x188
[ 1148.210036][T17331] __arm64_sys_getdents64+0x78/0x160
[ 1148.215161][T17331] invoke_syscall+0x78/0x108
[ 1148.219593][T17331] el0_svc_common.constprop.0+0x48/0xf0
[ 1148.224977][T17331] do_el0_svc+0x24/0x38
[ 1148.228974][T17331] el0_svc+0x40/0x168
[ 1148.232798][T17
---truncated---