mm: kmem: fix a NULL pointer dereference in obj_stock_flush_required()

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

mm: kmem: fix a NULL pointer dereference in obj_stock_flush_required()

KCSAN found an issue in obj_stock_flush_required():
stock->cached_objcg can be reset between the check and dereference:

==================================================================
BUG: KCSAN: data-race in drain_all_stock / drain_obj_stock

write to 0xffff888237c2a2f8 of 8 bytes by task 19625 on cpu 0:
drain_obj_stock+0x408/0x4e0 mm/memcontrol.c:3306
refill_obj_stock+0x9c/0x1e0 mm/memcontrol.c:3340
obj_cgroup_uncharge+0xe/0x10 mm/memcontrol.c:3408
memcg_slab_free_hook mm/slab.h:587 [inline]
__cache_free mm/slab.c:3373 [inline]
__do_kmem_cache_free mm/slab.c:3577 [inline]
kmem_cache_free+0x105/0x280 mm/slab.c:3602
__d_free fs/dcache.c:298 [inline]
dentry_free fs/dcache.c:375 [inline]
__dentry_kill+0x422/0x4a0 fs/dcache.c:621
dentry_kill+0x8d/0x1e0
dput+0x118/0x1f0 fs/dcache.c:913
__fput+0x3bf/0x570 fs/file_table.c:329
____fput+0x15/0x20 fs/file_table.c:349
task_work_run+0x123/0x160 kernel/task_work.c:179
resume_user_mode_work include/linux/resume_user_mode.h:49 [inline]
exit_to_user_mode_loop+0xcf/0xe0 kernel/entry/common.c:171
exit_to_user_mode_prepare+0x6a/0xa0 kernel/entry/common.c:203
__syscall_exit_to_user_mode_work kernel/entry/common.c:285 [inline]
syscall_exit_to_user_mode+0x26/0x140 kernel/entry/common.c:296
do_syscall_64+0x4d/0xc0 arch/x86/entry/common.c:86
entry_SYSCALL_64_after_hwframe+0x63/0xcd

read to 0xffff888237c2a2f8 of 8 bytes by task 19632 on cpu 1:
obj_stock_flush_required mm/memcontrol.c:3319 [inline]
drain_all_stock+0x174/0x2a0 mm/memcontrol.c:2361
try_charge_memcg+0x6d0/0xd10 mm/memcontrol.c:2703
try_charge mm/memcontrol.c:2837 [inline]
mem_cgroup_charge_skmem+0x51/0x140 mm/memcontrol.c:7290
sock_reserve_memory+0xb1/0x390 net/core/sock.c:1025
sk_setsockopt+0x800/0x1e70 net/core/sock.c:1525
udp_lib_setsockopt+0x99/0x6c0 net/ipv4/udp.c:2692
udp_setsockopt+0x73/0xa0 net/ipv4/udp.c:2817
sock_common_setsockopt+0x61/0x70 net/core/sock.c:3668
__sys_setsockopt+0x1c3/0x230 net/socket.c:2271
__do_sys_setsockopt net/socket.c:2282 [inline]
__se_sys_setsockopt net/socket.c:2279 [inline]
__x64_sys_setsockopt+0x66/0x80 net/socket.c:2279
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

value changed: 0xffff8881382d52c0 -> 0xffff888138893740

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 19632 Comm: syz-executor.0 Not tainted 6.3.0-rc2-syzkaller-00387-g534293368afa #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/02/2023

Fix it by using READ_ONCE()/WRITE_ONCE() for all accesses to
stock->cached_objcg.