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

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 reading the cached_objcg with READ_ONCE().

Link: https://lkml.kernel.org/r/20230421174054.3434533-1-roman.gushchin@linux.dev
Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API")
Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Reported-by: syzbot+774c29891415ab0fd29d@syzkaller.appspotmail.com
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Link: https://lore.kernel.org/linux-mm/CACT4Y+ZfucZhM60YPphWiCLJr6+SGFhT+jjm8k1P-a_8Kkxsjg@mail.gmail.com/T/#t
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>