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Date:   Tue, 2 May 2023 14:38:19 -0700
From:   Roman Gushchin <roman.gushchin@linux.dev>
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <muchun.song@linux.dev>,
        linux-kernel@vger.kernel.org,
        syzbot+774c29891415ab0fd29d@syzkaller.appspotmail.com,
        Dmitry Vyukov <dvyukov@google.com>
Subject: Re: [PATCH v2 1/2] mm: kmem: fix a NULL pointer dereference in
 obj_stock_flush_required()
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On Tue, May 02, 2023 at 01:15:02PM -0700, Yosry Ahmed wrote:
> On Tue, May 2, 2023 at 9:09 AM Roman Gushchin <roman.gushchin@linux.dev> wrote:
> >
> > 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.
> 
> I believe all read accesses other than obj_stock_flush_required() are
> done under the lock, so READ_ONCE() wouldn't be needed AFAICT. Having
> READ_ONCE() only around the racy read can be useful to document the
> racy read and differentiate it from others.
> 
> With that said, it's also inconvenient to keep track moving forward of
> which reading sites are racy, and it may be simpler to just annotate
> all readers with READ_ONCE().
> 
> I am not sure which approach is better, just thinking out loud.

Yeah, I wasn't sure either. I believe that all changes except the original
READ_ONCE() are not leading to any meaningful asm changes, so it's a matter
of taste.

The reason why I went with the "change them all" approach:
reads without READ_ONCE() and subsequent writes with WRITE_ONCE()
inside a single function looked really weird.

> 
> >
> > Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API")
> > 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
> > Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
> 
> With the above said, I don't feel strongly either way, the patch looks
> good AFAICT:
> Reviewed-by: Yosry Ahmed <yosryahmed@google.com>

Thanks!