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Date:   Fri, 21 Apr 2023 15:55:28 -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] mm: kmem: fix a NULL pointer dereference in
 obj_stock_flush_required()
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 <20230421174054.3434533-1-roman.gushchin@linux.dev>
 <CAJD7tkZRLsubybPesf65t1ATEksTtif8uztAFZ-gVmYCnjw0Bg@mail.gmail.com>
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On Fri, Apr 21, 2023 at 03:16:02PM -0700, Yosry Ahmed wrote:
> On Fri, Apr 21, 2023 at 10:41 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 reading the cached_objcg with READ_ONCE().
> 
> IIUC reading an outdated objcg is fine here, and the real problem is a
> potential NULL dereference, which isn't shown by this report. Is this
> correct?

My understanding of the problem:
1) we are reading stock->cached_objcg and checking for being NULL, it's not NULL.
2) we're reading it again and dereferencing it without a check.

If stock->cached_objcg is zeroed between 1) and 2), we're in trouble.

The fix makes sure we're dereferencing exactly what we check.

> 
> >
> > 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>
> > ---
> >  mm/memcontrol.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 5abffe6f8389..9426a1ddc190 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -3314,10 +3314,11 @@ static struct obj_cgroup *drain_obj_stock(struct memcg_stock_pcp *stock)
> >  static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
> >                                      struct mem_cgroup *root_memcg)
> >  {
> > +       struct obj_cgroup *objcg = READ_ONCE(stock->cached_objcg);
> >         struct mem_cgroup *memcg;
> >
> > -       if (stock->cached_objcg) {
> > -               memcg = obj_cgroup_memcg(stock->cached_objcg);
> > +       if (objcg) {
> > +               memcg = obj_cgroup_memcg(objcg);
> >                 if (memcg && mem_cgroup_is_descendant(memcg, root_memcg))
> >                         return true;
> >         }
> > --
> > 2.40.0
> >
> >