Re: [PATCH v2 1/2] mm: kmem: fix a NULL pointer dereference in obj_stock_flush_required()

[Yosry Ahmed]

On Tue, May 2, 2023 at 2:38 PM Roman Gushchin <roman.gushchin@xxxxxxxxx> wrote:
>
> 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@xxxxxxxxx> 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.
>

Agreed. It might be worth adding a comment somewhere documenting this.
It's not very hard to dig though, so whatever you think is best.

> >
> > >
> > > Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API")
> > > Reported-by: syzbot+774c29891415ab0fd29d@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > > Link:
> > > https://lore.kernel.org/linux-mm/CACT4Y+ZfucZhM60YPphWiCLJr6+SGFhT+jjm8k1P-a_8Kkxsjg@xxxxxxxxxxxxxx/T/#t
> > > Signed-off-by: Roman Gushchin <roman.gushchin@xxxxxxxxx>
> >
> > With the above said, I don't feel strongly either way, the patch looks
> > good AFAICT:
> > Reviewed-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
>
> Thanks!