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

Yosry Ahmed
2023-04-22T06:59:17Z

On Fri, Apr 21, 2023 at 3:55 PM Roman Gushchin <roman.gushchin@linux.dev> wrote:
>
> 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.

I see. This matches my understanding. The bug report does not show a
NULL dereference but it shows a data race that can very well lead to
one.

FWIW,
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>

>
> >
> > >
> > > 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
> > >
> > >