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NULL pointer dereference in obj_stock_flush_required() To: Roman Gushchin Cc: linux-mm@kvack.org, Andrew Morton , Johannes Weiner , Michal Hocko , Shakeel Butt , Muchun Song , linux-kernel@vger.kernel.org, syzbot+774c29891415ab0fd29d@syzkaller.appspotmail.com, Dmitry Vyukov Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 5B53B80005 X-Stat-Signature: 68o3ta7ds3edz3n5unqjjeewqcjxrwja X-Rspam-User: X-HE-Tag: 1682146710-750646 X-HE-Meta: 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 qENZyOAM 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Apr 21, 2023 at 3:55=E2=80=AFPM Roman Gushchin wrote: > > On Fri, Apr 21, 2023 at 03:16:02PM -0700, Yosry Ahmed wrote: > > On Fri, Apr 21, 2023 at 10:41=E2=80=AFAM Roman Gushchin > > wrote: > > > > > > KCSAN found an issue in obj_stock_flush_required(): > > > stock->cached_objcg can be reset between the check and dereference: > > > > > > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D > > > 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-syzkalle= r-00387-g534293368afa #0 > > > Hardware name: Google Google Compute Engine/Google Compute Engine, BI= OS 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 n= ot 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 > > > > > > > > > Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API") > > > Reported-by: syzbot+774c29891415ab0fd29d@syzkaller.appspotmail.com > > > Reported-by: Dmitry Vyukov > > > Link: https://lore.kernel.org/linux-mm/CACT4Y+ZfucZhM60YPphWiCLJr6+SG= FhT+jjm8k1P-a_8Kkxsjg@mail.gmail.com/T/#t > > > Signed-off-by: Roman Gushchin > > > --- > > > 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(str= uct memcg_stock_pcp *stock) > > > static bool obj_stock_flush_required(struct memcg_stock_pcp *stock, > > > struct mem_cgroup *root_memcg) > > > { > > > + struct obj_cgroup *objcg =3D READ_ONCE(stock->cached_objcg); > > > struct mem_cgroup *memcg; > > > > > > - if (stock->cached_objcg) { > > > - memcg =3D obj_cgroup_memcg(stock->cached_objcg); > > > + if (objcg) { > > > + memcg =3D obj_cgroup_memcg(objcg); > > > if (memcg && mem_cgroup_is_descendant(memcg, root_mem= cg)) > > > return true; > > > } > > > -- > > > 2.40.0 > > > > > >