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From: Roman Gushchin To: Yosry Ahmed 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 Subject: Re: [PATCH] mm: kmem: fix a NULL pointer dereference in obj_stock_flush_required() Message-ID: References: <00000000000058b63f05f9d98811@google.com> <20230421174054.3434533-1-roman.gushchin@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 7D8801A0004 X-Rspam-User: X-Stat-Signature: 6wucumid1oaaqogsmnr9fscj5y49estg X-HE-Tag: 1682117735-583938 X-HE-Meta: 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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 > > Link: https://lore.kernel.org/linux-mm/CACT4Y+ZfucZhM60YPphWiCLJr6+SGFhT+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(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 > > > >