Linux stable v5.10.226 suffers a lockdep warning when accessing /proc/PID/cpuset. cset_cgroup_from_root() is called without cgroup_mutex is held, which causes assertion failure.
Bisect blames 5.10.225 commit 688325078a8b ("cgroup/cpuset: Prevent UAF in proc_cpuset_show()"). I've have not easily reproduced the problem that this change fixes, so I'm not sure if it's best to revert the fix or adapt it to meet the 5.10 locking expectations.
The lockdep complaint:
$ cat /proc/1/cpuset $ dmesg [ 198.744891] ------------[ cut here ]------------ [ 198.744918] WARNING: CPU: 4 PID: 9301 at kernel/cgroup/cgroup.c:1395 cset_cgroup_from_root+0xb2/0xd0 [ 198.744957] RIP: 0010:cset_cgroup_from_root+0xb2/0xd0 [ 198.744960] Code: 02 00 00 74 11 48 8b 09 48 39 cb 75 eb eb 19 49 83 c6 10 4c 89 f0 48 85 c0 74 0d 5b 41 5e c3 48 8b 43 60 48 85 c0 75 f3 0f 0b <0f> 0b 83 3d 69 01 ee 01 00 0f 85 78 ff ff ff eb 8b 0f 0b eb 87 66 [ 198.744962] RSP: 0018:ffffb492608a7ce8 EFLAGS: 00010046 [ 198.744977] RAX: 0000000000000000 RBX: ffffffff8f4171b8 RCX: cc949de848c33e00 [ 198.744979] RDX: 0000000000001000 RSI: ffffffff8f415450 RDI: ffff92e5417c4dc0 [ 198.744981] RBP: ffff9303467e3f00 R08: 0000000000000008 R09: ffffffff9122d568 [ 198.744983] R10: ffff92e5417c4380 R11: 0000000000000000 R12: ffff92e3d9506000 [ 198.744984] R13: 0000000000000000 R14: ffff92e443a96000 R15: ffff92e3d9506000 [ 198.744987] FS: 00007f15d94ed740(0000) GS:ffff9302bf500000(0000) knlGS:0000000000000000 [ 198.744988] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 198.744990] CR2: 00007f15d94ca000 CR3: 00000002816ca003 CR4: 00000000001706e0 [ 198.744992] Call Trace: [ 198.744996] ? __warn+0xcd/0x1c0 [ 198.745000] ? cset_cgroup_from_root+0xb2/0xd0 [ 198.745008] ? report_bug+0x87/0xf0 [ 198.745015] ? handle_bug+0x42/0x80 [ 198.745017] ? exc_invalid_op+0x16/0x70 [ 198.745021] ? asm_exc_invalid_op+0x12/0x20 [ 198.745030] ? cset_cgroup_from_root+0xb2/0xd0 [ 198.745034] ? cset_cgroup_from_root+0x28/0xd0 [ 198.745038] cgroup_path_ns_locked+0x23/0x50 [ 198.745044] proc_cpuset_show+0x115/0x210 [ 198.745049] proc_single_show+0x4a/0xa0 [ 198.745056] seq_read_iter+0x14d/0x400 [ 198.745063] seq_read+0x103/0x130 [ 198.745074] vfs_read+0xea/0x320 [ 198.745078] ? do_user_addr_fault+0x25b/0x390 [ 198.745085] ? do_user_addr_fault+0x25b/0x390 [ 198.745090] ksys_read+0x70/0xe0 [ 198.745096] do_syscall_64+0x2d/0x40 [ 198.745099] entry_SYSCALL_64_after_hwframe+0x61/0xcb