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Date:   Fri, 21 Apr 2023 12:40:45 -0500
From:   Mike Christie <michael.christie@...cle.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Dmitry Vyukov <dvyukov@...gle.com>
Cc:     syzbot <syzbot+c2de99a72baaa06d31f3@...kaller.appspotmail.com>,
        adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, llvm@...ts.linux.dev, nathan@...nel.org,
        ndesaulniers@...gle.com, syzkaller-bugs@...glegroups.com,
        trix@...hat.com, tytso@....edu,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, martin.lau@...ux.dev,
        bpf <bpf@...r.kernel.org>, KP Singh <kpsingh@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Christian Brauner <brauner@...nel.org>
Subject: Re: [syzbot] [ext4?] [mm?] KCSAN: data-race in strscpy / strscpy (3)

cc'ing Christian, because I might have fixed this with a patch in
his tree.

On 4/21/23 9:58 AM, Matthew Wilcox wrote:
> I'm not sure how it is that bpf is able to see the task before comm is
> initialised; that seems to be the real race here, that comm is not set
> before the kthread is a schedulable entity?  Adding the scheduler people.
> 
>>> ==================================================================
>>> BUG: KCSAN: data-race in strscpy / strscpy
>>>
>>> write to 0xffff88812ed8b730 of 8 bytes by task 16157 on cpu 1:
>>>  strscpy+0xa9/0x170 lib/string.c:165
>>>  strscpy_pad+0x27/0x80 lib/string_helpers.c:835
>>>  __set_task_comm+0x46/0x140 fs/exec.c:1232
>>>  set_task_comm include/linux/sched.h:1984 [inline]
>>>  __kthread_create_on_node+0x2b2/0x320 kernel/kthread.c:474
>>>  kthread_create_on_node+0x8a/0xb0 kernel/kthread.c:512
>>>  ext4_run_lazyinit_thread fs/ext4/super.c:3848 [inline]
>>>  ext4_register_li_request+0x407/0x650 fs/ext4/super.c:3983
>>>  __ext4_fill_super fs/ext4/super.c:5480 [inline]
>>>  ext4_fill_super+0x3f4a/0x43f0 fs/ext4/super.c:5637
>>>  get_tree_bdev+0x2b1/0x3a0 fs/super.c:1303
>>>  ext4_get_tree+0x1c/0x20 fs/ext4/super.c:5668
>>>  vfs_get_tree+0x51/0x190 fs/super.c:1510
>>>  do_new_mount+0x200/0x650 fs/namespace.c:3042
>>>  path_mount+0x498/0xb40 fs/namespace.c:3372
>>>  do_mount fs/namespace.c:3385 [inline]
>>>  __do_sys_mount fs/namespace.c:3594 [inline]
>>>  __se_sys_mount+0x27f/0x2d0 fs/namespace.c:3571
>>>  __x64_sys_mount+0x67/0x80 fs/namespace.c:3571
>>>  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
>>>
>>> read to 0xffff88812ed8b733 of 1 bytes by task 16161 on cpu 0:
>>>  strscpy+0xde/0x170 lib/string.c:174
>>>  ____bpf_get_current_comm kernel/bpf/helpers.c:260 [inline]
>>>  bpf_get_current_comm+0x45/0x70 kernel/bpf/helpers.c:252
>>>  ___bpf_prog_run+0x281/0x3050 kernel/bpf/core.c:1822
>>>  __bpf_prog_run32+0x74/0xa0 kernel/bpf/core.c:2043
>>>  bpf_dispatcher_nop_func include/linux/bpf.h:1124 [inline]
>>>  __bpf_prog_run include/linux/filter.h:601 [inline]
>>>  bpf_prog_run include/linux/filter.h:608 [inline]
>>>  __bpf_trace_run kernel/trace/bpf_trace.c:2263 [inline]
>>>  bpf_trace_run4+0x9f/0x140 kernel/trace/bpf_trace.c:2304
>>>  __traceiter_sched_switch+0x3a/0x50 include/trace/events/sched.h:222
>>>  trace_sched_switch include/trace/events/sched.h:222 [inline]
>>>  __schedule+0x7e7/0x8e0 kernel/sched/core.c:6622
>>>  schedule+0x51/0x80 kernel/sched/core.c:6701
>>>  schedule_preempt_disabled+0x10/0x20 kernel/sched/core.c:6760
>>>  kthread+0x11c/0x1e0 kernel/kthread.c:369
>>>  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:308
>>>


I didn't see the beginning of this thread and I think the part of the
sysbot report that lists the patches/trees being used got cut off so
I'm not 100% sure what's in the kernel.

In Linus's current tree we do set_task_comm in __kthread_create_on_node
after waiting on the kthread_create_info completion which is completed by
threadd(). At this time, kthread() has already done the complete() on the
kthread_create_info completion and started to run the threadfn function and
that could be running. So we can hit the race that way.


In linux next, from 
https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/log/?h=kernel.user_worker
we have:

commit cf587db2ee0261c74d04f61f39783db88a0b65e4
Author: Mike Christie <michael.christie@...cle.com>
Date:   Fri Mar 10 16:03:23 2023 -0600

    kernel: Allow a kernel thread's name to be set in copy_process

and so now copy_process() sets the name before the taskfn is started, so we
shouldn't hit any races like above.

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