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Subject: Re: [PATCH net] net/9p: fix issue of list_del corruption in
 p9_fd_cancel()
To:     <asmadeus@codewreck.org>
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References: <20221110122606.383352-1-shaozhengchao@huawei.com>
 <Y2zz24jRIo9DdWw7@codewreck.org>
From:   shaozhengchao <shaozhengchao@huawei.com>
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On 2022/11/10 20:51, asmadeus@codewreck.org wrote:
> Zhengchao Shao wrote on Thu, Nov 10, 2022 at 08:26:06PM +0800:
>> Syz reported the following issue:
>> kernel BUG at lib/list_debug.c:53!
>> invalid opcode: 0000 [#1] PREEMPT SMP KASAN
>> RIP: 0010:__list_del_entry_valid.cold+0x5c/0x72
>> Call Trace:
>> <TASK>
>> p9_fd_cancel+0xb1/0x270
>> p9_client_rpc+0x8ea/0xba0
>> p9_client_create+0x9c0/0xed0
>> v9fs_session_init+0x1e0/0x1620
>> v9fs_mount+0xba/0xb80
>> legacy_get_tree+0x103/0x200
>> vfs_get_tree+0x89/0x2d0
>> path_mount+0x4c0/0x1ac0
>> __x64_sys_mount+0x33b/0x430
>> do_syscall_64+0x35/0x80
>> entry_SYSCALL_64_after_hwframe+0x46/0xb0
>> </TASK>
>>
>> The process is as follows:
>> Thread A:                       Thread B:
>> p9_poll_workfn()                p9_client_create()
>> ...                                 ...
>>      p9_conn_cancel()                p9_fd_cancel()
>>          list_del()                      ...
>>          ...                             list_del()  //list_del
>>                                                        corruption
>> There is no lock protection when deleting list in p9_conn_cancel(). After
>> deleting list in Thread A, thread B will delete the same list again. It
>> will cause issue of list_del corruption.
> 
> Thanks!
> 
> I'd add a couple of lines here describing the actual fix.
> Something like this?
> ---
> Setting req->status to REQ_STATUS_ERROR under lock prevents other
> cleanup paths from trying to manipulate req_list.
> The other thread can safely check req->status because it still holds a
> reference to req at this point.
> ---
> 
> With that out of the way, it's a good idea; I didn't remember that
> p9_fd_cancel (and cancelled) check for req status before acting on it.
> This really feels like whack-a-mole, but I'd say this is one step
> better.
> 
> Please tell me if you want to send a v2 with your words, or I'll just
> pick this up with my suggestion and submit to Linus in a week-ish after
> testing. No point in waiting a full cycle for this.
> 
> 
Hi Dominique:
	Thank you for your review. Your suggestion looks good to me, and
please add your suggestion. :)
>> Fixes: 52f1c45dde91 ("9p: trans_fd/p9_conn_cancel: drop client lock earlier")
>> Reported-by: syzbot+9b69b8d10ab4a7d88056@syzkaller.appspotmail.com
>> Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
>> ---
>> v2: set req status when removing list
> 
> (I don't recall seeing a v1?)
> 
Sorry, please ignore this notes.
>> ---
>>   net/9p/trans_fd.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
>> index 56a186768750..bd28e63d7666 100644
>> --- a/net/9p/trans_fd.c
>> +++ b/net/9p/trans_fd.c
>> @@ -202,9 +202,11 @@ static void p9_conn_cancel(struct p9_conn *m, int err)
>>   
>>   	list_for_each_entry_safe(req, rtmp, &m->req_list, req_list) {
>>   		list_move(&req->req_list, &cancel_list);
>> +		req->status = REQ_STATUS_ERROR;
>>   	}
>>   	list_for_each_entry_safe(req, rtmp, &m->unsent_req_list, req_list) {
>>   		list_move(&req->req_list, &cancel_list);
>> +		req->status = REQ_STATUS_ERROR;
>>   	}
>>   
>>   	spin_unlock(&m->req_lock);
> 
> --
> Dominique