netdev - Re: [PATCH net] net/smc: Initialize smc hashtables before registering users

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Message-ID: <64405058-23a9-49df-aed0-891fa0a19fbb@linux.ibm.com>
Date: Mon, 22 Dec 2025 10:50:37 +0100
From: Alexandra Winter <wintera@...ux.ibm.com>
To: dust.li@...ux.alibaba.com, David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Andrew Lunn <andrew+netdev@...n.ch>,
        "D. Wythe" <alibuda@...ux.alibaba.com>,
        Sidraya Jayagond <sidraya@...ux.ibm.com>,
        Wenjia Zhang
 <wenjia@...ux.ibm.com>,
        Wang Liang <wangliang74@...wei.com>
Cc: netdev@...r.kernel.org, linux-s390@...r.kernel.org,
        Aswin Karuvally <aswin@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev
 <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>, Simon Horman <horms@...nel.org>,
        Mahanta Jambigi <mjambigi@...ux.ibm.com>,
        Tony Lu
 <tonylu@...ux.alibaba.com>, Wen Gu <guwen@...ux.alibaba.com>,
        linux-rdma@...r.kernel.org, stable@...r.kernel.org,
        syzbot+f69bfae0a4eb29976e44@...kaller.appspotmail.com
Subject: Re: [PATCH net] net/smc: Initialize smc hashtables before registering
 users



On 17.12.25 16:25, Dust Li wrote:
> On 2025-12-17 12:48:19, Alexandra Winter wrote:
>> During initialisation of the SMC module initialize smc_v4/6_hashinfo before
>> calling smc_nl_init(), proto_register() or sock_register(), to avoid a race
>> that can cause use of an uninitialised pointer in case an smc protocol is
>> called before the module is done initialising.
>>
>> syzbot report:
>> KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
>> Call Trace:
>> <TASK>
>> smc_diag_dump+0x59/0xa0 net/smc/smc_diag.c:236
>> netlink_dump+0x647/0xd80 net/netlink/af_netlink.c:2325
>> __netlink_dump_start+0x59f/0x780 net/netlink/af_netlink.c:2440
>> netlink_dump_start include/linux/netlink.h:339 [inline]
>> smc_diag_handler_dump+0x1ab/0x250 net/smc/smc_diag.c:251
>> sock_diag_rcv_msg+0x3dc/0x5f0
>> netlink_rcv_skb+0x1e3/0x430 net/netlink/af_netlink.c:2550
>> netlink_unicast_kernel net/netlink/af_netlink.c:1331 [inline]
>> netlink_unicast+0x7f0/0x990 net/netlink/af_netlink.c:1357
>> netlink_sendmsg+0x8e4/0xcb0 net/netlink/af_netlink.c:1901
> 
> I don't think this is related to smc_nl_init().
> 
> Here the calltrace is smc_diag_dump(), which was registered in
> sock_diag_register(&smc_diag_handler).
> 
> But smc_nl_init() is registering the general netlink in SMC,
> which is unrelated to smc_diag_dump().


I had assumed some dependency between the smc netlink diag socket and smc_nl_init()
and wrongly assumed that the smc_diag_init() and smc_init() could race.
I now understand that modprobe will ensure smc_diag_init() is called before smc_init(),
so you are right: this patch is indeed NOT a fix for this sysbot report [1]


> I think the root cause should be related to the initializing between
> smc_diag.ko and smc_v4/6_hashinfo.ht.

Given modprobe initializes the modules sequentially, I do not see how these could race.

I guess this syszbot report was fixed by
f584239a9ed2 ("net/smc: fix general protection fault in __smc_diag_dump")
as reported in [2] .

I'm not sure about the correct procedure, if nobody recommends a better action, I'll send a

#syz dup: general protection fault in __smc_diag_dump
to
syzbot+f69bfae0a4eb29976e44@...kaller.appspotmail.com
(this one: general protection fault in smc_diag_dump_proto [1])


I still think initializing the hashtables before smc_nl_init()
makes sense. I'll resend this patch without mentioning syzbot.

-----
[1] https://syzkaller.appspot.com/bug?extid=f69bfae0a4eb29976e44
[2] https://syzkaller.appspot.com/bug?extid=f775be4458668f7d220e