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15.21.0270.012; Fri, 31 Jul 2026 05:54:42 +0000 Message-ID: Date: Fri, 31 Jul 2026 13:54:31 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] tty: ldisc: fix deadlock between ldisc_sem and rtnl_mutex To: Greg KH Cc: jirislaby@kernel.org, socketcan@hartkopp.net, linux-serial@vger.kernel.org, mkl@pengutronix.de, linux-can@vger.kernel.org, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org References: <20260724084648.3879356-1-yun.zhou@windriver.com> <2026073027-nervy-imposing-0068@gregkh> Content-Language: en-US From: "Zhou, Yun" In-Reply-To: <2026073027-nervy-imposing-0068@gregkh> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SI2P153CA0027.APCP153.PROD.OUTLOOK.COM (2603:1096:4:190::22) To CH0PR11MB5332.namprd11.prod.outlook.com (2603:10b6:610:bf::17) Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: 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engine=8.22.0-2606150000 definitions=main-2607310038 On 7/30/2026 10:50 PM, Greg KH wrote: > On Fri, Jul 24, 2026 at 04:46:48PM +0800, Yun Zhou wrote: >> syzbot reported a circular lock dependency involving tty ldisc_sem and >> the networking rtnl_mutex. The full chain is: >> >> rtnl_mutex --> nft_commit_mutex --> ... --> ep->mtx --> ldisc_sem --> rtnl_mutex >> >> The last edge (ldisc_sem -> rtnl_mutex) is created because tty line >> discipline .open() callbacks (slcan, slip) call register_netdev() which >> acquires rtnl_mutex, and .open() runs under ldisc_sem write lock in >> tty_set_ldisc(). >> >> Fix by moving the .open() call outside the ldisc_sem write lock. The >> ldisc .open() is initialization of the NEW discipline after the old one >> has been closed - there is no need for ldisc_sem protection at this >> point since: >> >> - tty_lock is held throughout, preventing concurrent tty_set_ldisc, >> hangup, or close >> - tty->ldisc is set to NULL during the window. tty_ldisc_ref_wait() >> waits for the transition to complete. tty_ldisc_ref() returns NULL >> which callers already handle. >> - tty buffer data stays queued until the ldisc is installed >> >> The sequence becomes: >> 1. Hold ldisc_sem(write): close old ldisc, set tty->ldisc = NULL >> 2. Release ldisc_sem(write) >> 3. Call new_ldisc->ops->open() without ldisc_sem >> 4. Re-acquire ldisc_sem(write): install new ldisc (or restore old) >> 5. Release ldisc_sem(write) >> >> Reported-by: syzbot+de610eeef174bd59a8a3@syzkaller.appspotmail.com >> Closes: https://syzkaller.appspot.com/bug?extid=de610eeef174bd59a8a3 >> Signed-off-by: Yun Zhou >> --- > > Sashiko has some comments: > https://sashiko.dev/#/patchset/20260724084648.3879356-1-yun.zhou@windriver.com > are they correct? > I've already responded to Sashiko about the "New issue" - this scenario does not exist [1] . I'm not sure why my reply didn't CC you - my bad. https://sashiko.dev/#/message/3894bbdd-1930-4ad4-8b28-431a162aece0%40windriver.com As for the pre-existing issue, it's very hard to trigger and practically impossible in real-world scenarios. If we really need to fix it, I'll send a separate patch series for it later. BR, Yun