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15.21.0223.017; Mon, 20 Jul 2026 12:21:08 +0000 Message-ID: Date: Mon, 20 Jul 2026 20:20:59 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] 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: <20260716064719.1401892-1-yun.zhou@windriver.com> <2026071718-yearbook-bloated-48ce@gregkh> Content-Language: en-US From: "Zhou, Yun" In-Reply-To: <2026071718-yearbook-bloated-48ce@gregkh> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SEWP216CA0015.KORP216.PROD.OUTLOOK.COM (2603:1096:101:2b6::9) 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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comes from a non Wind River email account! > Do not click links or open attachments unless you recognize the sender and know the content is safe. > > On Thu, Jul 16, 2026 at 02:47:19PM +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, so concurrent readers >> (tty_ldisc_ref, tty_ldisc_ref_wait) see NULL and return immediately, >> which callers already handle as a hangup condition >> - tty buffer data stays queued until the ldisc is installed > > Ah, but look at the review at: > https://sashiko.dev/#/patchset/20260716064719.1401892-1-yun.zhou@windriver.com > which says: > Does unlocking the semaphore while the ldisc pointer is NULL introduce a UAPI > break for concurrent operations? > > If a concurrent process calls read(), write(), or poll() during this unlocked > window, it can enter tty_ldisc_ref_wait() in drivers/tty/tty_io.c. Because the > semaphore was unlocked here, tty_ldisc_ref_wait() will successfully acquire > the read lock but observe tty->ldisc as NULL. > > This causes the reader to immediately return EOF or -EIO, potentially > aborting userspace applications unexpectedly during a line discipline > transition. > > Is that not true? > No, the issue reported by Sashiko does indeed exist. I will fix it in the v2.