From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 76FE13BB11C; Thu, 16 Jul 2026 07:58:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784188716; cv=none; b=WaxbfX5wRCYjomU3WRDS2pzxat48K3K8EcinFPJKWQUW0F0cqojmebUR7RIz5AronKuHG8WlCOL17FQ8RJN66fdjUTZ2iDGeOzp1K4/HceXBnSEGkdOabqmHX+rFkbqc94uf9UTvts2End1+9Yf0R9UxrnX2Bp+TN72tqQY7b3M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784188716; c=relaxed/simple; bh=DETAK/XdWNA5vfwdagrjglJ8kBtx/fUgC6Qwenu5IC8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=PeBsebYY8BHSFvGuxJuQqQ9X6i3xVKQt3WUeEWeNiIFRgz7W7Y/ziqMlgNqSbetHQZerD9sLssxMCLyg449tBfjP6vW/R/KXnutAQVtH9bLcnQYSyyxxt8gaNywWgGXXeDj35DX3DzUystS1oAYtEdEoKwRNNMj/tzACKG5lYS0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=DKOTLEED; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="DKOTLEED" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8FD121F00A3D; Thu, 16 Jul 2026 07:58:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1784188715; bh=ne3Ukv1uxkP2pFM8Hep2c/+j1jREZfaqYG+mNHKPZFE=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=DKOTLEEDJaklR7ahI5eiPrDILbQFqHAZn0NSEFLOBaE5Xi17Nik2Q/1lLX/XetH/+ +8KOKCY8SIY3s9XFEXq8qrKUBF9hjg+u8lnLWJFlZ0zLTK/vzc4iDiPq9aPXx8bDCM krWpKK+/fhK4X/eu5ruRNSHr29mskcarQWcTDFYI= Date: Thu, 16 Jul 2026 09:57:15 +0200 From: Greg KH To: Yun Zhou 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 Subject: Re: [PATCH] tty: ldisc: fix deadlock between ldisc_sem and rtnl_mutex Message-ID: <2026071655-steering-cardstock-cd06@gregkh> References: <20260716064719.1401892-1-yun.zhou@windriver.com> Precedence: bulk X-Mailing-List: linux-can@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260716064719.1401892-1-yun.zhou@windriver.com> 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 > > 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 > --- > drivers/tty/tty_ldisc.c | 17 +++++++++++++++-- > 1 file changed, 15 insertions(+), 2 deletions(-) What commit caused this to be a problem and why have we not seen this in any real-world usages? > > diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c > index 27fe8236f662..248a6995cc53 100644 > --- a/drivers/tty/tty_ldisc.c > +++ b/drivers/tty/tty_ldisc.c > @@ -556,15 +556,28 @@ int tty_set_ldisc(struct tty_struct *tty, int disc) > /* Shutdown the old discipline. */ > tty_ldisc_close(tty, old_ldisc); > > - /* Now set up the new line discipline. */ > - tty->ldisc = new_ldisc; > + /* Clear tty->ldisc so concurrent readers back off during transition */ > + tty->ldisc = NULL; > tty_set_termios_ldisc(tty, disc); > + tty_ldisc_unlock(tty); > > + /* > + * Open the new discipline outside ldisc_sem. The ldisc .open() > + * may acquire locks (e.g., rtnl_mutex) that would create circular > + * dependencies if taken under ldisc_sem. tty_lock is still held, > + * preventing concurrent ldisc changes and hangup. > + */ > retval = tty_ldisc_open(tty, new_ldisc); Now you are calling open when previously we were not, are you sure this isn't going to cause problems? > + > + tty_ldisc_lock(tty, MAX_SCHEDULE_TIMEOUT); Why that timeout? > + > if (retval < 0) { > /* Back to the old one or N_TTY if we can't */ > tty_ldisc_put(new_ldisc); > tty_ldisc_restore(tty, old_ldisc); > + } else { > + /* Success - install new ldisc */ > + tty->ldisc = new_ldisc; > } Does open cause anything else to be incremented that you have to clean up when done that you aren't doing here? thanks, greg k-h