From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-wr1-f50.google.com (mail-wr1-f50.google.com [209.85.221.50]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 46A4038B12A for ; Tue, 4 Aug 2026 14:03:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.221.50 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785852192; cv=none; b=qDgM49NpE2b/mssdrP2MjmMfJr9/MeMuj+zU/pnOmBciTJlxvV7n9k25F08ZxIvrLEuRXy+iF+XMuQcvVWZlw8ZH0kfKJFAvdaJQ4AfblZV1Z9WeOWf+U69SmDEnmv2+NLAmtT1H9Hw6koUPx70N9iYlLuL2SwJ/pJjcn8AsuWc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785852192; c=relaxed/simple; bh=c/jldCHsn1gXilDxTKLh+mbg50n1kYkXdD+wvLJiUUY=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version:Content-Type; b=SJX+EAQX8hnpLk7Nij3bNepne7LwF8JOM0liu9CWZKUjH3iCM9Yiopr00uHdUxAPXNz/LczvLt/MTGVF3fjyCvEcTsp6R1CiRfAdbz9Fj8cm/tICzfbULA8msea2fJ1V9/Y57kBt3tDs9nZTBGjDvV35u9gj3rosLxl/xPpdLos= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=RASRon6f; arc=none smtp.client-ip=209.85.221.50 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="RASRon6f" Received: by mail-wr1-f50.google.com with SMTP id ffacd0b85a97d-47f633e6058so4212025f8f.0 for ; Tue, 04 Aug 2026 07:03:11 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20251104; t=1785852189; x=1786456989; darn=vger.kernel.org; h=content-transfer-encoding:content-type:mime-version:message-id:date :subject:cc:to:from:from:to:cc:subject:date:message-id:reply-to :content-type; bh=p0ui5oK9AY/l/1r8JIeMGahp/pYXTVjMsBc64nxxCXo=; b=RASRon6f1fCzWbdnMOc5Nklmsislq6JnMeAiE/ZMlH9c4l2SycXJKiA5IE2dERILL3 VcY1A5PX6Dob7ZC4hqoY3fBUoF7e3JVzIeHKikT3bkZe1Rya51ShqWRZIXQYghR3zlGd dmc8+2RRHpZqsbpO5mcNfllSVqF+qXOlANCkdTJVwG32wVT8/1WeK81TP1brlw87Q8gM tgtIvssGSh+0sicPfGDL8EiPpr28q4O/IQXgDE2NQUlEJwljQrz9mJao2UWI89gqLJQ8 4K+xvQi+YrYBcOkwEkGs33Y6GjoIGmHkb5APO0Tgip3IbF3oLP1LJuQMPyD+0mDFobqa tasw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1785852189; x=1786456989; h=content-transfer-encoding:content-type:mime-version:message-id:date :subject:cc:to:from:x-gm-gg:x-gm-message-state:from:to:cc:subject :date:message-id:reply-to:content-type; bh=p0ui5oK9AY/l/1r8JIeMGahp/pYXTVjMsBc64nxxCXo=; b=kwgHHYXDdmG1EX9BEFDoDYUdbqeic/KrrbEVr5OLcu1fAgeoJ/Un+drQQtzjk1bQPO SVT6/kpx7in4JJ+n32oAq5vS4tvZMp4mNXL8byd+/0Wt09Uqh5jmcw5B/gSnoFjJvgsN qSRx/AzIgYAmIPwtw7M0FVzoeZjdwvehurBa4jTgBX+fYVH/r0fNFavXyPDDGTNdcy85 jQY9+2K1MQRIIinvBJ3UjtWWTDOdVFEPl6ycdHKAquPp2mNtHvbEpuniwRjF6X4gTX/P p2vfRszi2iDUclZJB6WKor1n4xvHx9ltbDSzyMtNRmFttbwNAH6g6xdCXby2Ja/2wn3V HMnA== X-Gm-Message-State: AOJu0YzYmP1aRsbmXTHz+bMntLobU1ej+jt+aGwPhtSiRIGd1wtih8re vx6GxYvbTB/VvIQ1OpNOJdMspllUoOpxjCUGrek7MyiUbeuyO7v/ZMZj X-Gm-Gg: AR+sD13rWmfl3GDh8/JpGZVU4qgTYvRbF1+tDjSu2Tvdv1dX5bTAtMNsnmPzK3/6CsL WqqTEO6WtDaBfWUczD1apPKiNV20ki48ec3CBjAvLwap1pUqPRi1iLrsNyJeGrMqirOXQC048kj JuAob1T1P8mCVSAzagmsYwTRM1DTNfsgwwitLdUMfltH/rUslQdbBnKk4mijGXy2rxIjvoWTclW wE/xNSlBc7oA9xshm9f+4LNAphb9RlNVM0Qbozl7auaN5i/S2RP+H9QRu+I0SItP96Adp1JuVBa D9p0G4v/II3e05zLzZKf78ZQxn/H+HNqHvbsZIPYjtR0XzHI8nAxep1N6GctIONe3GwQwEuFW/+ i5V3WVg6Fy9bn2dSvkBVDwKInchgtvzxdMOU1iFlL90p0MV67dwpsa5DODXXNfi0yYDS/tNUttE rnyqd8zBaJFEzMyV8v5ZPwT4e0xujKl8AJee22jkbMMAOf42OM6ULd3Vgz0eM= X-Received: by 2002:a05:6000:1372:b0:47f:93b4:2def with SMTP id ffacd0b85a97d-47fd72e062dmr27002961f8f.28.1785852189032; Tue, 04 Aug 2026 07:03:09 -0700 (PDT) Received: from metepc ([46.197.185.197]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-47fd41e2abbsm44236877f8f.9.2026.08.04.07.03.06 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 04 Aug 2026 07:03:07 -0700 (PDT) From: =?UTF-8?q?=C3=96mer=20Mete=20Kaya?= To: gregkh@linuxfoundation.org, jirislaby@kernel.org Cc: linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org, syzbot+d84934b4184d7f9a1aed@syzkaller.appspotmail.com, =?UTF-8?q?=C3=96mer=20Mete=20Kaya?= Subject: [PATCH] tty: fix lock order inversion in tty_buffer_flush() Date: Tue, 4 Aug 2026 17:02:51 +0300 Message-ID: <20260804140251.8197-1-omermetekaya0@gmail.com> X-Mailer: git-send-email 2.55.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit tty_buffer_flush() calls ld->ops->flush_buffer() while holding buf->lock. For the N_TTY line discipline, flush_buffer() is n_tty_flush_buffer(), which acquires termios_rwsem. However, the documented stable lock order (see tty.h) is: termios_rwsem -> buf->lock Calling flush_buffer() under buf->lock inverts this order and creates a circular locking dependency detected by lockdep: &buf->lock -> console_lock -> &port->mutex -> &tty->termios_rwsem -> &buf->lock Fix this by moving the flush_buffer() call outside buf->lock. The buffer data has already been discarded at this point, so the ordering change is safe. Reported-by: syzbot+d84934b4184d7f9a1aed@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=d84934b4184d7f9a1aed Signed-off-by: Ömer Mete Kaya --- drivers/tty/tty_buffer.c | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-) diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c index 96be90db5..bb7b7fbcb 100644 --- a/drivers/tty/tty_buffer.c +++ b/drivers/tty/tty_buffer.c @@ -244,11 +244,15 @@ void tty_buffer_flush(struct tty_struct *tty, struct tty_ldisc *ld) buf->head->read = buf->head->commit; buf->head->lookahead = buf->head->read; - if (ld && ld->ops->flush_buffer) - ld->ops->flush_buffer(tty); - atomic_dec(&buf->priority); mutex_unlock(&buf->lock); + /* + * Call flush_buffer() outside buf->lock: n_tty_flush_buffer() + * acquires termios_rwsem, but the required lock order is + * termios_rwsem -> buf->lock, not the other way around. + */ + if (ld && ld->ops->flush_buffer) + ld->ops->flush_buffer(tty); } /** -- 2.55.0