From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-wr1-f45.google.com (mail-wr1-f45.google.com [209.85.221.45]) (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 479D0463B6A for ; Tue, 4 Aug 2026 14:03:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.221.45 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785852193; cv=none; b=SWAFHs3oPDqfJoRUQ3l+R9ffwyz5YjZagJ8dO1jp2HKExVSZAJ7OUMWrDgZ2AUYu7jOEX3CbI+Wp5c14/c1x2I5DnKNRoLXX4qiO4x8v1QnOJfNSgdqV9rF1aYDTwZwZFJhZFWk7qBSEA887K4cu9yGHEoRZgcZA0TWVk5yKKG8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785852193; c=relaxed/simple; bh=c/jldCHsn1gXilDxTKLh+mbg50n1kYkXdD+wvLJiUUY=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version:Content-Type; b=kp0hOuJrTTtFV7UFMOZCd060frYpeRQuNkZqdAuoNQdFgtjUlBIDs33f14vxczXfoj+nijvcx+5j9d03+EJqSFkU3jc5J6JPCU8d/WZkesVOLG/o05iZSVI++bUsPXgDV0c/L9f16JthP6S+7Ez9tEtuMFu6ttzA5Ybqhr7mxQ0= 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=209.85.221.45 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-f45.google.com with SMTP id ffacd0b85a97d-471eeac43bfso4412603f8f.3 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=IXIQ5X+rYD1Hdvd6l9sq7SVupO00yYiqvmfflc+/+z+1qb8jLLmE96HWPWWKUhB+jZ 0egTHLcDQcBD8iMrvtpjzxLOpDBRjQiS+4D0/W3ZZTe3b2tWxXESco1meigN5g73z7du /S1+H/AKPADyixLMYWLaaRuSFk7OIovcj0ybes1p563O9sw1GH5tLeEWg655ajUX2QoD 8VEJwwRINuxj7uTNd2pFU3WGw+778Iw+m7vJ3c7YLFxzdjyLxKh0X8eE7sE1Jiuk67eS jYdtijZGq3G7gNG7pYKTwk1X73vXV8DJwBBkR1bkWOiBN4zqPvhbTwmZdVXDlnieSpzU wqfA== X-Forwarded-Encrypted: i=1; AHgh+RoHrX3oKCc2gcDuaXZdloKILbTv6yKc8OYSOa1ynR12W3fha3sKdYcHw1vKnIHKlaLfn4zUHTh7zi1w6Sc=@vger.kernel.org X-Gm-Message-State: AOJu0YyQQkuDS2JIKFyaTLa9A1KnsLzGByJKFBS5+nF9MKxbRGoRZCPV KBPBVLTgd5n9QW5DtsLt7JQpC0SjqSH4777rs12Y1xrN3FjovEmz2be9S2MSTYUpGfAf+w== X-Gm-Gg: AR+sD13r83mkmE2HEkvvjLEWqnyRXF0wHD1dr5r3wHnFEMcnl6KOUzFYIN8q9C/YlSp I+iovBvUN+UJwYGKjIlTNwCR2uxMSFid3u/nWEZyF5xOGf4QUx4j19FgCwN5zlZQwLu7WozdAmz DlywKcY/mJ9PWx2+HGFYrjCugSJKPlbys9IcHGar5KRrtV8DbDfY1VdlF8g6y2o4RDWeGCM/FYB 9mrzsUNskRP6qaItZOobBz52rLKx4wypgEYUV/UKtEf3JtdqhYxkcukKUcgqkG0uvaAxvCUsXJD adPu1rfNnsvUSd9jMkUKgL3Kz7xoFPUKxdxBk2EUETXnBU+i8R+rM5jxhJN70Eui7vGQu88iSgP FyierAlJagrbGb2uPZ9SrGmUx0FGW6wEBzkZuVR57T6AifCrTN3vkjBUoKwb8K4mzE6337+QqpE YQeTFUdcUIg1mDUq8rt0fa+UvUde+eEvkYdK+oqH4Wj9gQ5baBgOzIXbwl91c= 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-serial@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