linux-kernel - Re: [PATCH] staging: Fix atomicity violation in get_serial

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Message-ID: <bddd479b-8fa3-4e39-8ca5-f7f133a8b298@stanley.mountain>
Date: Mon, 30 Sep 2024 13:36:54 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Qiu-ji Chen <chenqiuji666@...il.com>
Cc: dtwlin@...il.com, johan@...nel.org, elder@...nel.org,
	gregkh@...uxfoundation.org, greybus-dev@...ts.linaro.org,
	linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org,
	baijiaju1990@...il.com, stable@...r.kernel.org
Subject: Re: [PATCH] staging: Fix atomicity violation in get_serial_info()

On Mon, Sep 30, 2024 at 06:14:03PM +0800, Qiu-ji Chen wrote:
> Atomicity violation occurs during consecutive reads of the members of 
> gb_tty. Consider a scenario where, because the consecutive reads of gb_tty
> members are not protected by a lock, the value of gb_tty may still be 
> changing during the read process. 
> 
> gb_tty->port.close_delay and gb_tty->port.closing_wait are updated
> together, such as in the set_serial_info() function. If during the
> read process, gb_tty->port.close_delay and gb_tty->port.closing_wait
> are still being updated, it is possible that gb_tty->port.close_delay
> is updated while gb_tty->port.closing_wait is not. In this case,
> the code first reads gb_tty->port.close_delay and then
> gb_tty->port.closing_wait. A new gb_tty->port.close_delay and an
> old gb_tty->port.closing_wait could be read. Such values, whether
> before or after the update, should not coexist as they represent an
> intermediate state.
> 
> This could result in a mismatch of the values read for gb_tty->minor, 
> gb_tty->port.close_delay, and gb_tty->port.closing_wait, which in turn 
> could cause ss->close_delay and ss->closing_wait to be mismatched.
> 
> To address this issue, we have enclosed all sequential read operations of 
> the gb_tty variable within a lock. This ensures that the value of gb_tty 
> remains unchanged throughout the process, guaranteeing its validity.
> 
> This possible bug is found by an experimental static analysis tool
> developed by our team. This tool analyzes the locking APIs
> to extract function pairs that can be concurrently executed, and then
> analyzes the instructions in the paired functions to identify possible
> concurrency bugs including data races and atomicity violations.
> 

Ideally a commit message should say what the bug looks like to the user.
Obviously when you're doing static analysis and not using the code, it's more
difficult to tell the impact.

I would say that this commit message is confusing and makes it seem like a
bigger deal than it is.  The "ss" struct is information that we're going to send
to the user.  It's not used again in the kernel.

Could you re-write the commit message to say something like, "Our static checker
found a bug where set serial takes a mutex and get serial doesn't.  Fortunately,
the impact of this is relatively minor.  It doesn't cause a crash or anything.
If the user races set serial and get serial there is a chance that the get
serial information will be garbage."

regards,
dan carpenter