linux-kernel - Re: [PATCH v2] staging: greybus: audio: fix loop cursor use after iteration

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Date:   Fri, 10 Jun 2022 10:00:03 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Jared Kangas <kangas.jd@...il.com>
Cc:     vaibhav.sr@...il.com, elder@...nel.org, gregkh@...uxfoundation.org,
        greybus-dev@...ts.linaro.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, linux-staging@...ts.linux.dev,
        mgreer@...malcreek.com, Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [PATCH v2] staging: greybus: audio: fix loop cursor use after
 iteration

On Thu, Jun 09, 2022 at 02:45:18PM -0700, Jared Kangas wrote:
> gbaudio_dapm_free_controls() iterates over widgets using the
> list_for_each_entry*() family of macros from <linux/list.h>, which
> leaves the loop cursor pointing to a meaningless structure if it
> completes a traversal of the list. The cursor was set to NULL at the end
> of the loop body, but would be overwritten by the final loop cursor
> update.
> 
> Because of this behavior, the widget could be non-null after the loop
> even if the widget wasn't found, and the cleanup logic would treat the
> pointer as a valid widget to free.
> 
> To fix this, introduce a temporary variable to act as the loop cursor
> and copy it to a variable that can be accessed after the loop finishes.
> Due to not removing any list elements, use list_for_each_entry() instead
> of list_for_each_entry_safe() in the revised loop.
> 
> This was detected with the help of Coccinelle.
> 
> Fixes: 510e340efe0c ("staging: greybus: audio: Add helper APIs for dynamic audio modules")
> Cc: stable@...r.kernel.org
> Reviewed-by: Dan Carpenter <dan.carpenter@...cle.com>
> Reviewed-by: Johan Hovold <johan@...nel.org>
> Signed-off-by: Jared Kangas <kangas.jd@...il.com>
> ---
> 
> Changes since v1:
>  * Removed safe list iteration as suggested by Johan Hovold <johan@...nel.org>
>  * Updated patch changelog to explain the list iteration change
>  * Added tags to changelog based on feedback (Cc:, Fixes:, Reviewed-by:)

Apparently Greg applied this to staging-next before we had a change to
look at it. You should have received a notification from Greg when he
did so.

	https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging.git/commit/?h=staging-next&id=80c968a04a381dc0e690960c60ffd6b6aee7e157

It seems unlikely that this would cause any issues in real life, but
there's still a chance it will be picked up by the stable team despite
the lack of a CC stable tag.

I've just sent a follow-up patch to replace the list macro.

Johan