linux-kernel - Re: [PATCH] soc: meson: fix a missing check on list iterator

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Date:   Sun, 27 Mar 2022 21:08:01 +0800
From:   Xiaomeng Tong <xiam0nd.tong@...il.com>
To:     jbrunet@...libre.com
Cc:     alsa-devel@...a-project.org, broonie@...nel.org,
        khilman@...libre.com, lgirdwood@...il.com,
        linux-amlogic@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        martin.blumenstingl@...glemail.com, narmstrong@...libre.com,
        perex@...ex.cz, stable@...r.kernel.org, tiwai@...e.com,
        xiam0nd.tong@...il.com
Subject: Re: [PATCH] soc: meson: fix a missing check on list iterator

On Sun, 27 Mar 2022 13:03:14 +0200, Jerome Brunet <jbrunet@...libre.com> wrote:
> On Sun 27 Mar 2022 at 16:18, Xiaomeng Tong <xiam0nd.tong@...il.com> wrote:
> 
> > The bug is here:
> > 	*dai_name = dai->driver->name;
> >
> > For for_each_component_dais(), just like list_for_each_entry,
> > the list iterator 'runtime' will point to a bogus position
> > containing HEAD if the list is empty or no element is found.
> > This case must be checked before any use of the iterator,
> > otherwise it will lead to a invalid memory access.
> >
> > To fix the bug, just move the assignment into loop and return
> > 0 when element is found, otherwise return -EINVAL;
> 
> Except we already checked that the id is valid and know an element will
> be be found once we enter the loop. No bug here and this patch does not
> seem necessary to me.

Yea, you should be right, it is not a bug here. id already be checked before
enter the loop:

if (id < 0 || id >= component->num_dai)
                return -EINVAL;

but if component->num_dai is not correct due to miscaculation or others reason
and the door is reopened, this patch can avoid a invalid memory access. Anyway,
it is a good choice to use the list iterator only inside the loop, as linus
suggested[1]. and we are on the way to change all these use-after-iter cases.

[1]https://lore.kernel.org/lkml/20220217184829.1991035-1-jakobkoschel@gmail.com/

--
Xiaomeng Tong