linux-kernel - Re: [PATCH] ALSA: isa: gus: Fix a possible null-pointer dereference in snd_gf1_mem

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Date:   Tue, 23 Jul 2019 15:47:47 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     "Jia-Ju Bai" <baijiaju1990@...il.com>
Cc:     <huangfq.daxian@...il.com>, <tglx@...utronix.de>,
        <allison@...utok.net>, <perex@...ex.cz>,
        <alsa-devel@...a-project.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ALSA: isa: gus: Fix a possible null-pointer dereference in snd_gf1_mem_xfree()

On Tue, 23 Jul 2019 15:40:20 +0200,
Jia-Ju Bai wrote:
> 
> In snd_gf1_mem_xfree(), there is an if statement on line 72 and line 74
> to check whether block->next is NULL:
>     if (block->next)
> 
> When block->next is NULL, block->next is used on line 84:
>     block->next->prev = block->prev;
> 
> Thus, a possible null-pointer dereference may occur in this case.

There is already a check beforehand:

	if (alloc->last == block) {

and the code path you're referring to is only after this check fails,
i.e. it's no last entry, hence block->next can be never NULL.

So the current code is OK.


thanks,

Takashi

> 
> To fix this possible bug, block->next is checked before using it.
> 
> This bug is found by a static analysis tool STCheck written by us.
> 
> Signed-off-by: Jia-Ju Bai <baijiaju1990@...il.com>
> ---
>  sound/isa/gus/gus_mem.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/sound/isa/gus/gus_mem.c b/sound/isa/gus/gus_mem.c
> index cb02d18dde60..ed6205b88057 100644
> --- a/sound/isa/gus/gus_mem.c
> +++ b/sound/isa/gus/gus_mem.c
> @@ -81,7 +81,8 @@ int snd_gf1_mem_xfree(struct snd_gf1_mem * alloc, struct snd_gf1_mem_block * blo
>  		if (block->prev)
>  			block->prev->next = NULL;
>  	} else {
> -		block->next->prev = block->prev;
> +		if (block->next)
> +			block->next->prev = block->prev;
>  		if (block->prev)
>  			block->prev->next = block->next;
>  	}
> -- 
> 2.17.0
> 
>