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Subject: Re: [PATCH] mac80211: fix incorrect strlen of .write in debugfs
From:   Johannes Berg <johannes@sipsolutions.net>
To:     Shayne Chen <shayne.chen@mediatek.com>,
        linux-wireless <linux-wireless@vger.kernel.org>
Cc:     Toke =?ISO-8859-1?Q?H=F8iland-J=F8rgensen?= <toke@toke.dk>,
        Felix Fietkau <nbd@nbd.name>,
        Lorenzo Bianconi <lorenzo.bianconi@redhat.com>,
        Ryder Lee <ryder.lee@mediatek.com>,
        linux-mediatek <linux-mediatek@lists.infradead.org>,
        Sujuan Chen <sujuan.chen@mediatek.com>
Date:   Fri, 08 Jan 2021 21:02:53 +0100
In-Reply-To: <20210108105643.10834-1-shayne.chen@mediatek.com>
References: <20210108105643.10834-1-shayne.chen@mediatek.com>
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This looks wrong to me, am I missing something?

> diff --git a/net/mac80211/debugfs.c b/net/mac80211/debugfs.c
> index 9135b6f..9991a6a 100644
> --- a/net/mac80211/debugfs.c
> +++ b/net/mac80211/debugfs.c
> @@ -120,7 +120,6 @@ static ssize_t aqm_write(struct file *file,
>  {
>  	struct ieee80211_local *local = file->private_data;
>  	char buf[100];
> -	size_t len;
>  
>  	if (count > sizeof(buf))
>  		return -EINVAL;

This ensures that count <= sizeof(buf)

> @@ -128,10 +127,10 @@ static ssize_t aqm_write(struct file *file,
>  	if (copy_from_user(buf, user_buf, count))
>  		return -EFAULT;

We copy, that's fine.
 
> -	buf[sizeof(buf) - 1] = '\0';
> -	len = strlen(buf);
> -	if (len > 0 && buf[len-1] == '\n')
> -		buf[len-1] = 0;
> +	if (count && buf[count - 1] == '\n')
> +		buf[count - 1] = '\0';

This I think really was meant as strlen, because if you write something
like

 10\n\0\0\0\0

before it would have parsed it as 10 still, now it gets confused?

I guess I'm not worried about that though.

> +	buf[count] = '\0';

But if count == sizeof(buf) then this is an out-of-bounds write.

Same for all the other copied instances.

johannes