linux-kernel - Re: [PATCH] mcb: fix a missing-check bug

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Message-ID: <5d09862f-e903-169e-9b11-310d4a1fd01c@kernel.org>
Date:   Mon, 29 Oct 2018 09:44:23 +0100
From:   Johannes Thumshirn <jth@...nel.org>
To:     Wenwen Wang <wang6495@....edu>
Cc:     Kangjie Lu <kjlu@....edu>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mcb: fix a missing-check bug

Hi Wenwen,

Sorry for the late reply:

On 19/10/18 17:11, Wenwen Wang wrote:
> In chameleon_parse_cells(), to parse each cell, the descriptor type 'dtype'
> is acquired from the IO memory region pointed by 'p' through readl() in
> get_next_dtype(). Then 'dtype' is checked to see whether it is
> CHAMELEON_DTYPE_GENERAL. If yes, chameleon_parse_gdd() is invoked to parse
> Chameleon general device descriptor. In chameleon_parse_gdd(), the data in
> the IO memory region is read again through readl() field by field.
> Specifically, the 'reg1' field contains the type information. That means
> the type is read twice. More importantly, no check is re-enforced after the
> second read. Given that the IO memory region can also be accessed by the
> device, it is possible that a malicious device controlled by an attacker
> can modify the type information between the two reads. This can cause
> undefined behavior of the kernel and introduce potential security risk.

Yes but this doesn't really mitigate the problem, does it? If a
malicious attacker controlling the MMIO space can change the register
contents after the first read, what stops him/her from doing it after
the second, third, 4096th read?

>  
>  	reg1 = readl(&gdd->reg1);
> +	if ((reg1 >> 28) != CHAMELEON_DTYPE_GENERAL) {
> +		ret = -EINVAL;
> +		goto err;
> +	}


Just an advice for your next submission, give that 'magic' 28 a define
(like CHAMELEON_DTYPE_SHIFT or whatever), this makes the code nicer to read.