linux-kernel - Re: [PATCH] cdc-acm: prevent infinite loop when parsing CDC headers.

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Message-ID: <552BECD6.9090406@cogentembedded.com>
Date:	Mon, 13 Apr 2015 19:20:38 +0300
From:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:	Quentin Casasnovas <quentin.casasnovas@...cle.com>,
	lkml <linux-kernel@...r.kernel.org>
CC:	linux-usb <linux-usb@...r.kernel.org>,
	Phil Turnbull <phil.turnbull@...cle.com>,
	Oliver Neukum <oneukum@...e.de>, Adam Lee <adam8157@...il.com>
Subject: Re: [PATCH] cdc-acm: prevent infinite loop when parsing CDC headers.

Hello.

On 04/13/2015 06:24 PM, Quentin Casasnovas wrote:

> Phil and I found out a problem with commit:

>    7e860a6e ("cdc-acm: add sanity checks")

> It added some sanity checks to ignore potential garbage in CDC headers but
> also introduced a potential infinite loop.  This can happen at the first
> loop iteration (elength = 0 in that case) if the description isn't a
> DT_CS_INTERFACE or later if 'buffer[0]' is zero.

> It should also be noted that the wrong length was being added to 'buffer'
> in case 'buffer[1]' was not a DT_CS_INTERFACE descriptor, since elength was
> assigned after that check in the loop.

> A specially crafted USB device could be used to trigger this infinite loop.

> Fixes: 7e860a6e ("cdc-acm: add sanity checks")

    12-digit SHA1 hash is required here.

> Signed-off-by: Phil Turnbull <phil.turnbull@...cle.com>
> Signed-off-by: Quentin Casasnovas <quentin.casasnovas@...cle.com>
> CC: Oliver Neukum <oneukum@...e.de>
> CC: Adam Lee <adam8157@...il.com>
> ---
>   drivers/usb/class/cdc-acm.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)

> diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
> index 6836177..1ac4587 100644
> --- a/drivers/usb/class/cdc-acm.c
> +++ b/drivers/usb/class/cdc-acm.c
> @@ -1133,11 +1133,12 @@ static int acm_probe(struct usb_interface *intf,
>   	}
>
>   	while (buflen > 0) {
> +		if ((elength = buffer[0]) == 0)

    Please run your patches thru scripts/checkpatch.pl. Assignments in the 
*if* operator are not allowed.

[...]

WBR, Sergei

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