netdev - Re: possible integer overflow in CDC-NCM checks

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Message-ID: <YgU0imlDm4Otol6a@kroah.com>
Date:   Thu, 10 Feb 2022 16:51:38 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Oliver Neukum <oneukum@...e.com>
Cc:     Bjørn Mork <bjorn@...k.no>,
        USB list <linux-usb@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: possible integer overflow in CDC-NCM checks

On Thu, Feb 10, 2022 at 04:23:58PM +0100, Oliver Neukum wrote:
> Hi,
> 
> unfortunately there is no maintainer and you were among
> the last to send fixes for this driver, so I am going to ask
> you for review.
> 
> It looks to me like the sanity check in
> cdc_ncm_rx_fixup() can be fooled by abusing integer overflows.
> You cannot guarantee that the addition of offset and len will
> fit into an integer and this gets worse if offset can be
> negative.
> 
> As this is tricky, do you think this fix is correct?
> 
>     Regards
>         Oliver
> 
> CDC-NCM: avoid overflow in sanity checking A broken device may give an
> extreme offset like 0xFFF0 and a reasonable length for a fragment. In
> the sanity check as formulated now, this will create an integer
> overflow, defeating the sanity check. It needs to be rewritten as a
> subtraction and the variables should be unsigned. Signed-off-by: Oliver
> Neukum <oneukum@...e.com> --- drivers/net/usb/cdc_ncm.c | 6 +++--- 1
> file changed, 3 insertions(+), 3 deletions(-) diff --git
> a/drivers/net/usb/cdc_ncm.c b/drivers/net/usb/cdc_ncm.c index
> e303b522efb5..f78fccbc4b93 100644 --- a/drivers/net/usb/cdc_ncm.c +++
> b/drivers/net/usb/cdc_ncm.c @@ -1715,10 +1715,10 @@ int
> cdc_ncm_rx_fixup(struct usbnet *dev, struct sk_buff *skb_in) { struct
> sk_buff *skb; struct cdc_ncm_ctx *ctx = (struct cdc_ncm_ctx
> *)dev->data[0]; - int len; + unsigned int len; int nframes; int x; - int
> offset; + unsigned int offset; union { struct usb_cdc_ncm_ndp16 *ndp16;
> struct usb_cdc_ncm_ndp32 *ndp32; @@ -1791,7 +1791,7 @@ int
> cdc_ncm_rx_fixup(struct usbnet *dev, struct sk_buff *skb_in) } /* sanity
> checking */ - if (((offset + len) > skb_in->len) || + if ((offset >
> skb_in->len - len) || (len > ctx->rx_max) || (len < ETH_HLEN)) {
> netif_dbg(dev, rx_err, dev->net, "invalid frame detected (ignored)
> offset[%u]=%u, length=%u, skb=%p\n", -- 2.34.1
> 

Your fix is impossible to read :(