linux-kernel - Re: [PATCH] staging: rtl8188eu: Avoid null pointer arithmetic

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Date:   Thu, 27 Sep 2018 22:12:54 +0100
From:   Aymen Qader <qader.aymen@...il.com>
To:     Larry Finger <Larry.Finger@...inger.net>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: rtl8188eu: Avoid null pointer arithmetic

On Thu, Sep 27, 2018 at 03:52:53PM -0500, Larry Finger wrote:
> On 9/27/18 12:04 PM, Aymen Qader wrote:
> > Avoid null pointer arithmetic in rtw_mlme_ext.c by skipping other field
> > checks if the information element pointer is null.
> > 
> > Signed-off-by: Aymen Qader <qader.aymen@...il.com>
> > ---
> >   drivers/staging/rtl8188eu/core/rtw_mlme_ext.c | 4 +++-
> >   1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/staging/rtl8188eu/core/rtw_mlme_ext.c b/drivers/staging/rtl8188eu/core/rtw_mlme_ext.c
> > index 834053a0ae9d..8a3a71456cd0 100644
> > --- a/drivers/staging/rtl8188eu/core/rtw_mlme_ext.c
> > +++ b/drivers/staging/rtl8188eu/core/rtw_mlme_ext.c
> > @@ -2971,8 +2971,10 @@ static unsigned int OnAssocReq(struct adapter *padapter,
> >   	/*  checking SSID */
> >   	p = rtw_get_ie(pframe + WLAN_HDR_A3_LEN + ie_offset, _SSID_IE_, &ie_len,
> >   		pkt_len - WLAN_HDR_A3_LEN - ie_offset);
> > -	if (!p)
> > +	if (!p) {
> >   		status = _STATS_FAILURE_;
> > +		goto OnAssocReqFail;
> > +	}
> >   	if (ie_len == 0) { /*  broadcast ssid, however it is not allowed in assocreq */
> >   		status = _STATS_FAILURE_;
> 
> I do not think this patch avoids any pointer arithmetic. If p is NULL, then
> ie_len will be zero and the branch with the memcmp() call, where the pointer
> arithmetic is done, will be skipped.
I'm sincerely sorry, you're completely right--that was a bad oversight
from me, I should have checked more thoroughly.

> 
> That said, it is better to bail out with the first failure condition. I do
> not require the following, but the code would be even simpler if you test p
> and ie_len==0 in a single if statement and eliminate some code as in
> 
> diff --git a/drivers/staging/rtl8188eu/core/rtw_mlme_ext.c
> b/drivers/staging/rtl8188eu/core/rtw_mlme_ext.c
> index 1115050077e4..71722cec84a0 100644
> --- a/drivers/staging/rtl8188eu/core/rtw_mlme_ext.c
> +++ b/drivers/staging/rtl8188eu/core/rtw_mlme_ext.c
> @@ -2982,11 +2982,10 @@ static unsigned int OnAssocReq(struct adapter *padapter,
>         /*  checking SSID */
>         p = rtw_get_ie(pframe + WLAN_HDR_A3_LEN + ie_offset, _SSID_IE_, &ie_len,
>                 pkt_len - WLAN_HDR_A3_LEN - ie_offset);
> -       if (!p)
> -               status = _STATS_FAILURE_;
> 
> -       if (ie_len == 0) { /*  broadcast ssid, however it is not allowed in
> assocreq */
> +       if (!p || ie_len == 0) { /*  broadcast ssid, however it is not
> allowed in assocreq */
>                 status = _STATS_FAILURE_;
> +               goto OnAssocReqFail;
>         } else {
>                 /*  check if ssid match */
>                 if (memcmp((void *)(p+2), cur->Ssid.Ssid, cur->Ssid.SsidLength))
> 

Yep, I understand that would be a lot better. If it's alright, I'll send
this in with a v2 (w/ a more appropriate commit message).

> 
> ACKed-by: Larry Finger <Larry.Finger@...inger.net>
> 
> Thanks,
> 
> Larry
> 

Kind regards,
Aymen