Re: [PATCH 1/1] mwifiex: Fix possible buffer overflows in mwifiex_config_scan

[Brian Norris <briannorris@chromium.org>]

(Note: this is version 1; there's a later version posted, which does
not have a v2 tag...)

https://lore.kernel.org/linux-wireless/20201208150951.35866-1-ruc_zhangxiaohui@163.com/

On Sat, Jan 9, 2021 at 7:11 AM Peter Seiderer <ps.report@gmx.net> wrote:
> On Tue,  8 Dec 2020 20:45:23 +0800, Xiaohui Zhang <ruc_zhangxiaohui@163.com> wrote:
> > From: Zhang Xiaohui <ruc_zhangxiaohui@163.com>
> > mwifiex_config_scan() calls memcpy() without checking
> > the destination size may trigger a buffer overflower,
> > which a local user could use to cause denial of service
> > or the execution of arbitrary code.
> > Fix it by putting the length check before calling memcpy().
> >
> > Signed-off-by: Zhang Xiaohui <ruc_zhangxiaohui@163.com>
> > ---
> >  drivers/net/wireless/marvell/mwifiex/scan.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/drivers/net/wireless/marvell/mwifiex/scan.c b/drivers/net/wireless/marvell/mwifiex/scan.c
> > index c2a685f63..b1d90678f 100644
> > --- a/drivers/net/wireless/marvell/mwifiex/scan.c
> > +++ b/drivers/net/wireless/marvell/mwifiex/scan.c
> > @@ -930,6 +930,8 @@ mwifiex_config_scan(struct mwifiex_private *priv,
> >                                   "DIRECT-", 7))
> >                               wildcard_ssid_tlv->max_ssid_length = 0xfe;
> >
> > +                     if (ssid_len > 1)
> > +                             ssid_len = 1;
>
> Why do your believe the available size is only '1'? A SSID is expected
> to be of size IEEE80211_MAX_SSID_LE/32 and the wildcard_ssid_tlv pointer
> is casted from tlv_pos (some lines above) which is a pointer/index into
> scan_cfg_out->tlv_buf...

I pointed out this discrepancy already, taking a slightly different approach:

https://lore.kernel.org/linux-wireless/CA+ASDXPVu5S0Vm0aOcyqLN090u3BwA_nV358YwkpXuU223Ug9g@mail.gmail.com/

> And the define (line 44) indicates there should be enough space for a SSID:
>
>   42 /* Memory needed to store a max number/size WildCard SSID TLV for a firmware
>   43         scan */
>   44 #define WILDCARD_SSID_TLV_MAX_SIZE  \
>   45         (MWIFIEX_MAX_SSID_LIST_LENGTH *                                 \
>   46                 (sizeof(struct mwifiex_ie_types_wildcard_ssid_params)   \
>   47                         + IEEE80211_MAX_SSID_LEN))

Ah, good catch. So this may not be a true overflow issue at all, even
if it's confusing and bad code. The "problem" is that this piece of
the struct is variable-length, and unless we're going to dynamically
resize/reallocate the whole buffer, we have to assume the initial
allocation was large enough (and per your note, it is).

> For sure something to improve here instead of using a confusing 'u8 ssid[1]'
> in struct mwifiex_ie_types_wildcard_ssid_params...

Yep.

Brian