linux-kernel - Re: [PATCH] staging: rtl8723bs: Fix uninitialized variable

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Date:   Sun, 06 Jun 2021 10:45:58 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Wenli Looi <wlooi@...lgary.ca>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: rtl8723bs: Fix uninitialized variable

On Sunday, June 6, 2021 10:09:39 AM CEST Wenli Looi wrote:
> On Sun, Jun 6, 2021 at 1:00 AM Fabio M. De Francesco
> 
> <fmdefrancesco@...il.com> wrote:
> > On Sunday, June 6, 2021 9:51:35 AM CEST Wenli Looi wrote:
> > > On Sun, Jun 6, 2021 at 12:13 AM Greg Kroah-Hartman
> > > 
> > > <gregkh@...uxfoundation.org> wrote:
> > > > On Sun, Jun 06, 2021 at 12:00:21AM -0700, Wenli Looi wrote:
> > > > > Uninitialized struct with invalid pointer causes BUG and prevents 
access
> > > > > point from working. Access point works once I apply this patch.
> > > > > 
> > > > > https://forum.armbian.com/topic/14727-wifi-ap-kernel-bug-in-kernel-5444/
> > > > > has more details.
> > > > > 
> > > > > Signed-off-by: Wenli Looi <wlooi@...lgary.ca>
> > > > > ---
> > > > > 
> > > > >  drivers/staging/rtl8723bs/os_dep/ioctl_cfg80211.c | 2 +-
> > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/drivers/staging/rtl8723bs/os_dep/ioctl_cfg80211.c
> > > > > b/drivers/staging/rtl8723bs/os_dep/ioctl_cfg80211.c index 
2fb80b6eb..
> > 
> > 7308e1185
> > 
> > > > > 100644
> > > > > --- a/drivers/staging/rtl8723bs/os_dep/ioctl_cfg80211.c
> > > > > +++ b/drivers/staging/rtl8723bs/os_dep/ioctl_cfg80211.c
> > > > > @@ -2384,7 +2384,7 @@ void rtw_cfg80211_indicate_sta_assoc(struct
> > 
> > adapter *padapter,
> > 
> > > > > u8 *pmgmt_frame,> >
> > > > > 
> > > > >       DBG_871X(FUNC_ADPT_FMT"\n", FUNC_ADPT_ARG(padapter));
> > > > >       
> > > > >       {
> > > > > 
> > > > > -             struct station_info sinfo;
> > > > > +             struct station_info sinfo = {};
> > > > 
> > > > What caused this bug to show up?  Did it happen from some other 
commit?
> > > > 
> > > > Are you sure that all of the fields are being cleared properly here,
> > > > what about any "holes" in the structure?
> > > > 
> > > > thanks,
> > > > greg k-h
[CUT]
> > > 
> > > Do you think kzalloc() would be preferable?
> > 
> > You cannot use kzalloc there: 'sinfo' is instantiated automatically on the
> > stack. The example you took had a pointer to the struct.
> 
> The stack variable could be replaced with code like:
> 
> struct station_info *sinfo;if (!sinfo)
>
Why that "if (!sinfo" before kzalloc?
>
> sinfo = kzalloc(sizeof(*sinfo), GFP_KERNEL);
> ...
> cfg80211_new_sta(..., sinfo, ...);
> kfree(sinfo);
> 
> which is what the linked code basically does. I'm not sure if this is 
preferred?
>
I don't know that code, but I think that:

(1) It is generally preferred that big structures should be allocated 
dinamically: kernel stack is a scarce resource.
(2) Passing address of variables from the stack to other functions is 
generally a good source of troubles.

I think you are closer to the proper solution.

Fabio 
>
> > 
> > > Sorry, I'm not familiar with "holes" in the struct.