linux-hardening - Re: [PATCH] staging: rtl8723bs: Check for NULL header value

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Date:   Thu, 13 Jan 2022 10:29:04 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Larry Finger <Larry.Finger@...inger.net>,
        Phillip Potter <phil@...lpotter.co.uk>,
        Michael Straube <straube.linux@...il.com>,
        Fabio Aiuto <fabioaiuto83@...il.com>,
        linux-staging@...ts.linux.dev, Hans de Goede <hdegoede@...hat.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Marco Cesati <marcocesati@...il.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] staging: rtl8723bs: Check for NULL header value

On Thu, Jan 13, 2022 at 10:13:46AM +0100, Greg Kroah-Hartman wrote:
> On Wed, Jan 12, 2022 at 04:20:01PM -0800, Kees Cook wrote:
> > When building with -Warray-bounds, the following warning is emitted:
> > 
> > In file included from ./include/linux/string.h:253,
> >                  from ./arch/x86/include/asm/page_32.h:22,
> >                  from ./arch/x86/include/asm/page.h:14,
> >                  from ./arch/x86/include/asm/thread_info.h:12,
> >                  from ./include/linux/thread_info.h:60,
> >                  from ./arch/x86/include/asm/preempt.h:7,
> >                  from ./include/linux/preempt.h:78,
> >                  from ./include/linux/rcupdate.h:27,
> >                  from ./include/linux/rculist.h:11,
> >                  from ./include/linux/sched/signal.h:5,
> >                  from ./drivers/staging/rtl8723bs/include/drv_types.h:17,
> >                  from drivers/staging/rtl8723bs/core/rtw_recv.c:7:
> > In function 'memcpy',
> >     inlined from 'wlanhdr_to_ethhdr' at drivers/staging/rtl8723bs/core/rtw_recv.c:1554:2:
> > ./include/linux/fortify-string.h:41:33: warning: '__builtin_memcpy' offset [0, 5] is out of the bounds [0, 0] [-Warray-bounds]
> >    41 | #define __underlying_memcpy     __builtin_memcpy
> >       |                                 ^
> > 
> > This is because the compiler sees it is possible for "ptr" to be a NULL
> > value, and concludes that it has zero size and attempts to copy to it
> > would overflow. Instead, detect the NULL return and error out early.
> > 
> > Cc: Larry Finger <Larry.Finger@...inger.net>
> > Cc: Phillip Potter <phil@...lpotter.co.uk>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Cc: Michael Straube <straube.linux@...il.com>
> > Cc: Fabio Aiuto <fabioaiuto83@...il.com>
> > Cc: linux-staging@...ts.linux.dev
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> >  drivers/staging/rtl8723bs/core/rtw_recv.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/drivers/staging/rtl8723bs/core/rtw_recv.c b/drivers/staging/rtl8723bs/core/rtw_recv.c
> > index 41bfca549c64..61135c49322b 100644
> > --- a/drivers/staging/rtl8723bs/core/rtw_recv.c
> > +++ b/drivers/staging/rtl8723bs/core/rtw_recv.c
> > @@ -1513,6 +1513,9 @@ static signed int wlanhdr_to_ethhdr(union recv_frame *precvframe)
> >  	u8 *ptr = get_recvframe_data(precvframe) ; /*  point to frame_ctrl field */
> >  	struct rx_pkt_attrib *pattrib = &precvframe->u.hdr.attrib;
> >  
> > +	if (!ptr)
> > +		return _FAIL;
> 
> This will never happen, so let's not paper over compiler issues with
> stuff like this please.
> 
> As the call to get_recvframe_data() is only done in one place in this
> driver (in all drivers that look like this as well), it can just be
> replaced with the real code instead of the nonsensical test for NULL and
> then the compiler should be happy.
> 
> I'll gladly take that fix instead of this one, as that would be the
> correct solution here.

I changed it around, but it doesn't help. I assume this is because we
build with -fno-delete-null-pointer-checks, so the compiler continues
to assume it's possible for the incoming argument to be NULL.

Should I rearrange this to do a NULL check for precvframe before all the
assignments in addition to removing get_recvframe_data()?

-Kees

-- 
Kees Cook