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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Tue, 14 Jul 2020 09:52:24 +0200
Message-ID: <CAMuHMdVW_MzdDQk4f0RN-FsaedEr8WHREuTHxymWGpx3CmDX0Q@mail.gmail.com>
Subject: Re: [PATCH 01/10] misc: c2port: core: Ensure source size does not
 equal destination size in strncpy()
To:     Lee Jones <lee.jones@linaro.org>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Greg KH <gregkh@linuxfoundation.org>,
        "Eurotech S.p.A" <info@eurotech.it>,
        Rodolfo Giometti <giometti@linux.it>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
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Hi Lee,

On Tue, Jul 14, 2020 at 9:46 AM Lee Jones <lee.jones@linaro.org> wrote:
> On Mon, 13 Jul 2020, Geert Uytterhoeven wrote:
> > On Fri, Jun 26, 2020 at 3:06 PM Lee Jones <lee.jones@linaro.org> wrote:
> > > We need to ensure there's a place for the NULL terminator.
> >
> > But who's filling that space with a NUL (not NULL) terminator?
> >
> > > Fixes the following W=1 warning(s):
> > >
> > >  In file included from include/linux/bitmap.h:9,
> > >  from include/linux/nodemask.h:95,
> > >  from include/linux/mmzone.h:17,
> > >  from include/linux/gfp.h:6,
> > >  from include/linux/umh.h:4,
> > >  from include/linux/kmod.h:9,
> > >  from include/linux/module.h:16,
> > >  from drivers/misc/c2port/core.c:9:
> > >  In function ‘strncpy’,
> > >  inlined from ‘c2port_device_register’ at drivers/misc/c2port/core.c:926:2:
> > >  include/linux/string.h:297:30: warning: ‘__builtin_strncpy’ specified bound 32 equals destination size [-Wstringop-truncation]
> > >  297 | #define __underlying_strncpy __builtin_strncpy
> > >  | ^
> > >  include/linux/string.h:307:9: note: in expansion of macro ‘__underlying_strncpy’
> > >  307 | return __underlying_strncpy(p, q, size);
> > >  | ^~~~~~~~~~~~~~~~~~~~
> > >
> > > Cc: Rodolfo Giometti <giometti@linux.it>
> > > Cc: "Eurotech S.p.A" <info@eurotech.it>
> > > Signed-off-by: Lee Jones <lee.jones@linaro.org>
> > > ---
> > >  drivers/misc/c2port/core.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/misc/c2port/core.c b/drivers/misc/c2port/core.c
> > > index 33bba18022892..80d87e8a0bea9 100644
> > > --- a/drivers/misc/c2port/core.c
> > > +++ b/drivers/misc/c2port/core.c
> > > @@ -923,7 +923,7 @@ struct c2port_device *c2port_device_register(char *name,
> > >         }
> > >         dev_set_drvdata(c2dev->dev, c2dev);
> >
> > c2dev is allocated using:
> >
> >         c2dev = kmalloc(sizeof(struct c2port_device), GFP_KERNEL);
> >
> > hence the allocated memory is not zeroed.
> >
> > >
> > > -       strncpy(c2dev->name, name, C2PORT_NAME_LEN);
> > > +       strncpy(c2dev->name, name, C2PORT_NAME_LEN - 1);
> >
> > strncpy()
> >   1. does not terminate the destination with a NUL if the source length
> >       is C2PORT_NAME_LEN - 1,
> >   2. fills all remaining space in the destination buffer with NUL characters.
> >
> > So c2dev.name[C2PORT_NAME_LEN - 1] always contains an uninitialized
> > value.
> >
> > Now, it seems the only caller of c2port_device_register() passes
> > "uc" as the name.  Which means in practice c2dev.name[] will be
> > NUL-terminated. However, the last byte will still be uninitialized, and
> > if the buffer is ever copied to userspace, your patch will have introduced
> > a leak.
>
> Quite right.  Good spot.  I must have made the assumption that the
> destination buffer would be pre-initialised.  Not sure why it's not in
> this case.  Seems like an odd practice.
>
> So we have a choice.  We can either enlarge the destination buffer to
> *actually* allow a full length (32 byte in this case) naming string,
> or zero the buffer.
>
> Or even both!
>
> Do you have a preference?

Do we know if the buffer or full c2dev struct is ever copied to userspace?
If it may be copied => kalloc().
If it will never be copied => strlcpy() (no NUL-padding, only NUL-terminator).

Oh, and there is a newer one on the block (which I always have to lookup),
which is preferred over strlcpy() and strncpy(): strscpy().
And reading lib/string.c, there's strscpy_pad(), too ;-)

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds