Re: [PATCH 01/10] misc: c2port: core: Ensure source size does not equal destination size in strncpy()

Geert Uytterhoeven
2020-07-13T19:12:58Z

Hi Lee,

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.

> c2dev->ops = ops;
> mutex_init(&c2dev->mutex);

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