Re: [PATCH] net: dsa: lan9303: replace deprecated strncpy with memcpy

[Justin Stitt]

On Thu, Oct 5, 2023 at 7:54 AM Alexander Lobakin
<aleksander.lobakin@xxxxxxxxx> wrote:
>
> From: Kees Cook <keescook@xxxxxxxxxxxx>
> Date: Wed, 4 Oct 2023 22:02:00 -0700
>
> > On Wed, Oct 04, 2023 at 08:07:55PM -0700, Kees Cook wrote:
> >> On Thu, Oct 05, 2023 at 12:30:18AM +0000, Justin Stitt wrote:
> >>> `strncpy` is deprecated for use on NUL-terminated destination strings
> >>> [1] and as such we should prefer more robust and less ambiguous
> >>> interfaces.
> >>>
> >>> Let's opt for memcpy as we are copying strings into slices of length
> >>> `ETH_GSTRING_LEN` within the `data` buffer. Other similar get_strings()
> >>> implementations [2] [3] use memcpy().
> >>>
> >>> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> >>> Link: https://elixir.bootlin.com/linux/v6.3/source/drivers/infiniband/ulp/opa_vnic/opa_vnic_ethtool.c#L167 [2]
> >>> Link: https://elixir.bootlin.com/linux/v6.3/source/drivers/infiniband/ulp/ipoib/ipoib_ethtool.c#L137 [3]
> >>> Link: https://github.com/KSPP/linux/issues/90
> >>> Cc: linux-hardening@xxxxxxxxxxxxxxx
> >>> Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
> >>> ---
> >>> Note: build-tested only.
> >>> ---
> >>>  drivers/net/dsa/lan9303-core.c | 4 ++--
> >>>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/drivers/net/dsa/lan9303-core.c b/drivers/net/dsa/lan9303-core.c
> >>> index ee67adeb2cdb..665d69384b62 100644
> >>> --- a/drivers/net/dsa/lan9303-core.c
> >>> +++ b/drivers/net/dsa/lan9303-core.c
> >>> @@ -1013,8 +1013,8 @@ static void lan9303_get_strings(struct dsa_switch *ds, int port,
> >>>             return;
> >>>
> >>>     for (u = 0; u < ARRAY_SIZE(lan9303_mib); u++) {
> >>> -           strncpy(data + u * ETH_GSTRING_LEN, lan9303_mib[u].name,
> >>> -                   ETH_GSTRING_LEN);
> >>> +           memcpy(data + u * ETH_GSTRING_LEN, lan9303_mib[u].name,
> >>> +                  ETH_GSTRING_LEN);
> >>
> >> This won't work because lan9303_mib entries aren't ETH_GSTRING_LEN-long
> >> strings; they're string pointers:
> >>
> >> static const struct lan9303_mib_desc lan9303_mib[] = {
> >>         { .offset = LAN9303_MAC_RX_BRDCST_CNT_0, .name = "RxBroad", },
> >>
> >> So this really does need a strcpy-family function.
> >>
> >> And, I think the vnic_gstrings_stats and ipoib_gstrings_stats examples
> >> are actually buggy -- they're copying junk into userspace...
> >>
> >> I am reminded of this patch, which correctly uses strscpy_pad():
> >> https://lore.kernel.org/lkml/20230718-net-dsa-strncpy-v1-1-e84664747713@xxxxxxxxxx/
> >>
> >> I think you want to do the same here, and use strscpy_pad(). And perhaps
> >> send some fixes for the other memcpy() users?
> >
> > Meh, I think it's not worth fixing the memcpy() users of this. This
> > buggy pattern is very common, it seems:
> >
> > $ git grep 'data.*ETH_GSTRING_LEN' | grep memcpy | wc -l
> > 47
>
> We have ethtool_sprintf() precisely for the sake of filling the Ethtool
> statistics names.
>
> BTW this weird pattern "let's make the array of our stats names of fixed
> width (ETH_GSTRING_LEN), so that we could use memcpy() instead of
> strcpy()" was pretty common some time ago, no idea why, as it wastes
> memory for tons of \0 padding and provokes issues like the one you
> noticed here.

Just sent a v2 using ethtool_sprintf(). I'd appreciate some feedback on if I
used it correctly.

>
> >
>
> Thanks,
> Olek

[v2]: https://lore.kernel.org/r/20231005-strncpy-drivers-net-dsa-lan9303-core-c-v2-1-feb452a532db@xxxxxxxxxx

Thanks
Justin