Re: [PATCH v2] netlink: Return unsigned value for nla_len()

[Nicolas Dichtel <nicolas.dichtel@6wind.com>]

Le 02/12/2023 à 21:25, Kees Cook a écrit :
> The return value from nla_len() is never expected to be negative, and can
> never be more than struct nlattr::nla_len (a u16). Adjust the prototype
> on the function. This will let GCC's value range optimization passes
> know that the return can never be negative, and can never be larger than
> u16. As recently discussed[1], this silences the following warning in
> GCC 12+:
> 
> net/wireless/nl80211.c: In function 'nl80211_set_cqm_rssi.isra':
> net/wireless/nl80211.c:12892:17: warning: 'memcpy' specified bound 18446744073709551615 exceeds maximum object size 9223372036854775807 [-Wstringop-overflow=]
> 12892 |                 memcpy(cqm_config->rssi_thresholds, thresholds,
>       |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 12893 |                        flex_array_size(cqm_config, rssi_thresholds,
>       |                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 12894 |                                        n_thresholds));
>       |                                        ~~~~~~~~~~~~~~
> 
> A future change would be to clamp the subtraction to make sure it never
> wraps around if nla_len is somehow less than NLA_HDRLEN, which would
> have the additional benefit of being defensive in the face of nlattr
> corruption or logic errors.
> 
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202311090752.hWcJWAHL-lkp@intel.com/ [1]
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Paolo Abeni <pabeni@redhat.com>
> Cc: Johannes Berg <johannes@sipsolutions.net>
> Cc: Jeff Johnson <quic_jjohnson@quicinc.com>
> Cc: Michael Walle <mwalle@kernel.org>
> Cc: Max Schulze <max.schulze@online.de>
> Cc: netdev@vger.kernel.org
> Cc: linux-wireless@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  v2:
>  - do not clamp return value (kuba)
>  - adjust NLA_HDRLEN to be u16 also
>  v1: https://lore.kernel.org/all/20231130200058.work.520-kees@kernel.org/
> ---
>  include/net/netlink.h        | 2 +-
>  include/uapi/linux/netlink.h | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/net/netlink.h b/include/net/netlink.h
> index 83bdf787aeee..7678a596a86b 100644
> --- a/include/net/netlink.h
> +++ b/include/net/netlink.h
> @@ -1200,7 +1200,7 @@ static inline void *nla_data(const struct nlattr *nla)
>   * nla_len - length of payload
>   * @nla: netlink attribute
>   */
> -static inline int nla_len(const struct nlattr *nla)
> +static inline u16 nla_len(const struct nlattr *nla)
>  {
>  	return nla->nla_len - NLA_HDRLEN;
>  }
> diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
> index f87aaf28a649..270feed9fd63 100644
> --- a/include/uapi/linux/netlink.h
> +++ b/include/uapi/linux/netlink.h
> @@ -247,7 +247,7 @@ struct nlattr {
>  
>  #define NLA_ALIGNTO		4
>  #define NLA_ALIGN(len)		(((len) + NLA_ALIGNTO - 1) & ~(NLA_ALIGNTO - 1))
> -#define NLA_HDRLEN		((int) NLA_ALIGN(sizeof(struct nlattr)))
> +#define NLA_HDRLEN		((__u16) NLA_ALIGN(sizeof(struct nlattr)))
I wonder if this may break the compilation of some userspace tools with errors
like comparing signed and unsigned values.


Regards,
Nicolas