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

[Kees Cook]

On Mon, Dec 04, 2023 at 10:22:25AM +0100, Nicolas Dichtel wrote:
> 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@xxxxxxxxx>
> > Closes: https://lore.kernel.org/oe-kbuild-all/202311090752.hWcJWAHL-lkp@xxxxxxxxx/ [1]
> > Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
> > Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> > Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> > Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
> > Cc: Johannes Berg <johannes@xxxxxxxxxxxxxxxx>
> > Cc: Jeff Johnson <quic_jjohnson@xxxxxxxxxxx>
> > Cc: Michael Walle <mwalle@xxxxxxxxxx>
> > Cc: Max Schulze <max.schulze@xxxxxxxxx>
> > Cc: netdev@xxxxxxxxxxxxxxx
> > Cc: linux-wireless@xxxxxxxxxxxxxxx
> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > ---
> >  v2:
> >  - do not clamp return value (kuba)
> >  - adjust NLA_HDRLEN to be u16 also
> >  v1: https://lore.kernel.org/all/20231130200058.work.520-kees@xxxxxxxxxx/
> > ---
> >  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.

Should I drop this part, then?

-- 
Kees Cook