Re: [RFC PATCH] wifi: cfg80211: fix CQM for non-range use

[Kees Cook <keescook@chromium.org>]

On Thu, Nov 30, 2023 at 07:40:26PM +0100, Johannes Berg wrote:
> On Thu, 2023-11-30 at 10:32 -0800, Kees Cook wrote:
> > Yeah, I would expect this to mean that there is a code path that
> > GCC found where the value could overflow. It does this when a variable
> > "value range" gets bounded (e.g. an int isn't the full -INT_MAX to INT_MAX
> > range).And flex_array_size() was designed to saturate at SIZE_MIX rather
> > than wrapping around to an unexpected small value, so these are playing
> > together it seems.
> > 
> > However, I would have expected the kzalloc() to blow up _first_.
> 
> Hmm.
> 
> > Regardless, I suspect the addition of "if (n_thresholds > 1)" is what is
> > tripping GCC.
> > 
> >                 int len = nla_len(attrs[NL80211_ATTR_CQM_RSSI_THOLD]);
> > 		...
> >                 return nl80211_set_cqm_rssi(info, thresholds, len / 4,
> >                                             hysteresis);
> > 
> > Now it "knows" there is a path where n_threasholds could be [2,
> > INT_MAX].
> 
> Yeah, it's not _really_ bounded, apart from the message length? But then
> struct_size() should saturate and fail? But I guess it cannot know that,
> and limits the object size to 1<<63 - 1 whereas the copy is 1<<64 - 1...
> 
> > Does this warning go away if "len" is made unsigned?
> 
> Thing is, neither Kalle nor I can even reproduce the warning locally, so
> it's a bit hard to check ... not even with their config and gcc 12.2.0
> (nix, rather than debian though.)

Ah! Hmm. Let me see if I can coax out the warning locally...

-- 
Kees Cook