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Date: Thu, 30 Nov 2023 10:55:16 -0800
From: Kees Cook <keescook@chromium.org>
To: Johannes Berg <johannes@sipsolutions.net>
Cc: Jeff Johnson <quic_jjohnson@quicinc.com>,
	Michael Walle <mwalle@kernel.org>, lkp@intel.com,
	oe-kbuild-all@lists.linux.dev, linux-wireless@vger.kernel.org,
	Max Schulze <max.schulze@online.de>
Subject: Re: [RFC PATCH] wifi: cfg80211: fix CQM for non-range use
Message-ID: <202311301054.0049306B7@keescook>
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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?

Actually, this alone fixes it too:

diff --git a/include/net/netlink.h b/include/net/netlink.h
index 167b91348e57..c59679524705 100644
--- a/include/net/netlink.h
+++ b/include/net/netlink.h
@@ -1214,9 +1214,9 @@ 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;
+	return nla->nla_len > NLA_HDRLEN ? nla->nla_len - NLA_HDRLEN : 0;
 }
 
 /**

-- 
Kees Cook