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Message-ID: <313903974b14e94a2fb0dd5962362dba39bb3dea.camel@sipsolutions.net> Date: Fri, 01 Dec 2023 08:45:28 +0100 From: Johannes Berg <johannes@...solutions.net> To: Jakub Kicinski <kuba@...nel.org>, Kees Cook <keescook@...omium.org> Cc: kernel test robot <lkp@...el.com>, "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Jeff Johnson <quic_jjohnson@...cinc.com>, Michael Walle <mwalle@...nel.org>, Max Schulze <max.schulze@...ine.de>, netdev@...r.kernel.org, linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org Subject: Re: [PATCH] netlink: Return unsigned value for nla_len() On Thu, 2023-11-30 at 17:25 -0800, Jakub Kicinski wrote: > On Thu, 30 Nov 2023 12:01:01 -0800 Kees Cook wrote: > > This has the additional benefit of being defensive in the face of nlattr > > corruption or logic errors (i.e. nla_len being set smaller than > > NLA_HDRLEN). > > As Johannes predicted I'd rather not :( :) > The callers should put the nlattr thru nla_ok() during validation > (nla_validate()), or walking (nla_for_each_* call nla_ok()). Which we do, since we have just normal input validation on generic netlink. Actually nla_validate() only does it via walking either ;-) The thing is that's something the compiler can't really see, it happens out-of-line in completely different code (generic netlink) before you even get into nl80211. > > -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; > > } > > Note the the NLA_HDRLEN is the length of struct nlattr. > I mean of the @nla object that gets passed in as argument here. > So accepting that nla->nla_len may be < NLA_HDRLEN means > that we are okay with dereferencing a truncated object... > > We can consider making the return unsinged without the condition maybe? That seems problematic too though - better for an (unvalidated) attribute with a bad size to actually show up with a negative payload length rather than an underflow to a really big size. Anyway I really don't mind the workaround in nl80211 (which was to make the variables holding this unsigned), since we *do* know that we validated there, that's not an issue wrt. the length. johannes
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