On Fri, 1 Dec 2023 20:39:44 -0800 Kees Cook wrote: > > We are reading nla->nla_len, which is the first 2 bytes of the structure. > > And then we check if the structure is... there? > > I'm not debating whether it's there or not -- I'm saying the _contents_ of > "nlattr::nla_len", in the face of corruption or lack of initialization, > may be less than NLA_HDRLEN. (There's a lot of "but that's can't happen" > that _does_ happen in the kernel, so I'm extra paranoid.) nlattr is not an object someone has allocated. It's a header of a TLV in a byte stream of nested TLVs which comes from user space. If the attr did not go thru nla_ok() or some other careful validation we're toast regardless. > > If we don't trust that struct nlattr which gets passed here is at least > > NLA_HDRLEN (4B) then why do we think it's safe to read nla_len (the > > first 2B of it)? > > Type confusion (usually due to Use-after-Free flaws) means that a memory > region is valid (i.e. good pointer), but that the contents might have > gotten changed through other means. (To see examples of this with > struct msg_msg, see: https://syst3mfailure.io/wall-of-perdition/) A bit of a long read. > (On a related note, why does nla_len start at 4 instead of 0? i.e. why > does it include the size of nlattr? That seems redundant based on the > same logic you're using here.) Beats me.
