add mixed binary hardening rule

[polazarus]

open question: should the guide includes current hardening options?

closes #72

[polazarus]

-s

[hg-anssi]

@polazarus : could you rebase your commit on top of PR #83 ?

[hg-anssi]

@polazarus : Maybe this should be added to the FFI part ?

[polazarus]

I don't know. It should be enabled in product that do not directly use FFI... My understanding is that if a programmer does not use FFI, it should not have to look at the FFI section.

[hg-anssi]

In which cases can mixed binaries be used without FFI?

[polazarus]

• mettre un paragraphe hardening global avec une Recommandation de durcissement, en présence de code unsafe propre ou tiers
• changer la règle en warning
• ajouter un exemple simple

https://doc.rust-lang.org/beta/rustc/exploit-mitigations.html#stack-clashing-protection

[hg-anssi]

Here are some questions of mine regarding the subject of mixed binaries with hardening for the guide:

• This rule does not seem to be a Rust specificity
• However, it could be interesting to give some good practices to bypass attacks for the specific case of rust (function pointer)
  • Something similar is already written is the FFI part of the guide
• I think the problem of mixed binary should only be study in the "direction" Rust from C because
  • calling C function from Rust is unsafe, and thus is clearly tagged as dangerous by the unsafe annotation,
  • calling C from Rust has the same constraints than any other language calling C regarding preservation of hardening
• Thus I think that mixed binaries created by crates which bind to C library may not be study in this guide regarding the problem of hardening, since the constraints they have are not specific to Rust
• On the contrary, calling Rust from C, even though it involves the same requirements as any other language called from C, is interesting to study because the harmfulness of inclusion contrasts with the feeling of security provided while writing safe Rust code.

For these reasons, I think that the rules concerning the hardening of mixed binaries should be included in the FFI sections, possibly in the form of additional points insisting on the existing rules (such as the FFI-MARKEDFUNPTR rule, for example).