Re: [PATCH v10 25/27] x86: enable initial Rust support

[Ramon de C Valle]

On Thu, Oct 12, 2023 at 11:31 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Thu, Oct 12, 2023 at 10:50:36AM -0700, Sami Tolvanen wrote:
> > On Thu, Oct 12, 2023 at 3:47 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > >
> > > On Fri, Oct 14, 2022 at 11:34:30AM -0700, Sami Tolvanen wrote:
> > > > On Fri, Oct 14, 2022 at 11:05 AM Miguel Ojeda
> > > > <miguel.ojeda.sandonis@xxxxxxxxx> wrote:
> > > > >
> > > > > On Tue, Oct 11, 2022 at 1:16 AM Sami Tolvanen <samitolvanen@xxxxxxxxxx> wrote:
> > > > > >
> > > > > > Rust supports IBT with -Z cf-protection=branch, but I don't see this
> > > > > > option being enabled in the kernel yet. Cross-language CFI is going to
> > > > > > require a lot more work though because the type systems are not quite
> > > > > > compatible:
> > > > > >
> > > > > > https://github.com/rust-lang/rfcs/pull/3296
> > > > >
> > > > > I have pinged Ramon de C Valle as he is the author of the RFC above
> > > > > and implementation work too; since a month or so ago he also leads the
> > > > > Exploit Mitigations Project Group in Rust.
> > > >
> > > > Thanks, Miguel. I also talked to Ramon about KCFI earlier this week
> > > > and he expressed interest in helping with rustc support for it. In the
> > > > meanwhile, I think we can just add a depends on !CFI_CLANG to avoid
> > > > issues here.
> > >
> > > Having just read up on the thing it looks like the KCFI thing is
> > > resolved.
> > >
> > > I'm not sure I understand most of the objections in that thread through
> > > -- enabling CFI *will* break stuff, so what.
> > >
> > > Squashing the integer types seems a workable compromise I suppose. One
> > > thing that's been floated in the past is adding a 'seed' attribute to
> > > some functions in order to distinguish functions of otherwise identical
> > > signature.
> > >
> > > The Rust thing would then also need to support this attribute.
> > >
> > > Are there any concrete plans for this? It would allow, for example,
> > > to differentiate address_space_operations::swap_deactivate() from any
> > > other random function that takes only a file argument, say:
> > > locks_remove_file().
> >
> > I haven't really had time to look into it, so no concrete plans yet.
> > Adding an attribute shouldn't be terribly difficult, but Kees
> > expressed interest in automatic salting as well, which might be a more
> > involved project:
> >
> > https://github.com/ClangBuiltLinux/linux/issues/1736
>
> Automatic would be nice, but having an attribute would let us at least
> start the process manually (or apply salting from static analysis
> output, etc).

An idea would be to add something like the Rust cfi_encoding
attribute[1] and use it with something similar to the Newtype
Pattern[2], but in C[3], for aggregating function pointers that
otherwise would be aggregated in the same group in different groups.

[1]: https://doc.rust-lang.org/nightly/unstable-book/language-features/cfi-encoding.html
[2]: https://doc.rust-lang.org/book/ch19-04-advanced-types.html#using-the-newtype-pattern-for-type-safety-and-abstraction
[3]: Wrapping a type in a struct should achieve something similar even
without using the cfi_encoding attribute since the encoding for
structs is <length><name>, where <name> is <unscoped-name>.