Re: [PATCH] rust: add flags for shadow call stack sanitizer

[Alice Ryhl]

On Tue, Mar 5, 2024 at 12:31 PM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
>
> On Tue, Mar 5, 2024 at 12:28 PM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
> >
> > On Tue, Mar 5, 2024 at 12:20 PM Valentin Obst <kernel@xxxxxxxxxxxxxxx> wrote:
> > >
> > > > >>> It's not 100% clear to me whether this patch is enough for full SCS
> > > > >>> support in Rust. If there is some issue where this makes things compile
> > > > >>> and work without actually applying SCS to the Rust code, please let me
> > > > >>> know. Is there some way to verify that it is actually working?
> > > > >>
> > > > >> Perhaps you could write a Rust version of the CFI_BACKWARD test in LKDTM?
> > > > >>
> > > > >> Alternatively, the simplest way to verify this is to look at the
> > > > >> disassembly and verify that shadow stack instructions are emitted to
> > > > >> Rust functions too. In case of dynamic SCS, you might need to dump
> > > > >> function memory in a debugger to verify that PAC instructions were
> > > > >> patched correctly. If they're not, the code will just quietly continue
> > > > >> working without using shadow stacks.
> > > > >
> > > > > Was just in the process of doing that:
> > > > >
> > > > > - `paciasp`/`autiasp` pairs are emitted for functions in Rust modules.
> > > > > - Rust modules have no `.init.eh_frame` section, which implies that
> > > > >   `module_finalize` is _not_ rewriting the pac insns when SCS is dynamic.
> > > > >   - Confirmed that behavior in the debugger (C modules and the C part of the
> > > > >     kernel are correctly rewritten, Rust modules execute with
> > > > >     `paciasp`/`autiasp` still in place).
> > > > > - Kernel boots just fine with Rust kunit tests, tested with and without dynamic
> > > > >   SCS, i.e., on a CPU that supports PAC/BTI and one that does not.
> > > > > - Rust sample modules load and unload without problems as well.
> > > > > - `x18` is indeed not used in the codegen.
> > > > >
> > > > > I guess we might be able to get this working when we tweak the build system
> > > > > to emit the missing section for Rust modules.
> > > >
> > > > I suppose the -Cforce-unwind-tables=y flag will most likely do it.
> > >
> > > Yes, enabling this means that `.eh_frame` sections, which are converted to
> > > `.init.eh_frame` sections for loadable modules, are generated for Rust
> > > objects.
> > >
> > > Tested booting, kunit tests, sample modules (as builtin and loadable) for
> > > both, dynamic SCS active and inactive. Backtraces on Rust panicks also look
> > > normal.
> > >
> > > Confirmed that in the debugger that builtin and external modules are
> > > rewritten (or not rewritten if no dynamic SCS). Did not check that the
> > > `eh_frame` sections are exhaustive, i.e., cover all `paciasp`/`autiasp`
> > > pairs, only verified a few functions (in init text and normal text).
> >
> > Thank you for checking that!
> >
> > > > There's also an use_sync_unwind option, but it defaults to no, so it
> > > > doesn't seem like we need to set it.
> > >
> > > Are those defaults stable or will we notice if they change? If not it might
> > > make sense to set it explicitly anyways to avoid surprises in the future.
> >
> > The flag itself is unstable, so I imagine that nothing is promised about it.
> >
> > I tried it, but I wasn't actually able to find a way to set it. I can
> > see the flag in the rustc source code, but passing -Zuse-sync-unwind=n
> > results in "error: unknown unstable option: `use-sync-unwind`". Not
> > sure what the issue is.
>
> Oh, I understand now. It's really recent and not in 1.73.0, which is
> what I'm using in the Android build.

Sent v2 with what we have learned:
https://lore.kernel.org/rust-for-linux/20240305-shadow-call-stack-v2-1-c7b4a3f4d616@xxxxxxxxxx/

Thanks!