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From: Alice Ryhl <aliceryhl@google.com>
Date: Tue, 5 Mar 2024 12:28:58 +0100
Message-ID: <CAH5fLgis7Usg_cfWM5rBKjRKsB4857PkyuCMrCU6PmEqTOMFiw@mail.gmail.com>
Subject: Re: [PATCH] rust: add flags for shadow call stack sanitizer
To: Valentin Obst <kernel@valentinobst.de>
Cc: Jamie.Cunliffe@arm.com, a.hindborg@samsung.com, alex.gaynor@gmail.com, 
	ardb@kernel.org, benno.lossin@proton.me, bjorn3_gh@protonmail.com, 
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On Tue, Mar 5, 2024 at 12:20=E2=80=AFPM Valentin Obst <kernel@valentinobst.=
de> 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 com=
pile
> > >>> 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 L=
KDTM?
> > >>
> > >> 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 contin=
ue
> > >> 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 dyna=
mic.
> > >   - 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 witho=
ut 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=3Dy flag will most likely do it.
>
> Yes, enabling this means that `.eh_frame` sections, which are converted t=
o
> `.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 lo=
ok
> 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 mig=
ht
> 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=3Dn
results in "error: unknown unstable option: `use-sync-unwind`". Not
sure what the issue is.

Alice