Re: [PATCH 0/5] riscv: SCS support

Kees Cook
2023-08-14T18:33:33Z

On Mon, Aug 14, 2023 at 10:59:28AM -0700, Nathan Chancellor wrote:
> Hi Sami,
>
> On Fri, Aug 11, 2023 at 11:35:57PM +0000, Sami Tolvanen wrote:
> > Hi folks,
> >
> > This series adds Shadow Call Stack (SCS) support for RISC-V. SCS
> > uses compiler instrumentation to store return addresses in a
> > separate shadow stack to protect them against accidental or
> > malicious overwrites. More information about SCS can be found
> > here:
> >
> > https://clang.llvm.org/docs/ShadowCallStack.html
> >
> > Patch 1 is from Deepak, and it simplifies VMAP_STACK overflow
> > handling by adding support for accessing per-CPU variables
> > directly in assembly. The patch is included in this series to
> > make IRQ stack switching cleaner with SCS, and I've simply
> > rebased it. Patch 2 uses this functionality to clean up the stack
> > switching by moving duplicate code into a single function. On
> > RISC-V, the compiler uses the gp register for storing the current
> > shadow call stack pointer, which is incompatible with global
> > pointer relaxation. Patch 3 moves global pointer loading into a
> > macro that can be easily disabled with SCS. Patch 4 implements
> > SCS register loading and switching, and allows the feature to be
> > enabled, and patch 5 adds separate per-CPU IRQ shadow call stacks
> > when CONFIG_IRQ_STACKS is enabled.
> >
> > Note that this series requires Clang 17. Earlier Clang versions
> > support SCS on RISC-V, but use the x18 register instead of gp,
> > which isn't ideal. gcc has SCS support for arm64, but I'm not
> > aware of plans to support RISC-V. Once the Zicfiss extension is
> > ratified, it's probably preferable to use hardware-backed shadow
> > stacks instead of SCS on hardware that supports the extension,
> > and we may want to consider implementing CONFIG_DYNAMIC_SCS to
> > patch between the implementation at runtime (similarly to the
> > arm64 implementation, which switches to SCS when hardware PAC
> > support isn't available).
>
> I took this series for a spin on top of 6.5-rc6 with both LLVM 18 (built
> within the past couple of days) and LLVM 17.0.0-rc2 but it seems that
> the CFI_BACKWARDS LKDTM test does not pass with
> CONFIG_SHADOW_CALL_STACK=y.
>
> [ 73.324652] lkdtm: Performing direct entry CFI_BACKWARD
> [ 73.324900] lkdtm: Attempting unchecked stack return address redirection ...
> [ 73.325178] lkdtm: Eek: return address mismatch! 0000000000000002 != ffffffff80614982
> [ 73.325478] lkdtm: FAIL: stack return address manipulation failed!
>
> Does the test need to be adjusted or is there some other issue?

Does it pass without the series? I tried to write it to be
arch-agnostic, but I never tested it on RISC-V. It's very possible that
test needs adjusting for the architecture. Besides the label horrors,
the use of __builtin_frame_address may not work there either...

--
Kees Cook