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

[Nathan Chancellor]

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?

Cheers,
Nathan