Re: [PATCH v2] riscv: VMAP_STACK overflow detection thread-safe

[Sami Tolvanen <samitolvanen@google.com>]

Hi Deepak,

On Thu, Nov 24, 2022 at 01:48:45AM -0800, Deepak Gupta wrote:
> commit 31da94c25aea ("riscv: add VMAP_STACK overflow detection") added
> support for CONFIG_VMAP_STACK. If overflow is detected, CPU switches to
> `shadow_stack` temporarily before switching finally to per-cpu
> `overflow_stack`.
> 
> If two CPUs/harts are racing and end up in over flowing kernel stack, one
> or both will end up corrupting each other state because `shadow_stack` is
> not per-cpu. This patch optimizes per-cpu overflow stack switch by
> directly picking per-cpu `overflow_stack` and gets rid of `shadow_stack`.

Are you planning on resending this patch? I see it didn't gain much
traction last time, but this looks like a much cleaner solution for
selecting the overflow stack than having a `shadow_stack` and calling
to C to compute the per-CPU offset. The asm_per_cpu macro also would
come in handy when implementing CONFIG_SHADOW_CALL_STACK, which we'd
like to have on RISC-V too.

Sami