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

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Message-ID: <20230720001852.GA572618@google.com>
Date:   Wed, 19 Jul 2023 17:18:52 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Deepak Gupta <debug@...osinc.com>
Cc:     palmer@...belt.com, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org, Guo Ren <guoren@...nel.org>,
        Jisheng Zhang <jszhang@...nel.org>
Subject: Re: [PATCH v2] riscv: VMAP_STACK overflow detection thread-safe

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