linux-kernel - Re: [PATCH v2 0/6] riscv: SCS support

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Date:   Wed, 16 Aug 2023 09:42:31 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Kees Cook <keescook@...omium.org>, Guo Ren <guoren@...nel.org>,
        Deepak Gupta <debug@...osinc.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Fangrui Song <maskray@...gle.com>,
        linux-riscv@...ts.infradead.org, llvm@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/6] riscv: SCS support

Hi Sami,

On Tue, Aug 15, 2023 at 08:34:43PM +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. Patch 6 fixes the backward-
> edge CFI test in lkdtm for RISC-V.
> 
> 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 in QEMU with both LLVM 18.0.0 and
17.0.0-rc2 and the LKDTM test now passes with CONFIG_SHADOW_CALL_STACK=y
(and fails with LLVM 16.0.0, as CONFIG_SHADOW_CALL_STACK is not
selectable there).

Tested-by: Nathan Chancellor <nathan@...nel.org>

Cheers,
Nathan