linux-kernel - Re: [PATCH v12 22/28] riscv: enable kernel access to shadow stack memory via FWFT sbi call

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Message-Id: <D8LFQYX4EHF8.2AJ01XL34WK0W@ventanamicro.com>
Date: Thu, 20 Mar 2025 23:10:06 +0100
From: Radim Krčmář <rkrcmar@...tanamicro.com>
To: "Deepak Gupta" <debug@...osinc.com>, "Thomas Gleixner"
 <tglx@...utronix.de>, "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov"
 <bp@...en8.de>, "Dave Hansen" <dave.hansen@...ux.intel.com>,
 <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, "Andrew Morton"
 <akpm@...ux-foundation.org>, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 "Vlastimil Babka" <vbabka@...e.cz>, "Lorenzo Stoakes"
 <lorenzo.stoakes@...cle.com>, "Paul Walmsley" <paul.walmsley@...ive.com>,
 "Palmer Dabbelt" <palmer@...belt.com>, "Albert Ou" <aou@...s.berkeley.edu>,
 "Conor Dooley" <conor@...nel.org>, "Rob Herring" <robh@...nel.org>,
 "Krzysztof Kozlowski" <krzk+dt@...nel.org>, "Arnd Bergmann"
 <arnd@...db.de>, "Christian Brauner" <brauner@...nel.org>, "Peter Zijlstra"
 <peterz@...radead.org>, "Oleg Nesterov" <oleg@...hat.com>, "Eric Biederman"
 <ebiederm@...ssion.com>, "Kees Cook" <kees@...nel.org>, "Jonathan Corbet"
 <corbet@....net>, "Shuah Khan" <shuah@...nel.org>, "Jann Horn"
 <jannh@...gle.com>, "Conor Dooley" <conor+dt@...nel.org>
Cc: <linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
 <linux-mm@...ck.org>, <linux-riscv@...ts.infradead.org>,
 <devicetree@...r.kernel.org>, <linux-arch@...r.kernel.org>,
 <linux-doc@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
 <alistair.francis@....com>, <richard.henderson@...aro.org>,
 <jim.shu@...ive.com>, <andybnac@...il.com>, <kito.cheng@...ive.com>,
 <charlie@...osinc.com>, <atishp@...osinc.com>, <evan@...osinc.com>,
 <cleger@...osinc.com>, <alexghiti@...osinc.com>, <samitolvanen@...gle.com>,
 <broonie@...nel.org>, <rick.p.edgecombe@...el.com>, "Zong Li"
 <zong.li@...ive.com>, "linux-riscv"
 <linux-riscv-bounces@...ts.infradead.org>
Subject: Re: [PATCH v12 22/28] riscv: enable kernel access to shadow stack
 memory via FWFT sbi call

2025-03-14T14:39:41-07:00, Deepak Gupta <debug@...osinc.com>:
> Kernel will have to perform shadow stack operations on user shadow stack.
> Like during signal delivery and sigreturn, shadow stack token must be
> created and validated respectively. Thus shadow stack access for kernel
> must be enabled.

Why can't kernel access the user shadow stack through an aliased WR
mapping?

> In future when kernel shadow stacks are enabled for linux kernel, it must
> be enabled as early as possible for better coverage and prevent imbalance
> between regular stack and shadow stack. After `relocate_enable_mmu` has
> been done, this is as early as possible it can enabled.
>
> Reviewed-by: Zong Li <zong.li@...ive.com>
> Signed-off-by: Deepak Gupta <debug@...osinc.com>
> ---
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> @@ -320,6 +326,12 @@ SYM_CODE_START(_start_kernel)
>  	la tp, init_task
>  	la sp, init_thread_union + THREAD_SIZE
>  	addi sp, sp, -PT_SIZE_ON_STACK
> +	li a7, SBI_EXT_FWFT
> +	li a6, SBI_EXT_FWFT_SET
> +	li a0, SBI_FWFT_SHADOW_STACK
> +	li a1, 1 /* enable supervisor to access shadow stack access */
> +	li a2, SBI_FWFT_SET_FLAG_LOCK
> +	ecall

I think the ecall can fail even on machines that have Zicfiss, so it
would be good to disable user shadow stack if that happens.