linux-kernel - [PATCH v9 21/26] riscv: enable kernel access to shadow stack memory via FWFT sbi call

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Message-Id: <20250204-v5_user_cfi_series-v9-21-b37a49c5205c@rivosinc.com>
Date: Tue, 04 Feb 2025 17:22:08 -0800
From: Deepak Gupta <debug@...osinc.com>
To: 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, Deepak Gupta <debug@...osinc.com>
Subject: [PATCH v9 21/26] riscv: enable kernel access to shadow stack
 memory via FWFT sbi call

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.

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.

Signed-off-by: Deepak Gupta <debug@...osinc.com>
---
 arch/riscv/kernel/asm-offsets.c |  4 ++++
 arch/riscv/kernel/head.S        | 12 ++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index 0c188aaf3925..21f99d5757b6 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -515,4 +515,8 @@ void asm_offsets(void)
 	DEFINE(FREGS_A6,	    offsetof(struct __arch_ftrace_regs, a6));
 	DEFINE(FREGS_A7,	    offsetof(struct __arch_ftrace_regs, a7));
 #endif
+	DEFINE(SBI_EXT_FWFT, SBI_EXT_FWFT);
+	DEFINE(SBI_EXT_FWFT_SET, SBI_EXT_FWFT_SET);
+	DEFINE(SBI_FWFT_SHADOW_STACK, SBI_FWFT_SHADOW_STACK);
+	DEFINE(SBI_FWFT_SET_FLAG_LOCK, SBI_FWFT_SET_FLAG_LOCK);
 }
diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index 356d5397b2a2..6244408ca917 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -164,6 +164,12 @@ secondary_start_sbi:
 	call relocate_enable_mmu
 #endif
 	call .Lsetup_trap_vector
+	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
 	scs_load_current
 	call smp_callin
 #endif /* CONFIG_SMP */
@@ -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
 	scs_load_current

 #ifdef CONFIG_KASAN

-- 
2.34.1