linux-kernel - [RFC PATCH 10/12] riscv/traps: Introduce software check exception

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Date: Mon,  8 Apr 2024 23:10:41 -0700
From: Deepak Gupta <debug@...osinc.com>
To: linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev
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Subject: [RFC PATCH 10/12] riscv/traps: Introduce software check exception

zicfiss / zicfilp introduces a new exception to priv isa `software check
exception` with cause code = 18. This patch implements software check
exception.

If sw check exception was triggered while in usermode, unknown trap is
triggered for usermode. If sw check exception was triggered for kernel
mode, kernel dies.

Signed-off-by: Deepak Gupta <debug@...osinc.com>
---
 arch/riscv/include/asm/asm-prototypes.h |  1 +
 arch/riscv/kernel/entry.S               |  3 +++
 arch/riscv/kernel/traps.c               | 20 ++++++++++++++++++++
 3 files changed, 24 insertions(+)

diff --git a/arch/riscv/include/asm/asm-prototypes.h b/arch/riscv/include/asm/asm-prototypes.h
index cd627ec289f1..5a27cefd7805 100644
--- a/arch/riscv/include/asm/asm-prototypes.h
+++ b/arch/riscv/include/asm/asm-prototypes.h
@@ -51,6 +51,7 @@ DECLARE_DO_ERROR_INFO(do_trap_ecall_u);
 DECLARE_DO_ERROR_INFO(do_trap_ecall_s);
 DECLARE_DO_ERROR_INFO(do_trap_ecall_m);
 DECLARE_DO_ERROR_INFO(do_trap_break);
+DECLARE_DO_ERROR_INFO(do_trap_software_check);
 
 asmlinkage void handle_bad_stack(struct pt_regs *regs);
 asmlinkage void do_page_fault(struct pt_regs *regs);
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index 0262b46ab064..89aeae803702 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -353,6 +353,9 @@ SYM_DATA_START_LOCAL(excp_vect_table)
 	RISCV_PTR do_page_fault   /* load page fault */
 	RISCV_PTR do_trap_unknown
 	RISCV_PTR do_page_fault   /* store page fault */
+	RISCV_PTR do_trap_unknown /* cause=16 */
+	RISCV_PTR do_trap_unknown /* cause=17 */
+	RISCV_PTR do_trap_software_check /* cause=18 is sw check exception */
 SYM_DATA_END_LABEL(excp_vect_table, SYM_L_LOCAL, excp_vect_table_end)
 
 #ifndef CONFIG_MMU
diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index 05a16b1f0aee..b464355f62b2 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -354,6 +354,26 @@ void do_trap_ecall_u(struct pt_regs *regs)
 
 }
 
+/*
+ * software check exception is defined with risc-v cfi spec. Software check
+ * exception is raised when:-
+ * a) An indirect branch doesn't land on 4 byte aligned PC or `lpad`
+ *    instruction or `label` value programmed in `lpad` instr doesn't
+ *    match with value setup in `x7`. reported code in `xtval` is 2.
+ * b) `sspopchk` instruction finds a mismatch between top of shadow stack (ssp)
+ *    and x1/x5. reported code in `xtval` is 3.
+ */
+asmlinkage __visible __trap_section void do_trap_software_check(struct pt_regs *regs)
+{
+	if (user_mode(regs)) {
+		/* deliver unknown trap to usermode */
+		do_trap_unknown(regs);
+	} else {
+		/* sw check exception coming from kernel is a bug in kernel, die */
+		die(regs, "Kernel BUG");
+	}
+}
+
 #ifdef CONFIG_MMU
 asmlinkage __visible noinstr void do_page_fault(struct pt_regs *regs)
 {
-- 
2.43.2