[RFC PATCH v1 21/28] riscv/traps: Introduce software check exception

[debug]

From: Deepak Gupta <debug@xxxxxxxxxxxx>

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

Additionally it implements a cfi violation handler which checks for code in xtval
If xtval=2, it means that sw check exception happened because of an indirect
branch not landing on 4 byte aligned PC or not landing on `lpad` instruction or
label value embedded in `lpad` not matching label value setup in `x7`.
If xtval=3, it means that sw check exception happened because of mismatch between
link register (x1 or x5) and top of shadow stack (on execution of `sspopchk`)

In case of cfi violation, SIGSEGV is raised with code=SEGV_CPERR. SEGV_CPERR was
introduced by x86 shadow stack patches.

Signed-off-by: Deepak Gupta <debug@xxxxxxxxxxxx>
---
 arch/riscv/include/asm/asm-prototypes.h |  1 +
 arch/riscv/kernel/entry.S               |  3 ++
 arch/riscv/kernel/traps.c               | 38 +++++++++++++++++++++++++
 3 files changed, 42 insertions(+)

diff --git a/arch/riscv/include/asm/asm-prototypes.h b/arch/riscv/include/asm/asm-prototypes.h
index 36b955c762ba..4ba8aea58dd0 100644
--- a/arch/riscv/include/asm/asm-prototypes.h
+++ b/arch/riscv/include/asm/asm-prototypes.h
@@ -24,6 +24,7 @@ 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 410659e2eadb..56dfe04094c1 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -369,6 +369,9 @@ 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 a1b9be3c4332..9fba263428a1 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -339,6 +339,44 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
 
 }
 
+#define CFI_TVAL_FCFI_CODE	2
+#define CFI_TVAL_BCFI_CODE	3
+/* handle cfi violations */
+bool handle_user_cfi_violation(struct pt_regs *regs)
+{
+	bool ret = false;
+	unsigned long tval = csr_read(CSR_TVAL);
+
+	if (((tval == CFI_TVAL_FCFI_CODE) && cpu_supports_indirect_br_lp_instr()) ||
+		((tval == CFI_TVAL_BCFI_CODE) && cpu_supports_shadow_stack())) {
+		do_trap_error(regs, SIGSEGV, SEGV_CPERR, regs->epc,
+					  "Oops - control flow violation");
+		ret = true;
+	}
+
+	return ret;
+}
+/*
+ * 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)) {
+		/* not a cfi violation, then merge into flow of unknown trap handler */
+		if (!handle_user_cfi_violation(regs))
+			do_trap_unknown(regs);
+	} else {
+		/* sw check exception coming from kernel is a bug in kernel */
+		die(regs, "Kernel BUG");
+	}
+}
+
 #ifdef CONFIG_MMU
 asmlinkage __visible noinstr void do_page_fault(struct pt_regs *regs)
 {
-- 
2.43.0