[PATCH v3 21/29] riscv/traps: Introduce software check exception

Deepak Gupta
2024-04-03T23:43:57Z

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@rivosinc.com>
---
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 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 7245a0ea25c1..f97af4ff5237 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -374,6 +374,9 @@ 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)
{
--
2.43.2