[PATCH v1 RFC Zisslpcfi 14/20] riscv: audit mode for cfi violations

Deepak Gupta
2023-02-13T04:55:21Z

Adding an audit mode per task which suppresses cfi violations reported
as illegal instruction exception.

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
---
arch/riscv/include/asm/processor.h | 3 ++-
arch/riscv/kernel/process.c | 2 ++
arch/riscv/kernel/traps.c | 7 ++++++-
3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index f065309927b1..39c36f739ebb 100644
--- a/arch/riscv/include/asm/processor.h
+++ b/arch/riscv/include/asm/processor.h
@@ -45,7 +45,8 @@ struct thread_struct {
struct cfi_status {
unsigned int ufcfi_en : 1; /* Enable for forward cfi. Note that ELP goes in sstatus */
unsigned int ubcfi_en : 1; /* Enable for backward cfi. */
- unsigned int rsvd1 : 30;
+ unsigned int audit_mode : 1;
+ unsigned int rsvd1 : 29;
unsigned int lp_label; /* saved label value (25bit) */
long user_shdw_stk; /* Current user shadow stack pointer */
long shdw_stk_base; /* Base address of shadow stack */
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index db676262e61e..bfd8511914d9 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -246,11 +246,13 @@ int arch_elf_setup_cfi_state(const struct arch_elf_state *state)

info->user_cfi_state.user_shdw_stk = (shadow_stack_base + shadow_stk_size);
info->user_cfi_state.shdw_stk_base = shadow_stack_base;
+ info->user_cfi_state.audit_mode = 1;
}
/* setup forward cfi state */
if (arch_supports_indirect_br_lp_instr() && (state->flags & RISCV_ELF_FCFI)) {
info->user_cfi_state.ufcfi_en = 1;
info->user_cfi_state.lp_label = 0;
+ info->user_cfi_state.audit_mode = 1;
}

return ret;
diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index a292699f4f25..1901a8b73de5 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -165,6 +165,7 @@ int handle_illegal_instruction(struct pt_regs *regs)
if (arch_supports_indirect_br_lp_instr() &&
#ifdef CONFIG_USER_INDIRECT_BR_LP
info->user_cfi_state.ufcfi_en &&
+ info->user_cfi_state.audit_mode &&
#endif
(regs->status & SR_ELP)) {
pr_warn("cfi violation (elp): comm = %s, task = %p\n", task->comm, task);
@@ -172,7 +173,11 @@ int handle_illegal_instruction(struct pt_regs *regs)
return 0;
}
/* if faulting opcode is sscheckra/lpcll/lpcml/lpcll, advance PC and resume */
- if (is_cfi_violation_insn(insn)) {
+ if (is_cfi_violation_insn(insn)
+#if defined(CONFIG_USER_SHADOW_STACK) || defined(CONFIG_USER_INDIRECT_BR_LP)
+ && info->user_cfi_state.audit_mode
+#endif
+ ) {
/* no compressed form for zisslpcfi instructions */
regs->epc += 4;
return 0;
--
2.25.1