[PATCH v17 15/27] riscv/traps: Introduce software check exception and uprobe handling

[Deepak Gupta]

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.

To keep uprobes working, handle the uprobe event first before reporting
the CFI violation in software-check exception handler. Because when the
landing pad is activated, if the uprobe point is set at the lpad
instruction at the beginning of a function, the system triggers a software
-check exception instead of an ebreak exception due to the exception
priority, then uprobe can't work successfully.

Co-developed-by: Zong Li <zong.li at sifive.com>
Reviewed-by: Zong Li <zong.li at sifive.com>
Signed-off-by: Zong Li <zong.li at sifive.com>
Signed-off-by: Deepak Gupta <debug at rivosinc.com>
---
 arch/riscv/include/asm/asm-prototypes.h |  1 +
 arch/riscv/include/asm/entry-common.h   |  2 ++
 arch/riscv/kernel/entry.S               |  3 ++
 arch/riscv/kernel/traps.c               | 51 +++++++++++++++++++++++++++++++++
 4 files changed, 57 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/include/asm/entry-common.h b/arch/riscv/include/asm/entry-common.h
index b28ccc6cdeea..34ed149af5d1 100644
--- a/arch/riscv/include/asm/entry-common.h
+++ b/arch/riscv/include/asm/entry-common.h
@@ -40,4 +40,6 @@ static inline int handle_misaligned_store(struct pt_regs *regs)
 }
 #endif
 
+bool handle_user_cfi_violation(struct pt_regs *regs);
+
 #endif /* _ASM_RISCV_ENTRY_COMMON_H */
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index 978115567bca..8d25837a9384 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -474,6 +474,9 @@ 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 8ff8e8b36524..64388370e1ad 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -354,6 +354,57 @@ 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)
+{
+	unsigned long tval = csr_read(CSR_TVAL);
+	bool is_fcfi = (tval == CFI_TVAL_FCFI_CODE && cpu_supports_indirect_br_lp_instr());
+	bool is_bcfi = (tval == CFI_TVAL_BCFI_CODE && cpu_supports_shadow_stack());
+
+	/*
+	 * Handle uprobe event first. The probe point can be a valid target
+	 * of indirect jumps or calls, in this case, forward cfi violation
+	 * will be triggered instead of breakpoint exception.
+	 */
+	if (is_fcfi && probe_breakpoint_handler(regs))
+		return true;
+
+	if (is_fcfi || is_bcfi) {
+		do_trap_error(regs, SIGSEGV, SEGV_CPERR, regs->epc,
+			      "Oops - control flow violation");
+		return true;
+	}
+
+	return false;
+}
+
+/*
+ * 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)) {
+		irqentry_enter_from_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);
+
+		irqentry_exit_to_user_mode(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