[RFC PATCH v1 25/28] riscv/ptrace: riscv cfi status and state via ptrace and in core files

[debug at rivosinc.com]

From: Deepak Gupta <debug at rivosinc.com>

Expose a new register type NT_RISCV_USER_CFI for risc-v cfi status and
state. Intentionally both landing pad and shadow stack status and state
are rolled into cfi state. Creating two different NT_RISCV_USER_XXX would
not be useful and wastage of a note type. Enabling or disabling of feature
is not allowed via ptrace set interface. However setting `elp` state or
setting shadow stack pointer are allowed via ptrace set interface. It is
expected `gdb` might have use to fixup `elp` state or `shadow stack`
pointer.

Signed-off-by: Deepak Gupta <debug at rivosinc.com>
---
 arch/riscv/include/uapi/asm/ptrace.h | 18 ++++++
 arch/riscv/kernel/ptrace.c           | 83 ++++++++++++++++++++++++++++
 include/uapi/linux/elf.h             |  1 +
 3 files changed, 102 insertions(+)

diff --git a/arch/riscv/include/uapi/asm/ptrace.h b/arch/riscv/include/uapi/asm/ptrace.h
index a38268b19c3d..512be06a8661 100644
--- a/arch/riscv/include/uapi/asm/ptrace.h
+++ b/arch/riscv/include/uapi/asm/ptrace.h
@@ -127,6 +127,24 @@ struct __riscv_v_regset_state {
  */
 #define RISCV_MAX_VLENB (8192)
 
+struct __cfi_status {
+	/* indirect branch tracking state */
+	__u64 lp_en : 1;
+	__u64 lp_lock : 1;
+	__u64 elp_state : 1;
+
+	/* shadow stack status */
+	__u64 shstk_en : 1;
+	__u64 shstk_lock : 1;
+
+	__u64 rsvd : sizeof(__u64) - 5;
+};
+
+struct user_cfi_state {
+	struct __cfi_status	cfi_status;
+	__u64 shstk_ptr;
+};
+
 #endif /* __ASSEMBLY__ */
 
 #endif /* _UAPI_ASM_RISCV_PTRACE_H */
diff --git a/arch/riscv/kernel/ptrace.c b/arch/riscv/kernel/ptrace.c
index 2afe460de16a..8ddd529bef0b 100644
--- a/arch/riscv/kernel/ptrace.c
+++ b/arch/riscv/kernel/ptrace.c
@@ -19,6 +19,7 @@
 #include <linux/regset.h>
 #include <linux/sched.h>
 #include <linux/sched/task_stack.h>
+#include <asm/usercfi.h>
 
 enum riscv_regset {
 	REGSET_X,
@@ -28,6 +29,9 @@ enum riscv_regset {
 #ifdef CONFIG_RISCV_ISA_V
 	REGSET_V,
 #endif
+#ifdef CONFIG_RISCV_USER_CFI
+	REGSET_CFI,
+#endif
 };
 
 static int riscv_gpr_get(struct task_struct *target,
@@ -149,6 +153,75 @@ static int riscv_vr_set(struct task_struct *target,
 }
 #endif
 
+#ifdef CONFIG_RISCV_USER_CFI
+static int riscv_cfi_get(struct task_struct *target,
+			const struct user_regset *regset,
+			struct membuf to)
+{
+	struct user_cfi_state user_cfi;
+	struct pt_regs *regs;
+
+	regs = task_pt_regs(target);
+
+	user_cfi.cfi_status.lp_en = is_indir_lp_enabled(target);
+	user_cfi.cfi_status.lp_lock = is_indir_lp_locked(target);
+	user_cfi.cfi_status.elp_state = (regs->status & SR_ELP);
+
+	user_cfi.cfi_status.shstk_en = is_shstk_enabled(target);
+	user_cfi.cfi_status.shstk_lock = is_shstk_locked(target);
+	user_cfi.shstk_ptr = get_active_shstk(target);
+
+	return membuf_write(&to, &user_cfi, sizeof(user_cfi));
+}
+
+/*
+ * Does it make sense to allowing enable / disable of cfi via ptrace?
+ * Not allowing enable / disable / locking control via ptrace for now.
+ * Setting shadow stack pointer is allowed. GDB might use it to unwind or
+ * some other fixup. Similarly gdb might want to suppress elp and may want
+ * to reset elp state.
+ */
+static int riscv_cfi_set(struct task_struct *target,
+			const struct user_regset *regset,
+			unsigned int pos, unsigned int count,
+			const void *kbuf, const void __user *ubuf)
+{
+	int ret;
+	struct user_cfi_state user_cfi;
+	struct pt_regs *regs;
+
+	regs = task_pt_regs(target);
+
+	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &user_cfi, 0, -1);
+	if (ret)
+		return ret;
+
+	/*
+	 * Not allowing enabling or locking shadow stack or landing pad
+	 * There is no disabling of shadow stack or landing pad via ptrace
+	 * rsvd field should be set to zero so that if those fields are needed in future
+	 */
+	if (user_cfi.cfi_status.lp_en || user_cfi.cfi_status.lp_lock ||
+		user_cfi.cfi_status.shstk_en || user_cfi.cfi_status.shstk_lock ||
+		!user_cfi.cfi_status.rsvd)
+		return -EINVAL;
+
+	/* If lpad is enabled on target and ptrace requests to set / clear elp, do that */
+	if (is_indir_lp_enabled(target)) {
+		if (user_cfi.cfi_status.elp_state) /* set elp state */
+			regs->status |= SR_ELP;
+		else
+			regs->status &= ~SR_ELP; /* clear elp state */
+	}
+
+	/* If shadow stack enabled on target, set new shadow stack pointer */
+	if (is_shstk_enabled(target))
+		set_active_shstk(target, user_cfi.shstk_ptr);
+
+	return 0;
+}
+#endif
+
 static const struct user_regset riscv_user_regset[] = {
 	[REGSET_X] = {
 		.core_note_type = NT_PRSTATUS,
@@ -179,6 +252,16 @@ static const struct user_regset riscv_user_regset[] = {
 		.set = riscv_vr_set,
 	},
 #endif
+#ifdef CONFIG_RISCV_USER_CFI
+	[REGSET_CFI] = {
+		.core_note_type = NT_RISCV_USER_CFI,
+		.align = sizeof(__u64),
+		.n = sizeof(struct user_cfi_state) / sizeof(__u64),
+		.size = sizeof(__u64),
+		.regset_get = riscv_cfi_get,
+		.set = riscv_cfi_set,
+	}
+#endif
 };
 
 static const struct user_regset_view riscv_user_native_view = {
diff --git a/include/uapi/linux/elf.h b/include/uapi/linux/elf.h
index 9417309b7230..f60b2de66b1c 100644
--- a/include/uapi/linux/elf.h
+++ b/include/uapi/linux/elf.h
@@ -447,6 +447,7 @@ typedef struct elf64_shdr {
 #define NT_MIPS_MSA	0x802		/* MIPS SIMD registers */
 #define NT_RISCV_CSR	0x900		/* RISC-V Control and Status Registers */
 #define NT_RISCV_VECTOR	0x901		/* RISC-V vector registers */
+#define NT_RISCV_USER_CFI	0x902		/* RISC-V shadow stack state */
 #define NT_LOONGARCH_CPUCFG	0xa00	/* LoongArch CPU config registers */
 #define NT_LOONGARCH_CSR	0xa01	/* LoongArch control and status registers */
 #define NT_LOONGARCH_LSX	0xa02	/* LoongArch Loongson SIMD Extension registers */
-- 
2.43.0