riscv: add kernel command line option to opt out of user CFI

Add a kernel command line option to disable part or all
of user CFI.  User backward CFI and forward CFI can be controlled
independently.  The kernel command line parameter "riscv_nousercfi" can
take the following values:
 - "all" : Disable forward and backward cfi both
 - "bcfi" : Disable backward cfi
 - "fcfi" : Disable forward cfi

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
Tested-by: Andreas Korb <andreas.korb@aisec.fraunhofer.de> # QEMU, custom CVA6
Tested-by: Valentin Haudiquet <valentin.haudiquet@canonical.com>
Link: https://patch.msgid.link/20251112-v5_user_cfi_series-v23-21-b55691eacf4f@rivosinc.com
[pjw@kernel.org: fixed warnings from checkpatch; cleaned up patch description, doc, printk text]
Signed-off-by: Paul Walmsley <pjw@kernel.org>

Author: deepak0414

Documentation/admin-guide/kernel-parameters.txt

@@ -6606,6 +6606,14 @@ Kernel parameters
 			replacement properties are not found. See the Kconfig
 			entry for RISCV_ISA_FALLBACK.
 
+	riscv_nousercfi=
+		all	Disable user CFI ABI to userspace even if cpu extension
+			are available.
+		bcfi	Disable user backward CFI ABI to userspace even if
+			the shadow stack extension is available.
+		fcfi	Disable user forward CFI ABI to userspace even if the
+			landing pad extension is available.
+
 	ro		[KNL] Mount root device read-only on boot
 
 	rodata=		[KNL,EARLY]

arch/riscv/include/asm/usercfi.h

@@ -5,6 +5,10 @@
 #ifndef _ASM_RISCV_USERCFI_H
 #define _ASM_RISCV_USERCFI_H
 
+#define CMDLINE_DISABLE_RISCV_USERCFI_FCFI	1
+#define CMDLINE_DISABLE_RISCV_USERCFI_BCFI	2
+#define CMDLINE_DISABLE_RISCV_USERCFI		3
+
 #ifndef __ASSEMBLER__
 #include <linux/types.h>
 #include <linux/prctl.h>
@@ -13,6 +17,8 @@
 struct task_struct;
 struct kernel_clone_args;
 
+extern unsigned long riscv_nousercfi;
+
 #ifdef CONFIG_RISCV_USER_CFI
 struct cfi_state {
 	unsigned long ubcfi_en : 1; /* Enable for backward cfi. */
@@ -83,6 +89,9 @@ void set_indir_lp_lock(struct task_struct *task);
 
 #endif /* CONFIG_RISCV_USER_CFI */
 
+bool is_user_shstk_enabled(void);
+bool is_user_lpad_enabled(void);
+
 #endif /* __ASSEMBLER__ */
 
 #endif /* _ASM_RISCV_USERCFI_H */

arch/riscv/kernel/cpufeature.c

@@ -28,6 +28,7 @@
 #include <asm/vector.h>
 #include <asm/vendor_extensions.h>
 #include <asm/vendor_extensions/thead.h>
+#include <asm/usercfi.h>
 
 #define NUM_ALPHA_EXTS ('z' - 'a' + 1)
 
@@ -299,7 +300,8 @@ static int riscv_ext_svadu_validate(const struct riscv_isa_ext_data *data,
 static int riscv_cfilp_validate(const struct riscv_isa_ext_data *data,
 				const unsigned long *isa_bitmap)
 {
-	if (!IS_ENABLED(CONFIG_RISCV_USER_CFI))
+	if (!IS_ENABLED(CONFIG_RISCV_USER_CFI) ||
+	    (riscv_nousercfi & CMDLINE_DISABLE_RISCV_USERCFI_FCFI))
 		return -EINVAL;
 
 	return 0;
@@ -308,7 +310,8 @@ static int riscv_cfilp_validate(const struct riscv_isa_ext_data *data,
 static int riscv_cfiss_validate(const struct riscv_isa_ext_data *data,
 				const unsigned long *isa_bitmap)
 {
-	if (!IS_ENABLED(CONFIG_RISCV_USER_CFI))
+	if (!IS_ENABLED(CONFIG_RISCV_USER_CFI) ||
+	    (riscv_nousercfi & CMDLINE_DISABLE_RISCV_USERCFI_BCFI))
 		return -EINVAL;
 
 	return 0;

arch/riscv/kernel/usercfi.c

@@ -17,6 +17,8 @@
 #include <asm/csr.h>
 #include <asm/usercfi.h>
 
+unsigned long riscv_nousercfi __read_mostly;
+
 #define SHSTK_ENTRY_SIZE sizeof(void *)
 
 bool is_shstk_enabled(struct task_struct *task)
@@ -59,7 +61,7 @@ unsigned long get_active_shstk(struct task_struct *task)
 
 void set_shstk_status(struct task_struct *task, bool enable)
 {
-	if (!cpu_supports_shadow_stack())
+	if (!is_user_shstk_enabled())
 		return;
 
 	task->thread_info.user_cfi_state.ubcfi_en = enable ? 1 : 0;
@@ -89,7 +91,7 @@ bool is_indir_lp_locked(struct task_struct *task)
 
 void set_indir_lp_status(struct task_struct *task, bool enable)
 {
-	if (!cpu_supports_indirect_br_lp_instr())
+	if (!is_user_lpad_enabled())
 		return;
 
 	task->thread_info.user_cfi_state.ufcfi_en = enable ? 1 : 0;
@@ -257,7 +259,7 @@ SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsi
 	bool set_tok = flags & SHADOW_STACK_SET_TOKEN;
 	unsigned long aligned_size = 0;
 
-	if (!cpu_supports_shadow_stack())
+	if (!is_user_shstk_enabled())
 		return -EOPNOTSUPP;
 
 	/* Anything other than set token should result in invalid param */
@@ -304,7 +306,7 @@ unsigned long shstk_alloc_thread_stack(struct task_struct *tsk,
 	unsigned long addr, size;
 
 	/* If shadow stack is not supported, return 0 */
-	if (!cpu_supports_shadow_stack())
+	if (!is_user_shstk_enabled())
 		return 0;
 
 	/*
@@ -350,7 +352,7 @@ void shstk_release(struct task_struct *tsk)
 {
 	unsigned long base = 0, size = 0;
 	/* If shadow stack is not supported or not enabled, nothing to release */
-	if (!cpu_supports_shadow_stack() || !is_shstk_enabled(tsk))
+	if (!is_user_shstk_enabled() || !is_shstk_enabled(tsk))
 		return;
 
 	/*
@@ -379,7 +381,7 @@ int arch_get_shadow_stack_status(struct task_struct *t, unsigned long __user *st
 {
 	unsigned long bcfi_status = 0;
 
-	if (!cpu_supports_shadow_stack())
+	if (!is_user_shstk_enabled())
 		return -EINVAL;
 
 	/* this means shadow stack is enabled on the task */
@@ -393,7 +395,7 @@ int arch_set_shadow_stack_status(struct task_struct *t, unsigned long status)
 	unsigned long size = 0, addr = 0;
 	bool enable_shstk = false;
 
-	if (!cpu_supports_shadow_stack())
+	if (!is_user_shstk_enabled())
 		return -EINVAL;
 
 	/* Reject unknown flags */
@@ -446,7 +448,7 @@ int arch_lock_shadow_stack_status(struct task_struct *task,
 				  unsigned long arg)
 {
 	/* If shtstk not supported or not enabled on task, nothing to lock here */
-	if (!cpu_supports_shadow_stack() ||
+	if (!is_user_shstk_enabled() ||
 	    !is_shstk_enabled(task) || arg != 0)
 		return -EINVAL;
 
@@ -459,7 +461,7 @@ int arch_get_indir_br_lp_status(struct task_struct *t, unsigned long __user *sta
 {
 	unsigned long fcfi_status = 0;
 
-	if (!cpu_supports_indirect_br_lp_instr())
+	if (!is_user_lpad_enabled())
 		return -EINVAL;
 
 	/* indirect branch tracking is enabled on the task or not */
@@ -472,7 +474,7 @@ int arch_set_indir_br_lp_status(struct task_struct *t, unsigned long status)
 {
 	bool enable_indir_lp = false;
 
-	if (!cpu_supports_indirect_br_lp_instr())
+	if (!is_user_lpad_enabled())
 		return -EINVAL;
 
 	/* indirect branch tracking is locked and further can't be modified by user */
@@ -496,11 +498,45 @@ int arch_lock_indir_br_lp_status(struct task_struct *task,
 	 * If indirect branch tracking is not supported or not enabled on task,
 	 * nothing to lock here
 	 */
-	if (!cpu_supports_indirect_br_lp_instr() ||
+	if (!is_user_lpad_enabled() ||
 	    !is_indir_lp_enabled(task) || arg != 0)
 		return -EINVAL;
 
 	set_indir_lp_lock(task);
 
 	return 0;
 }
+
+bool is_user_shstk_enabled(void)
+{
+	return (cpu_supports_shadow_stack() &&
+		!(riscv_nousercfi & CMDLINE_DISABLE_RISCV_USERCFI_BCFI));
+}
+
+bool is_user_lpad_enabled(void)
+{
+	return (cpu_supports_indirect_br_lp_instr() &&
+		!(riscv_nousercfi & CMDLINE_DISABLE_RISCV_USERCFI_FCFI));
+}
+
+static int __init setup_global_riscv_enable(char *str)
+{
+	if (strcmp(str, "all") == 0)
+		riscv_nousercfi = CMDLINE_DISABLE_RISCV_USERCFI;
+
+	if (strcmp(str, "fcfi") == 0)
+		riscv_nousercfi |= CMDLINE_DISABLE_RISCV_USERCFI_FCFI;
+
+	if (strcmp(str, "bcfi") == 0)
+		riscv_nousercfi |= CMDLINE_DISABLE_RISCV_USERCFI_BCFI;
+
+	if (riscv_nousercfi)
+		pr_info("RISC-V user CFI disabled via cmdline - shadow stack status : %s, landing pad status : %s\n",
+			(riscv_nousercfi & CMDLINE_DISABLE_RISCV_USERCFI_BCFI) ? "disabled" :
+			"enabled", (riscv_nousercfi & CMDLINE_DISABLE_RISCV_USERCFI_FCFI) ?
+			"disabled" : "enabled");
+
+	return 1;
+}
+
+__setup("riscv_nousercfi=", setup_global_riscv_enable);