riscv: Implement syscall wrappers

Commit f0bddf5058 ("riscv: entry: Convert to generic entry") moved
syscall handling to C code, which exposed function pointer type
mismatches that trip fine-grained forward-edge Control-Flow Integrity
(CFI) checks as syscall handlers are all called through the same
syscall_t pointer type. To fix the type mismatches, implement pt_regs
based syscall wrappers similarly to x86 and arm64.

This patch is based on arm64 syscall wrappers added in commit
4378a7d4be ("arm64: implement syscall wrappers"), where the main goal
was to minimize the risk of userspace-controlled values being used
under speculation. This may be a concern for riscv in future as well.

Following other architectures, the syscall wrappers generate three
functions for each syscall; __riscv_<compat_>sys_<name> takes a pt_regs
pointer and extracts arguments from registers, __se_<compat_>sys_<name>
is a sign-extension wrapper that casts the long arguments to the
correct types for the real syscall implementation, which is named
__do_<compat_>sys_<name>.

Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/r/20230710183544.999540-9-samitolvanen@google.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Kconfig

@@ -35,6 +35,7 @@ config RISCV
 	select ARCH_HAS_SET_MEMORY if MMU
 	select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
 	select ARCH_HAS_STRICT_MODULE_RWX if MMU && !XIP_KERNEL
+	select ARCH_HAS_SYSCALL_WRAPPER
 	select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
 	select ARCH_HAS_UBSAN_SANITIZE_ALL
 	select ARCH_HAS_VDSO_DATA

arch/riscv/include/asm/syscall.h

@@ -75,7 +75,7 @@ static inline int syscall_get_arch(struct task_struct *task)
 #endif
 }
 
-typedef long (*syscall_t)(ulong, ulong, ulong, ulong, ulong, ulong, ulong);
+typedef long (*syscall_t)(const struct pt_regs *);
 static inline void syscall_handler(struct pt_regs *regs, ulong syscall)
 {
 	syscall_t fn;
@@ -87,8 +87,7 @@ static inline void syscall_handler(struct pt_regs *regs, ulong syscall)
 #endif
 		fn = sys_call_table[syscall];
 
-	regs->a0 = fn(regs->orig_a0, regs->a1, regs->a2,
+	regs->a0 = fn(regs);
-		      regs->a3, regs->a4, regs->a5, regs->a6);
 }
 
 static inline bool arch_syscall_is_vdso_sigreturn(struct pt_regs *regs)

arch/riscv/include/asm/syscall_wrapper.h

@@ -0,0 +1,87 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * syscall_wrapper.h - riscv specific wrappers to syscall definitions
+ *
+ * Based on arch/arm64/include/syscall_wrapper.h
+ */
+
+#ifndef __ASM_SYSCALL_WRAPPER_H
+#define __ASM_SYSCALL_WRAPPER_H
+
+#include <asm/ptrace.h>
+
+asmlinkage long __riscv_sys_ni_syscall(const struct pt_regs *);
+
+#define SC_RISCV_REGS_TO_ARGS(x, ...)				\
+	__MAP(x,__SC_ARGS					\
+	      ,,regs->orig_a0,,regs->a1,,regs->a2		\
+	      ,,regs->a3,,regs->a4,,regs->a5,,regs->a6)
+
+#ifdef CONFIG_COMPAT
+
+#define COMPAT_SYSCALL_DEFINEx(x, name, ...)						\
+	asmlinkage long __riscv_compat_sys##name(const struct pt_regs *regs);		\
+	ALLOW_ERROR_INJECTION(__riscv_compat_sys##name, ERRNO);				\
+	static long __se_compat_sys##name(__MAP(x,__SC_LONG,__VA_ARGS__));		\
+	static inline long __do_compat_sys##name(__MAP(x,__SC_DECL,__VA_ARGS__));	\
+	asmlinkage long __riscv_compat_sys##name(const struct pt_regs *regs)		\
+	{										\
+		return __se_compat_sys##name(SC_RISCV_REGS_TO_ARGS(x,__VA_ARGS__));	\
+	}										\
+	static long __se_compat_sys##name(__MAP(x,__SC_LONG,__VA_ARGS__))		\
+	{										\
+		return __do_compat_sys##name(__MAP(x,__SC_DELOUSE,__VA_ARGS__));	\
+	}										\
+	static inline long __do_compat_sys##name(__MAP(x,__SC_DECL,__VA_ARGS__))
+
+#define COMPAT_SYSCALL_DEFINE0(sname)							\
+	asmlinkage long __riscv_compat_sys_##sname(const struct pt_regs *__unused);	\
+	ALLOW_ERROR_INJECTION(__riscv_compat_sys_##sname, ERRNO);			\
+	asmlinkage long __riscv_compat_sys_##sname(const struct pt_regs *__unused)
+
+#define COND_SYSCALL_COMPAT(name) 							\
+	asmlinkage long __weak __riscv_compat_sys_##name(const struct pt_regs *regs);	\
+	asmlinkage long __weak __riscv_compat_sys_##name(const struct pt_regs *regs)	\
+	{										\
+		return sys_ni_syscall();						\
+	}
+
+#define COMPAT_SYS_NI(name) \
+	SYSCALL_ALIAS(__riscv_compat_sys_##name, sys_ni_posix_timers);
+
+#endif /* CONFIG_COMPAT */
+
+#define __SYSCALL_DEFINEx(x, name, ...)						\
+	asmlinkage long __riscv_sys##name(const struct pt_regs *regs);		\
+	ALLOW_ERROR_INJECTION(__riscv_sys##name, ERRNO);			\
+	static long __se_sys##name(__MAP(x,__SC_LONG,__VA_ARGS__));		\
+	static inline long __do_sys##name(__MAP(x,__SC_DECL,__VA_ARGS__));	\
+	asmlinkage long __riscv_sys##name(const struct pt_regs *regs)		\
+	{									\
+		return __se_sys##name(SC_RISCV_REGS_TO_ARGS(x,__VA_ARGS__));	\
+	}									\
+	static long __se_sys##name(__MAP(x,__SC_LONG,__VA_ARGS__))		\
+	{									\
+		long ret = __do_sys##name(__MAP(x,__SC_CAST,__VA_ARGS__));	\
+		__MAP(x,__SC_TEST,__VA_ARGS__);					\
+		__PROTECT(x, ret,__MAP(x,__SC_ARGS,__VA_ARGS__));		\
+		return ret;							\
+	}									\
+	static inline long __do_sys##name(__MAP(x,__SC_DECL,__VA_ARGS__))
+
+#define SYSCALL_DEFINE0(sname)							\
+	SYSCALL_METADATA(_##sname, 0);						\
+	asmlinkage long __riscv_sys_##sname(const struct pt_regs *__unused);	\
+	ALLOW_ERROR_INJECTION(__riscv_sys_##sname, ERRNO);			\
+	asmlinkage long __riscv_sys_##sname(const struct pt_regs *__unused)
+
+#define COND_SYSCALL(name)							\
+	asmlinkage long __weak __riscv_sys_##name(const struct pt_regs *regs);	\
+	asmlinkage long __weak __riscv_sys_##name(const struct pt_regs *regs)	\
+	{									\
+		return sys_ni_syscall();					\
+	}
+
+#define SYS_NI(name) SYSCALL_ALIAS(__riscv_sys_##name, sys_ni_posix_timers);
+
+#endif /* __ASM_SYSCALL_WRAPPER_H */

arch/riscv/kernel/compat_syscall_table.c

@@ -9,11 +9,15 @@
 #include <asm/syscall.h>
 
 #undef __SYSCALL
-#define __SYSCALL(nr, call)      [nr] = (call),
+#define __SYSCALL(nr, call)	asmlinkage long __riscv_##call(const struct pt_regs *);
+#include <asm/unistd.h>
+
+#undef __SYSCALL
+#define __SYSCALL(nr, call)      [nr] = __riscv_##call,
 
 asmlinkage long compat_sys_rt_sigreturn(void);
 
 void * const compat_sys_call_table[__NR_syscalls] = {
-	[0 ... __NR_syscalls - 1] = sys_ni_syscall,
+	[0 ... __NR_syscalls - 1] = __riscv_sys_ni_syscall,
 #include <asm/unistd.h>
 };

arch/riscv/kernel/sys_riscv.c

@@ -335,3 +335,9 @@ SYSCALL_DEFINE5(riscv_hwprobe, struct riscv_hwprobe __user *, pairs,
 	return do_riscv_hwprobe(pairs, pair_count, cpu_count,
 				cpus, flags);
 }
+
+/* Not defined using SYSCALL_DEFINE0 to avoid error injection */
+asmlinkage long __riscv_sys_ni_syscall(const struct pt_regs *__unused)
+{
+	return -ENOSYS;
+}

arch/riscv/kernel/syscall_table.c

@@ -10,9 +10,13 @@
 #include <asm/syscall.h>
 
 #undef __SYSCALL
-#define __SYSCALL(nr, call)	[nr] = (call),
+#define __SYSCALL(nr, call)	asmlinkage long __riscv_##call(const struct pt_regs *);
+#include <asm/unistd.h>
+
+#undef __SYSCALL
+#define __SYSCALL(nr, call)	[nr] = __riscv_##call,
 
 void * const sys_call_table[__NR_syscalls] = {
-	[0 ... __NR_syscalls - 1] = sys_ni_syscall,
+	[0 ... __NR_syscalls - 1] = __riscv_sys_ni_syscall,
 #include <asm/unistd.h>
 };