riscv: Implement syscall wrappers

Commit f0bddf50586d ("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
4378a7d4be30 ("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>

1 files changed, 6 insertions, 2 deletions

diff --git a/arch/riscv/kernel/syscall_table.c b/arch/riscv/kernel/syscall_table.c
index 44b1420a2270..dda913764903 100644
--- a/arch/riscv/kernel/syscall_table.c
+++ b/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>
 };