riscv: Support RANDOMIZE_KSTACK_OFFSET

Inspired from arm64's implement -- commit 70918779aec9
("arm64: entry: Enable random_kstack_offset support")

Add support of kernel stack offset randomization while handling syscall,
the offset is defaultly limited by KSTACK_OFFSET_MAX() (i.e. 10 bits).

In order to avoid trigger stack canaries (due to __builtin_alloca) and
slowing down the entry path, use __no_stack_protector attribute to
disable stack protector for do_trap_ecall_u() at the function level.

Acked-by: Palmer Dabbelt <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Signed-off-by: Song Shuai <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Palmer Dabbelt <[e-mail]>

arch/riscv/Kconfig

···
 	select HAVE_ARCH_KGDB_QXFER_PKT
 	select HAVE_ARCH_MMAP_RND_BITS if MMU
 	select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
+	select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
 	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ARCH_THREAD_STRUCT_WHITELIST
 	select HAVE_ARCH_TRACEHOOK

arch/riscv/kernel/traps.c

···
 #include <linux/cpu.h>
 #include <linux/kernel.h>
 #include <linux/init.h>
+#include <linux/randomize_kstack.h>
 #include <linux/sched.h>
 #include <linux/sched/debug.h>
 #include <linux/sched/signal.h>
···
 	}
 }
 
-asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
+asmlinkage __visible __trap_section  __no_stack_protector
+void do_trap_ecall_u(struct pt_regs *regs)
 {
 	if (user_mode(regs)) {
 		long syscall = regs->a7;
···
 
 		syscall = syscall_enter_from_user_mode(regs, syscall);
 
+		add_random_kstack_offset();
+
 		if (syscall >= 0 && syscall < NR_syscalls)
 			syscall_handler(regs, syscall);
 		else if (syscall != -1)
 			regs->a0 = -ENOSYS;
+		/*
+		 * Ultimately, this value will get limited by KSTACK_OFFSET_MAX(),
+		 * so the maximum stack offset is 1k bytes (10 bits).
+		 *
+		 * The actual entropy will be further reduced by the compiler when
+		 * applying stack alignment constraints: 16-byte (i.e. 4-bit) aligned
+		 * for RV32I or RV64I.
+		 *
+		 * The resulting 6 bits of entropy is seen in SP[9:4].
+		 */
+		choose_random_kstack_offset(get_random_u16());
 
 		syscall_exit_to_user_mode(regs);
 	} else {