riscv: Randomize lower bits of stack address

Implement arch_align_stack() to randomize the lower bits
of the stack address.
Signed-off-by: Yunhui Cui <cuiyunhui@bytedance.com>
Link: https://lore.kernel.org/r/20240625030502.68988-1-cuiyunhui@bytedance.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/asm/exec.h

+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef __ASM_EXEC_H
+#define __ASM_EXEC_H
+
+extern unsigned long arch_align_stack(unsigned long sp);
+
+#endif	/* __ASM_EXEC_H */

arch/riscv/kernel/process.c

@@ -15,6 +15,7 @@
 #include <linux/tick.h>
 #include <linux/ptrace.h>
 #include <linux/uaccess.h>
+#include <linux/personality.h>
 
 #include <asm/unistd.h>
 #include <asm/processor.h>
@@ -26,6 +27,7 @@
 #include <asm/cpuidle.h>
 #include <asm/vector.h>
 #include <asm/cpufeature.h>
+#include <asm/exec.h>
 
 #if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_STACKPROTECTOR_PER_TASK)
 #include <linux/stackprotector.h>
@@ -99,6 +101,13 @@ void show_regs(struct pt_regs *regs)
 		dump_backtrace(regs, NULL, KERN_DEFAULT);
 }
 
+unsigned long arch_align_stack(unsigned long sp)
+{
+	if (!(current->personality & ADDR_NO_RANDOMIZE) && randomize_va_space)
+		sp -= get_random_u32_below(PAGE_SIZE);
+	return sp & ~0xf;
+}
+
 #ifdef CONFIG_COMPAT
 static bool compat_mode_supported __read_mostly;