riscv: process: fix kernel info leakage

thread_struct's s[12] may contain random kernel memory content, which
may be finally leaked to userspace. This is a security hole. Fix it
by clearing the s[12] array in thread_struct when fork.

As for kthread case, it's better to clear the s[12] array as well.

Fixes: 7db91e5 ("RISC-V: Task implementation")
Signed-off-by: Jisheng Zhang <[email protected]>
Tested-by: Guo Ren <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Guo Ren <[email protected]>
Link: https://lore.kernel.org/r/CAJF2gTSdVyAaM12T%[email protected]/
Signed-off-by: Palmer Dabbelt <[email protected]>

Author: xhackerustc

arch/riscv/kernel/process.c

@@ -164,6 +164,8 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 	unsigned long tls = args->tls;
 	struct pt_regs *childregs = task_pt_regs(p);
 
+	memset(&p->thread.s, 0, sizeof(p->thread.s));
+
 	/* p->thread holds context to be restored by __switch_to() */
 	if (unlikely(args->fn)) {
 		/* Kernel thread */