LoongArch: Add spectre boundry for syscall dispatch table

[ Upstream commit 0c965d2 ]

The LoongArch syscall number is directly controlled by userspace, but
does not have a array_index_nospec() boundry to prevent access past the
syscall function pointer tables.

Cc: stable@vger.kernel.org
Assisted-by: gkh_clanker_2000
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: gregkh

arch/loongarch/kernel/syscall.c

@@ -9,6 +9,7 @@
 #include <linux/entry-common.h>
 #include <linux/errno.h>
 #include <linux/linkage.h>
+#include <linux/nospec.h>
 #include <linux/syscalls.h>
 #include <linux/unistd.h>
 
@@ -55,7 +56,7 @@ void noinstr do_syscall(struct pt_regs *regs)
 	nr = syscall_enter_from_user_mode(regs, nr);
 
 	if (nr < NR_syscalls) {
-		syscall_fn = sys_call_table[nr];
+		syscall_fn = sys_call_table[array_index_nospec(nr, NR_syscalls)];
 		regs->regs[4] = syscall_fn(regs->orig_a0, regs->regs[5], regs->regs[6],
 					   regs->regs[7], regs->regs[8], regs->regs[9]);
 	}