From 7ef26f26e95bdff0596543aea37ccc652cf36cd3 Mon Sep 17 00:00:00 2001
From: fef <owo@fef.moe>
Date: Sun, 1 Aug 2021 23:28:25 +0200
Subject: [PATCH] syscall: add config option to check syscall source

---
 arch/at91sam3x8e/entry.c   | 19 +++++++++++++++++++
 arch/at91sam3x8e/syscall.S |  2 ++
 include/config.h.in        |  1 +
 options.cmake              |  2 ++
 4 files changed, 24 insertions(+)

diff --git a/arch/at91sam3x8e/entry.c b/arch/at91sam3x8e/entry.c
index fea7513..ee819a5 100644
--- a/arch/at91sam3x8e/entry.c
+++ b/arch/at91sam3x8e/entry.c
@@ -9,6 +9,13 @@
 #include <errno.h>
 #include <stddef.h>
 
+#include <config.h>
+
+#ifdef CONFIG_CHECK_SYSCALL_SOURCE
+/* syscall.S */
+extern uintptr_t __syscall_entry_point;
+#endif
+
 void arch_enter(void *sp)
 {
 	struct reg_snapshot *regs = sp;
@@ -17,6 +24,18 @@ void arch_enter(void *sp)
 		       sysarg_t arg4, sysarg_t arg5, sysarg_t arg6);
 	int sc_ret;
 
+#	ifdef CONFIG_CHECK_SYSCALL_SOURCE
+	/*
+	 * We need to ignore the program counter's LSB because the CPU uses
+	 * that as a flag for whether it's operating in ARM or Thumb mode
+	 * (1 for Thumb); the instructions are always 2-byte aligned.
+	 */
+	if ((regs->hw.pc & 0xfffffffe) != __syscall_entry_point) {
+		arch_syscall_set_rval(regs, -EACCES);
+		return;
+	}
+#	endif
+
 	if (sc_num > NSYSCALLS) {
 		arch_syscall_set_rval(regs, -ENOSYS);
 		return;
diff --git a/arch/at91sam3x8e/syscall.S b/arch/at91sam3x8e/syscall.S
index b22b597..10e552d 100644
--- a/arch/at91sam3x8e/syscall.S
+++ b/arch/at91sam3x8e/syscall.S
@@ -32,6 +32,8 @@ func_begin syscall
 	ldr	r4,	[sp, #16]	/* arg5 */
 	ldr	r5,	[sp, #20]	/* arg6 */
 
+.global __syscall_entry_point
+__syscall_entry_point:
 	svc	#0
 
 	pop	{r4-r5,r7}
diff --git a/include/config.h.in b/include/config.h.in
index 36afac0..b07b93f 100644
--- a/include/config.h.in
+++ b/include/config.h.in
@@ -13,6 +13,7 @@
 #define ARDIX_VERSION_STR "@ardix_VERSION@@ardix_VERSION_SUFFIX@"
 
 #cmakedefine DEBUG
+#cmakedefine CONFIG_CHECK_SYSCALL_SOURCE
 
 #define ARCH "@ARCH@"
 #define ARCH_@ARCH_UPPERCASE@
diff --git a/options.cmake b/options.cmake
index ff2e16d..119b43b 100644
--- a/options.cmake
+++ b/options.cmake
@@ -25,6 +25,8 @@ set(CONFIG_SERIAL_BUFSZ 256 CACHE STRING "Default serial buffer size in bytes")
 
 set(CONFIG_PRINTF_BUFSZ 64 CACHE STRING "Default buffer size for printf() and friends")
 
+option(CONFIG_CHECK_SYSCALL_SOURCE "Prohibit inline syscalls" OFF)
+
 # This file is part of Ardix.
 # Copyright (c) 2021 Felix Kopp <owo@fef.moe>.
 #