lkdtm: add function for testing .rodata section

This adds a function that lives in the .rodata section. The section
flags are corrected using objcopy since there is no way with gcc to
declare section flags in an architecture-agnostic way.

Signed-off-by: Kees Cook <keescook@chromium.org>

4 files changed, 41 insertions, 7 deletions

diff --git a/drivers/misc/Makefile b/drivers/misc/Makefile
index c3cb6ad8cc37..7d45ed4a1549 100644
--- a/drivers/misc/Makefile
+++ b/drivers/misc/Makefile
@@ -59,3 +59,11 @@ obj-$(CONFIG_CXL_BASE)		+= cxl/
 obj-$(CONFIG_PANEL)             += panel.o
 
 lkdtm-$(CONFIG_LKDTM)           += lkdtm_core.o
+lkdtm-$(CONFIG_LKDTM)           += lkdtm_rodata_objcopy.o
+
+OBJCOPYFLAGS :=
+OBJCOPYFLAGS_lkdtm_rodata_objcopy.o := \
+                        --set-section-flags .text=alloc,readonly \
+                        --rename-section .text=.rodata
+$(obj)/lkdtm_rodata_objcopy.o: $(obj)/lkdtm_rodata.o
+        $(call if_changed,objcopy)
diff --git a/drivers/misc/lkdtm.h b/drivers/misc/lkdtm.h
new file mode 100644
index 000000000000..9531fa3be4c3
--- /dev/null
+++ b/drivers/misc/lkdtm.h
@@ -0,0 +1,6 @@
+#ifndef __LKDTM_H
+#define __LKDTM_H
+
+void lkdtm_rodata_do_nothing(void);
+
+#endif
diff --git a/drivers/misc/lkdtm_core.c b/drivers/misc/lkdtm_core.c
index 605050c60f10..187cd9b63e9a 100644
--- a/drivers/misc/lkdtm_core.c
+++ b/drivers/misc/lkdtm_core.c
@@ -52,6 +52,8 @@
 #include <linux/ide.h>
 #endif
 
+#include "lkdtm.h"
+
 /*
  * Make sure our attempts to over run the kernel stack doesn't trigger
  * a compiler warning when CONFIG_FRAME_WARN is set. Then make sure we
@@ -103,6 +105,7 @@ enum ctype {
         CT_EXEC_STACK,
         CT_EXEC_KMALLOC,
         CT_EXEC_VMALLOC,
+        CT_EXEC_RODATA,
         CT_EXEC_USERSPACE,
         CT_ACCESS_USERSPACE,
         CT_WRITE_RO,
@@ -145,6 +148,7 @@ static char* cp_type[] = {
         "EXEC_STACK",
         "EXEC_KMALLOC",
         "EXEC_VMALLOC",
+        "EXEC_RODATA",
         "EXEC_USERSPACE",
         "ACCESS_USERSPACE",
         "WRITE_RO",
@@ -346,15 +350,18 @@ static noinline void corrupt_stack(void)
         memset((void *)data, 0, 64);
 }
 
-static void noinline execute_location(void *dst)
+static noinline void execute_location(void *dst, bool write)
 {
         void (*func)(void) = dst;
 
         pr_info("attempting ok execution at %p\n", do_nothing);
         do_nothing();
 
-        memcpy(dst, do_nothing, EXEC_SIZE);
-        flush_icache_range((unsigned long)dst, (unsigned long)dst + EXEC_SIZE);
+        if (write) {
+                memcpy(dst, do_nothing, EXEC_SIZE);
+                flush_icache_range((unsigned long)dst,
+                                   (unsigned long)dst + EXEC_SIZE);
+        }
         pr_info("attempting bad execution at %p\n", func);
         func();
 }
@@ -551,25 +558,28 @@ static void lkdtm_do_action(enum ctype which)
                 schedule();
                 break;
         case CT_EXEC_DATA:
-                execute_location(data_area);
+                execute_location(data_area, true);
                 break;
         case CT_EXEC_STACK: {
                 u8 stack_area[EXEC_SIZE];
-                execute_location(stack_area);
+                execute_location(stack_area, true);
                 break;
         }
         case CT_EXEC_KMALLOC: {
                 u32 *kmalloc_area = kmalloc(EXEC_SIZE, GFP_KERNEL);
-                execute_location(kmalloc_area);
+                execute_location(kmalloc_area, true);
                 kfree(kmalloc_area);
                 break;
         }
         case CT_EXEC_VMALLOC: {
                 u32 *vmalloc_area = vmalloc(EXEC_SIZE);
-                execute_location(vmalloc_area);
+                execute_location(vmalloc_area, true);
                 vfree(vmalloc_area);
                 break;
         }
+        case CT_EXEC_RODATA:
+                execute_location(lkdtm_rodata_do_nothing, false);
+                break;
         case CT_EXEC_USERSPACE: {
                 unsigned long user_addr;
 
diff --git a/drivers/misc/lkdtm_rodata.c b/drivers/misc/lkdtm_rodata.c
new file mode 100644
index 000000000000..4d0d851f02b9
--- /dev/null
+++ b/drivers/misc/lkdtm_rodata.c
@@ -0,0 +1,10 @@
+/*
+ * This includes functions that are meant to live entirely in .rodata
+ * (via objcopy tricks), to validate the non-executability of .rodata.
+ */
+#include <linux/kernel.h>
+
+void lkdtm_rodata_do_nothing(void)
+{
+        /* Does nothing. We just want an architecture agnostic "return". */
+}