[PATCH] lkdtm: fix memory copy size for WRITE_KERN

[Candle Sun]

From: Candle Sun <candle.sun@xxxxxxxxxx>

Though do_overwritten() follows do_nothing() in source code, the final
memory address order is determined by compiler. We can't always assume
address of do_overwritten() is bigger than do_nothing(). At least the
Clang we are using places do_overwritten() before do_nothing() in the
object. This causes the copy size in lkdtm_WRITE_KERN() is *really*
big and WRITE_KERN test on ARM32 arch will fail.

Compare the address order before doing the subtraction.

Signed-off-by: Candle Sun <candle.sun@xxxxxxxxxx>
---
 drivers/misc/lkdtm/perms.c | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index 2dede2ef658f..fbfbdf89d668 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c
@@ -31,13 +31,13 @@ static unsigned long ro_after_init __ro_after_init = 0x55AA5500;
  * This just returns to the caller. It is designed to be copied into
  * non-executable memory regions.
  */
-static void do_nothing(void)
+static noinline void do_nothing(void)
 {
 	return;
 }
 
 /* Must immediately follow do_nothing for size calculuations to work out. */
-static void do_overwritten(void)
+static noinline void do_overwritten(void)
 {
 	pr_info("do_overwritten wasn't overwritten!\n");
 	return;
@@ -110,15 +110,14 @@ void lkdtm_WRITE_RO_AFTER_INIT(void)
 
 void lkdtm_WRITE_KERN(void)
 {
-	size_t size;
-	volatile unsigned char *ptr;
+	unsigned long value_dow = (unsigned long)do_overwritten;
+	unsigned long value_do =  (unsigned long)do_nothing;
+	size_t size = (size_t)(value_dow > value_do ?
+			value_dow - value_do : value_do - value_dow);
 
-	size = (unsigned long)do_overwritten - (unsigned long)do_nothing;
-	ptr = (unsigned char *)do_overwritten;
-
-	pr_info("attempting bad %zu byte write at %px\n", size, ptr);
-	memcpy((void *)ptr, (unsigned char *)do_nothing, size);
-	flush_icache_range((unsigned long)ptr, (unsigned long)(ptr + size));
+	pr_info("attempting bad %zu byte write at %px\n", size, do_overwritten);
+	memcpy((void *)value_dow, (void *)value_do, size);
+	flush_icache_range(value_dow, value_dow + (unsigned long)size);
 	pr_err("FAIL: survived bad write\n");
 
 	do_overwritten();
-- 
2.17.0