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Subject[PATCH v2] lkdtm: fix memory copy size for WRITE_KERN
Date
From: Candle Sun <candle.sun@unisoc.com>

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.

Get absolute value of the address substraction for memcpy() size.

Signed-off-by: Candle Sun <candle.sun@unisoc.com>
---
Changes in v2:
- Use abs() in place of address comparison.
---
drivers/misc/lkdtm/perms.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index 2dede2ef658f..fbb7f4554054 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;
@@ -113,7 +113,7 @@ void lkdtm_WRITE_KERN(void)
size_t size;
volatile unsigned char *ptr;

- size = (unsigned long)do_overwritten - (unsigned long)do_nothing;
+ size = (size_t)abs((uintptr_t)do_overwritten - (uintptr_t)do_nothing);
ptr = (unsigned char *)do_overwritten;

pr_info("attempting bad %zu byte write at %px\n", size, ptr);
--
2.17.0
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