[PATCH 5.1 031/121] lkdtm/usercopy: Moves the KERNEL_DS test to non-canonical

[Greg Kroah-Hartman]

[ Upstream commit 2bf8496f6e9b7e9a557f65eb95eab16fea7958c7 ]

The prior implementation of the KERNEL_DS fault checking would work on
any unmapped kernel address, but this was narrowed to the non-canonical
range instead. This adjusts the LKDTM test to match.

Fixes: 00c42373d397 ("x86-64: add warning for non-canonical user access address dereferences")
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 drivers/misc/lkdtm/usercopy.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/misc/lkdtm/usercopy.c b/drivers/misc/lkdtm/usercopy.c
index d5a0e7f1813b..e172719dd86d 100644
--- a/drivers/misc/lkdtm/usercopy.c
+++ b/drivers/misc/lkdtm/usercopy.c
@@ -324,14 +324,16 @@ free_user:
 
 void lkdtm_USERCOPY_KERNEL_DS(void)
 {
-	char __user *user_ptr = (char __user *)ERR_PTR(-EINVAL);
+	char __user *user_ptr =
+		(char __user *)(0xFUL << (sizeof(unsigned long) * 8 - 4));
 	mm_segment_t old_fs = get_fs();
 	char buf[10] = {0};
 
-	pr_info("attempting copy_to_user on unmapped kernel address\n");
+	pr_info("attempting copy_to_user() to noncanonical address: %px\n",
+		user_ptr);
 	set_fs(KERNEL_DS);
-	if (copy_to_user(user_ptr, buf, sizeof(buf)))
-		pr_info("copy_to_user un unmapped kernel address failed\n");
+	if (copy_to_user(user_ptr, buf, sizeof(buf)) == 0)
+		pr_err("copy_to_user() to noncanonical address succeeded!?\n");
 	set_fs(old_fs);
 }
 
-- 
2.20.1