lkdtm: Avoid more compiler optimizations for bad writes

It seems at least Clang is able to throw away writes it knows are
destined for read-only memory, which makes things like the WRITE_RO test
fail, as the write gets elided. Instead, force the variable to be
volatile, and make similar changes through-out other tests in an effort
to avoid needing to repeat fixing these kinds of problems. Also includes
pr_err() calls in failure paths so that kernel logs are more clear in
the failure case.

Reported-by: Prasad Sodagudi <psodagud@codeaurora.org>
Suggested-by: Sami Tolvanen <samitolvanen@google.com>
Fixes: 9ae113ce5faf ("lkdtm: add tests for additional page permissions")
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200625203704.317097-2-keescook@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 5 insertions, 2 deletions

diff --git a/drivers/misc/lkdtm/usercopy.c b/drivers/misc/lkdtm/usercopy.c
index e172719dd86d..b833367a45d0 100644
--- a/drivers/misc/lkdtm/usercopy.c
+++ b/drivers/misc/lkdtm/usercopy.c
@@ -304,19 +304,22 @@ void lkdtm_USERCOPY_KERNEL(void)
 		return;
 	}
 
-	pr_info("attempting good copy_to_user from kernel rodata\n");
+	pr_info("attempting good copy_to_user from kernel rodata: %px\n",
+		test_text);
 	if (copy_to_user((void __user *)user_addr, test_text,
 			 unconst + sizeof(test_text))) {
 		pr_warn("copy_to_user failed unexpectedly?!\n");
 		goto free_user;
 	}
 
-	pr_info("attempting bad copy_to_user from kernel text\n");
+	pr_info("attempting bad copy_to_user from kernel text: %px\n",
+		vm_mmap);
 	if (copy_to_user((void __user *)user_addr, vm_mmap,
 			 unconst + PAGE_SIZE)) {
 		pr_warn("copy_to_user failed, but lacked Oops\n");
 		goto free_user;
 	}
+	pr_err("FAIL: survived bad copy_to_user()\n");
 
 free_user:
 	vm_munmap(user_addr, PAGE_SIZE);