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 <[e-mail]>
Suggested-by: Sami Tolvanen <[e-mail]>
Fixes: 9ae113ce5faf ("lkdtm: add tests for additional page permissions")
Signed-off-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/misc/lkdtm/bugs.c

···
 	/* Use default char array length that triggers stack protection. */
 	char data[8] __aligned(sizeof(void *));
 
-	__lkdtm_CORRUPT_STACK(&data);
-
-	pr_info("Corrupted stack containing char array ...\n");
+	pr_info("Corrupting stack containing char array ...\n");
+	__lkdtm_CORRUPT_STACK((void *)&data);
 }
 
 /* Same as above but will only get a canary with -fstack-protector-strong */
···
 		unsigned long *ptr;
 	} data __aligned(sizeof(void *));
 
-	__lkdtm_CORRUPT_STACK(&data);
-
-	pr_info("Corrupted stack containing union ...\n");
+	pr_info("Corrupting stack containing union ...\n");
+	__lkdtm_CORRUPT_STACK((void *)&data);
 }
 
 void lkdtm_UNALIGNED_LOAD_STORE_WRITE(void)
···
 
 	kfree(not_checked);
 	kfree(checked);
+	pr_err("FAIL: survived array bounds overflow!\n");
 }
 
 void lkdtm_CORRUPT_LIST_ADD(void)

drivers/misc/lkdtm/perms.c

···
 	}
 	pr_info("attempting bad execution at %px\n", func);
 	func();
+	pr_err("FAIL: func returned\n");
 }
 
 static void execute_user_location(void *dst)
···
 		return;
 	pr_info("attempting bad execution at %px\n", func);
 	func();
+	pr_err("FAIL: func returned\n");
 }
 
 void lkdtm_WRITE_RO(void)
 {
-	/* Explicitly cast away "const" for the test. */
-	unsigned long *ptr = (unsigned long *)&rodata;
+	/* Explicitly cast away "const" for the test and make volatile. */
+	volatile unsigned long *ptr = (unsigned long *)&rodata;
 
 	pr_info("attempting bad rodata write at %px\n", ptr);
 	*ptr ^= 0xabcd1234;
+	pr_err("FAIL: survived bad write\n");
 }
 
 void lkdtm_WRITE_RO_AFTER_INIT(void)
 {
-	unsigned long *ptr = &ro_after_init;
+	volatile unsigned long *ptr = &ro_after_init;
 
 	/*
 	 * Verify we were written to during init. Since an Oops
···
 
 	pr_info("attempting bad ro_after_init write at %px\n", ptr);
 	*ptr ^= 0xabcd1234;
+	pr_err("FAIL: survived bad write\n");
 }
 
 void lkdtm_WRITE_KERN(void)
 {
 	size_t size;
-	unsigned char *ptr;
+	volatile unsigned char *ptr;
 
 	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(ptr, (unsigned char *)do_nothing, size);
+	memcpy((void *)ptr, (unsigned char *)do_nothing, size);
 	flush_icache_range((unsigned long)ptr, (unsigned long)(ptr + size));
+	pr_err("FAIL: survived bad write\n");
 
 	do_overwritten();
 }
···
 	pr_info("attempting bad read at %px\n", ptr);
 	tmp = *ptr;
 	tmp += 0xc0dec0de;
+	pr_err("FAIL: survived bad read\n");
 
 	pr_info("attempting bad write at %px\n", ptr);
 	*ptr = tmp;
+	pr_err("FAIL: survived bad write\n");
 
 	vm_munmap(user_addr, PAGE_SIZE);
 }
···
 void lkdtm_ACCESS_NULL(void)
 {
 	unsigned long tmp;
-	unsigned long *ptr = (unsigned long *)NULL;
+	volatile unsigned long *ptr = (unsigned long *)NULL;
 
 	pr_info("attempting bad read at %px\n", ptr);
 	tmp = *ptr;
 	tmp += 0xc0dec0de;
+	pr_err("FAIL: survived bad read\n");
 
 	pr_info("attempting bad write at %px\n", ptr);
 	*ptr = tmp;
+	pr_err("FAIL: survived bad write\n");
 }
 
 void __init lkdtm_perms_init(void)
 {
 	/* Make sure we can write to __ro_after_init values during __init */
 	ro_after_init |= 0xAA;
-
 }

drivers/misc/lkdtm/usercopy.c

···
 		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);