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: 9ae113ce5f ("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>

drivers/misc/lkdtm/bugs.c

@@ -118,9 +118,8 @@ noinline void lkdtm_CORRUPT_STACK(void)
 	/* Use default char array length that triggers stack protection. */
 	char data[8] __aligned(sizeof(void *));
 
-	__lkdtm_CORRUPT_STACK(&data);
+	pr_info("Corrupting stack containing char array ...\n");
-
+	__lkdtm_CORRUPT_STACK((void *)&data);
-	pr_info("Corrupted stack containing char array ...\n");
 }
 
 /* Same as above but will only get a canary with -fstack-protector-strong */
@@ -131,9 +130,8 @@ noinline void lkdtm_CORRUPT_STACK_STRONG(void)
 		unsigned long *ptr;
 	} data __aligned(sizeof(void *));
 
-	__lkdtm_CORRUPT_STACK(&data);
+	pr_info("Corrupting stack containing union ...\n");
-
+	__lkdtm_CORRUPT_STACK((void *)&data);
-	pr_info("Corrupted stack containing union ...\n");
 }
 
 void lkdtm_UNALIGNED_LOAD_STORE_WRITE(void)
@@ -248,6 +246,7 @@ void lkdtm_ARRAY_BOUNDS(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

@@ -57,6 +57,7 @@ static noinline void execute_location(void *dst, bool write)
 	}
 	pr_info("attempting bad execution at %px\n", func);
 	func();
+	pr_err("FAIL: func returned\n");
 }
 
 static void execute_user_location(void *dst)
@@ -75,20 +76,22 @@ 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. */
+	/* Explicitly cast away "const" for the test and make volatile. */
-	unsigned long *ptr = (unsigned long *)&rodata;
+	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
@@ -102,19 +105,21 @@ void lkdtm_WRITE_RO_AFTER_INIT(void)
 
 	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();
 }
@@ -193,9 +198,11 @@ void lkdtm_ACCESS_USERSPACE(void)
 	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);
 }
@@ -203,19 +210,20 @@ void lkdtm_ACCESS_USERSPACE(void)
 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

@@ -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);