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, 15 insertions, 7 deletions

diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index 62f76d506f04..2dede2ef658f 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/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. */
-	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
@@ -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;
-
 }