[PATCH v2 3/4] lkdtm: verify that __ro_after_init works correctly

[Kees Cook]

The new __ro_after_init section should be writable before init, but
not after. Validate that it gets updated at init and can't be written
to afterwards.

Signed-off-by: Kees Cook <keesc...@chromium.org>
---
 drivers/misc/lkdtm.c | 29 ++++++++++++++++++++++++++---
 1 file changed, 26 insertions(+), 3 deletions(-)

diff --git a/drivers/misc/lkdtm.c b/drivers/misc/lkdtm.c
index 11fdadc68e53..2a6eaf1122b4 100644
--- a/drivers/misc/lkdtm.c
+++ b/drivers/misc/lkdtm.c
@@ -103,6 +103,7 @@ enum ctype {
        CT_EXEC_USERSPACE,
        CT_ACCESS_USERSPACE,
        CT_WRITE_RO,
+       CT_WRITE_RO_AFTER_INIT,
        CT_WRITE_KERN,
 };
 
@@ -140,6 +141,7 @@ static char* cp_type[] = {
        "EXEC_USERSPACE",
        "ACCESS_USERSPACE",
        "WRITE_RO",
+       "WRITE_RO_AFTER_INIT",
        "WRITE_KERN",
 };
 
@@ -162,6 +164,7 @@ static DEFINE_SPINLOCK(lock_me_up);
 static u8 data_area[EXEC_SIZE];
 
 static const unsigned long rodata = 0xAA55AA55;
+static unsigned long ro_after_init __ro_after_init = 0x55AA5500;
 
 module_param(recur_count, int, 0644);
 MODULE_PARM_DESC(recur_count, " Recursion level for the stack overflow test");
@@ -503,11 +506,28 @@ static void lkdtm_do_action(enum ctype which)
                break;
        }
        case CT_WRITE_RO: {
-               unsigned long *ptr;
+               /* Explicitly cast away "const" for the test. */
+               unsigned long *ptr = (unsigned long *)&rodata;
 
-               ptr = (unsigned long *)&rodata;
+               pr_info("attempting bad rodata write at %p\n", ptr);
+               *ptr ^= 0xabcd1234;
 
-               pr_info("attempting bad write at %p\n", ptr);
+               break;
+       }
+       case CT_WRITE_RO_AFTER_INIT: {
+               unsigned long *ptr = &ro_after_init;
+
+               /*
+                * Verify we were written to during init. Since an Oops
+                * is considered a "success", a failure is to just skip the
+                * real test.
+                */
+               if ((*ptr & 0xAA) != 0xAA) {
+                       pr_info("%p was NOT written during init!?\n", ptr);
+                       break;
+               }
+
+               pr_info("attempting bad ro_after_init write at %p\n", ptr);
                *ptr ^= 0xabcd1234;
 
                break;
@@ -817,6 +837,9 @@ static int __init lkdtm_module_init(void)
        int n_debugfs_entries = 1; /* Assume only the direct entry */
        int i;
 
+       /* Make sure we can write to __ro_after_init values during __init */
+       ro_after_init |= 0xAA;
+
        /* Register debugfs interface */
        lkdtm_debugfs_root = debugfs_create_dir("provoke-crash", NULL);
        if (!lkdtm_debugfs_root) {
-- 
1.9.1

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