Le 13/10/2021 à 09:48, Christophe Leroy a écrit :

Le 13/10/2021 à 09:39, Christophe Leroy a écrit :

quoted

Le 13/10/2021 à 09:23, Kees Cook a écrit :

quoted

On Mon, Oct 11, 2021 at 05:25:36PM +0200, Christophe Leroy wrote:

quoted

Behind a location, lkdtm_EXEC_RODATA() executes a real function,
not a copy of do_nothing().

So do it directly instead of using execute_location().

And fix displayed addresses by dereferencing the function descriptors.

Signed-off-by: Christophe Leroy <redacted>
---
  drivers/misc/lkdtm/perms.c | 9 ++++++++-
  1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index 442d60ed25ef..da16564e1ecd 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c

@@ -153,7 +153,14 @@ void lkdtm_EXEC_VMALLOC(void)

  void lkdtm_EXEC_RODATA(void)
  {
-    execute_location(lkdtm_rodata_do_nothing, CODE_AS_IS);
+    pr_info("attempting ok execution at %px\n",
+        dereference_symbol_descriptor(do_nothing));
+    do_nothing();
+
+    pr_info("attempting bad execution at %px\n",
+        dereference_symbol_descriptor(lkdtm_rodata_do_nothing));
+    lkdtm_rodata_do_nothing();
+    pr_err("FAIL: func returned\n");
  }

(In re-reading this more carefully, I see now why kallsyms.h is used
earlier: _function_ vs _symbol_ descriptor.)

In the next patch:

static noinline void execute_location(void *dst, bool write)
{
...
        func = setup_function_descriptor(&fdesc, dst);
        if (IS_ERR(func))
                return;

        pr_info("attempting bad execution at %px\n", dst);
        func();
        pr_err("FAIL: func returned\n");
}

What are the conditions for which dereference_symbol_descriptor works
but dereference _function_descriptor doesn't?

When LKDTM is built as a module I guess ?

To be more precise, dereference_symbol_descriptor() calls either 
dereference_kernel_function_descriptor() or 
dereference_module_function_descriptor()

Both functions call dereference_function_descriptor() after checking 
that we want to dereference something that is in the OPD section.

If we call dereference_function_descriptor() directly instead of 
dereference_symbol_descriptor() we skip the range verification and may 
dereference something that is not a function descriptor.

Should we do that ?

Indeed we are using it only for well known functions so using 
dereference_function_descriptor() is good enough. I'll use that in v2.