Le 13/10/2021 à 09:05, Kees Cook a écrit :

On Mon, Oct 11, 2021 at 05:25:35PM +0200, Christophe Leroy wrote:

quoted

WRITE_KERN is supposed to overwrite some kernel text, namely
do_overwritten() function.

But at the time being it overwrites do_overwritten() function
descriptor, not function text.

Fix it by dereferencing the function descriptor to obtain
function text pointer.

And make do_overwritten() noinline so that it is really
do_overwritten() which is called by lkdtm_WRITE_KERN().

Signed-off-by: Christophe Leroy <redacted>
---
  drivers/misc/lkdtm/perms.c | 8 +++++---
  1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index 60b3b2fe929d..442d60ed25ef 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c

@@ -5,6 +5,7 @@
   * even non-readable regions.
   */
  #include "lkdtm.h"
+#include <linux/kallsyms.h>

Why not #include <asm/sections.h> instead here?

dereference_symbol_descriptor() is defined in linux/kallsyms.h

quoted

  #include <linux/slab.h>
  #include <linux/vmalloc.h>
  #include <linux/mman.h>

@@ -37,7 +38,7 @@ static noinline void do_nothing(void)
  }
  
  /* Must immediately follow do_nothing for size calculuations to work out. */
-static void do_overwritten(void)
+static noinline void do_overwritten(void)
  {
  	pr_info("do_overwritten wasn't overwritten!\n");
  	return;

@@ -113,8 +114,9 @@ void lkdtm_WRITE_KERN(void)
  	size_t size;
  	volatile unsigned char *ptr;
  
-	size = (unsigned long)do_overwritten - (unsigned long)do_nothing;
-	ptr = (unsigned char *)do_overwritten;
+	size = (unsigned long)dereference_symbol_descriptor(do_overwritten) -
+	       (unsigned long)dereference_symbol_descriptor(do_nothing);
+	ptr = dereference_symbol_descriptor(do_overwritten);

But otherwise, yup, I expect there will be a bunch of things like this
to clean up in LKDTM. :| Sorry about that!

Acked-by: Kees Cook <redacted>

quoted

  
  	pr_info("attempting bad %zu byte write at %px\n", size, ptr);
  	memcpy((void *)ptr, (unsigned char *)do_nothing, size);
-- 
2.31.1