Re: [PATCH v1 10/10] lkdtm: Fix execute_[user]_location()

Le 13/10/2021 à 09:16, Kees Cook a écrit :

On Mon, Oct 11, 2021 at 05:25:37PM +0200, Christophe Leroy wrote:

quoted

execute_location() and execute_user_location() intent
to copy do_nothing() text and execute it at a new location.
However, at the time being it doesn't copy do_nothing() function
but do_nothing() function descriptor which still points to the
original text. So at the end it still executes do_nothing() at
its original location allthough using a copied function descriptor.

So, fix that by really copying do_nothing() text and build a new
function descriptor by copying do_nothing() function descriptor and
updating the target address with the new location.

Also fix the displayed addresses by dereferencing do_nothing()
function descriptor.

Signed-off-by: Christophe Leroy <redacted>
---
  drivers/misc/lkdtm/perms.c | 45 +++++++++++++++++++++++++++++++-------
  1 file changed, 37 insertions(+), 8 deletions(-)

diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index da16564e1ecd..1d03cd44c21d 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c

@@ -44,19 +44,42 @@ static noinline void do_overwritten(void)
  	return;
  }
  
+static void *setup_function_descriptor(funct_descr_t *fdesc, void *dst)
+{
+	int err;
+
+	if (!__is_defined(HAVE_DEREFERENCE_FUNCTION_DESCRIPTOR))
+		return dst;
+
+	err = copy_from_kernel_nofault(fdesc, do_nothing, sizeof(*fdesc));
+	if (err < 0)
+		return ERR_PTR(err);
+
+	fdesc->addr = (unsigned long)dst;
+	barrier();
+
+	return fdesc;
+}
+
  static noinline void execute_location(void *dst, bool write)
  {
-	void (*func)(void) = dst;
+	void (*func)(void);
+	funct_descr_t fdesc;
+	void *do_nothing_text = dereference_symbol_descriptor(do_nothing);
  
-	pr_info("attempting ok execution at %px\n", do_nothing);
+	pr_info("attempting ok execution at %px\n", do_nothing_text);
  	do_nothing();
  
  	if (write == CODE_WRITE) {
-		memcpy(dst, do_nothing, EXEC_SIZE);
+		memcpy(dst, do_nothing_text, EXEC_SIZE);
  		flush_icache_range((unsigned long)dst,
  				   (unsigned long)dst + EXEC_SIZE);
  	}
-	pr_info("attempting bad execution at %px\n", func);
+	func = setup_function_descriptor(&fdesc, dst);
+	if (IS_ERR(func))
+		return;

I think this error case should complain with some details. :) Maybe:

	pr_err("FAIL: could not build function descriptor for %px\n", dst);

Ok, I was going to add that in v2, but after one more thought I realise that there's no need to use copy_from_kernel_nofault() here, we are copying a static object into our stack, so a memcpy() will be good enough.

quoted

+
+	pr_info("attempting bad execution at %px\n", dst);

And then leave this pr_info() as it was, before the
setup_function_descriptor() call.

quoted

  	func();
  	pr_err("FAIL: func returned\n");
  }

@@ -66,16 +89,22 @@ static void execute_user_location(void *dst)
  	int copied;
  
  	/* Intentionally crossing kernel/user memory boundary. */
-	void (*func)(void) = dst;
+	void (*func)(void);
+	funct_descr_t fdesc;
+	void *do_nothing_text = dereference_symbol_descriptor(do_nothing);
  
-	pr_info("attempting ok execution at %px\n", do_nothing);
+	pr_info("attempting ok execution at %px\n", do_nothing_text);
  	do_nothing();
  
-	copied = access_process_vm(current, (unsigned long)dst, do_nothing,
+	copied = access_process_vm(current, (unsigned long)dst, do_nothing_text,
  				   EXEC_SIZE, FOLL_WRITE);
  	if (copied < EXEC_SIZE)
  		return;
-	pr_info("attempting bad execution at %px\n", func);
+	func = setup_function_descriptor(&fdesc, dst);
+	if (IS_ERR(func))
+		return;
+
+	pr_info("attempting bad execution at %px\n", dst);

Same here.

quoted

  	func();
  	pr_err("FAIL: func returned\n");
  }
-- 
2.31.1