Re: [PATCH v2 12/13] lkdtm: Fix execute_[user]_location()

From: Christophe Leroy
Date: Tue Nov 16 2021 - 10:08:59 EST

Hi Kees,

Le 16/10/2021 à 08:42, Christophe Leroy a écrit :

Le 15/10/2021 à 23:31, Kees Cook a écrit :
On Thu, Oct 14, 2021 at 07:50:01AM +0200, Christophe Leroy wrote:
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 <christophe.leroy@xxxxxxxxxx>
  drivers/misc/lkdtm/perms.c     | 25 +++++++++++++++++++++----
  include/asm-generic/sections.h |  5 +++++
  2 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index 5266dc28df6e..96b3ebfcb8ed 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c
@@ -44,19 +44,32 @@ static noinline void do_overwritten(void)
+static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
+    memcpy(fdesc, do_nothing, sizeof(*fdesc));
+    fdesc->addr = (unsigned long)dst;
+    barrier();
+    return fdesc;

How about collapsing the "have_function_descriptors()" check into

static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
    if (__is_defined(HAVE_FUNCTION_DESCRIPTORS)) {
        memcpy(fdesc, do_nothing, sizeof(*fdesc));
        fdesc->addr = (unsigned long)dst;
        return fdesc;
    } else {
        return dst;



diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
index 76163883c6ff..d225318538bd 100644
--- a/include/asm-generic/sections.h
+++ b/include/asm-generic/sections.h
@@ -70,6 +70,11 @@ typedef struct {
  } func_desc_t;
+static inline bool have_function_descriptors(void)
+    return __is_defined(HAVE_FUNCTION_DESCRIPTORS);
  /* random extra sections (if any).  Override
   * in asm/sections.h */
  #ifndef arch_is_kernel_text

This hunk seems like it should live in a separate patch.

Ok I move it in a previous patch.

Do you have any additional feedback or comment on series v3 ?

What's the way forward, should it go via LKDTM tree or via powerpc tree or another tree ? I see there are neither Ack-by nor Reviewed-by for the last 2 patches.