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

Christophe Leroy
2021-10-14T06:25:15Z

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@csgroup.eu>
---
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)
return;
}

+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;
+}
+
static noinline void execute_location(void *dst, bool write)
{
void (*func)(void) = dst;
+ func_desc_t fdesc;
+ void *do_nothing_text = dereference_function_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);
+ if (have_function_descriptors())
+ func = setup_function_descriptor(&fdesc, dst);
func();
pr_err("FAIL: func returned\n");
}
@@ -67,15 +80,19 @@ static void execute_user_location(void *dst)

/* Intentionally crossing kernel/user memory boundary. */
void (*func)(void) = dst;
+ func_desc_t fdesc;
+ void *do_nothing_text = dereference_function_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);
+ if (have_function_descriptors())
+ func = setup_function_descriptor(&fdesc, dst);
func();
pr_err("FAIL: func returned\n");
}
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;
#endif

+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
--
2.31.1