[PATCH v4 13/13] lkdtm: Add a test for function descriptors protection

Christophe Leroy
2022-02-15T12:50:45Z

Add WRITE_OPD to check that you can't modify function
descriptors.

Gives the following result when function descriptors are
not protected:

lkdtm: Performing direct entry WRITE_OPD
lkdtm: attempting bad 16 bytes write at c00000000269b358
lkdtm: FAIL: survived bad write
lkdtm: do_nothing was hijacked!

Looks like a standard compiler barrier() is not enough to force
GCC to use the modified function descriptor. Had to add a fake empty
inline assembly to force GCC to reload the function descriptor.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Kees Cook <keescook@chromium.org>
---
drivers/misc/lkdtm/core.c | 1 +
drivers/misc/lkdtm/lkdtm.h | 1 +
drivers/misc/lkdtm/perms.c | 22 ++++++++++++++++++++++
tools/testing/selftests/lkdtm/tests.txt | 1 +
4 files changed, 25 insertions(+)

diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
index f69b964b9952..e2228b6fc09b 100644
--- a/drivers/misc/lkdtm/core.c
+++ b/drivers/misc/lkdtm/core.c
@@ -149,6 +149,7 @@ static const struct crashtype crashtypes[] = {
CRASHTYPE(WRITE_RO),
CRASHTYPE(WRITE_RO_AFTER_INIT),
CRASHTYPE(WRITE_KERN),
+ CRASHTYPE(WRITE_OPD),
CRASHTYPE(REFCOUNT_INC_OVERFLOW),
CRASHTYPE(REFCOUNT_ADD_OVERFLOW),
CRASHTYPE(REFCOUNT_INC_NOT_ZERO_OVERFLOW),
diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
index d6137c70ebbe..305fc2ec3f25 100644
--- a/drivers/misc/lkdtm/lkdtm.h
+++ b/drivers/misc/lkdtm/lkdtm.h
@@ -106,6 +106,7 @@ void __init lkdtm_perms_init(void);
void lkdtm_WRITE_RO(void);
void lkdtm_WRITE_RO_AFTER_INIT(void);
void lkdtm_WRITE_KERN(void);
+void lkdtm_WRITE_OPD(void);
void lkdtm_EXEC_DATA(void);
void lkdtm_EXEC_STACK(void);
void lkdtm_EXEC_KMALLOC(void);
diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index 1cf24c4a79e9..2c6aba3ff32b 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c
@@ -44,6 +44,11 @@ static noinline void do_overwritten(void)
return;
}

+static noinline void do_almost_nothing(void)
+{
+ pr_info("do_nothing was hijacked!\n");
+}
+
static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
{
if (!have_function_descriptors())
@@ -144,6 +149,23 @@ void lkdtm_WRITE_KERN(void)
do_overwritten();
}

+void lkdtm_WRITE_OPD(void)
+{
+ size_t size = sizeof(func_desc_t);
+ void (*func)(void) = do_nothing;
+
+ if (!have_function_descriptors()) {
+ pr_info("XFAIL: Platform doesn't use function descriptors.\n");
+ return;
+ }
+ pr_info("attempting bad %zu bytes write at %px\n", size, do_nothing);
+ memcpy(do_nothing, do_almost_nothing, size);
+ pr_err("FAIL: survived bad write\n");
+
+ asm("" : "=m"(func));
+ func();
+}
+
void lkdtm_EXEC_DATA(void)
{
execute_location(data_area, CODE_WRITE);
diff --git a/tools/testing/selftests/lkdtm/tests.txt b/tools/testing/selftests/lkdtm/tests.txt
index 6b36b7f5dcf9..243c781f0780 100644
--- a/tools/testing/selftests/lkdtm/tests.txt
+++ b/tools/testing/selftests/lkdtm/tests.txt
@@ -44,6 +44,7 @@ ACCESS_NULL
WRITE_RO
WRITE_RO_AFTER_INIT
WRITE_KERN
+WRITE_OPD
REFCOUNT_INC_OVERFLOW
REFCOUNT_ADD_OVERFLOW
REFCOUNT_INC_NOT_ZERO_OVERFLOW
--
2.34.1