Re: [PATCH v2 13/13] lkdtm: Add a test for function descriptors protection

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Le 15/10/2021 à 23:35, Kees Cook a écrit :
On Thu, Oct 14, 2021 at 07:50:02AM +0200, Christophe Leroy wrote:
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. Add to add a fake empty
inline assembly to force GCC to reload the function descriptor.

Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
---
  drivers/misc/lkdtm/core.c  |  1 +
  drivers/misc/lkdtm/lkdtm.h |  1 +
  drivers/misc/lkdtm/perms.c | 22 ++++++++++++++++++++++
  3 files changed, 24 insertions(+)

diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
index fe6fd34b8caf..de092aa03b5d 100644
--- a/drivers/misc/lkdtm/core.c
+++ b/drivers/misc/lkdtm/core.c
@@ -148,6 +148,7 @@ 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 c212a253edde..188bd0fd6575 100644
--- a/drivers/misc/lkdtm/lkdtm.h
+++ b/drivers/misc/lkdtm/lkdtm.h
@@ -105,6 +105,7 @@ 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 96b3ebfcb8ed..3870bc82d40d 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)
  {
  	memcpy(fdesc, do_nothing, sizeof(*fdesc));
@@ -143,6 +148,23 @@ 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("Platform doesn't have function descriptors.\n");

This should be more explicit ('xfail'):

	pr_info("XFAIL: platform doesn't use function descriptors.\n");

Ok



+		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));

Since this is a descriptor, I assume no icache flush is needed. Are
function descriptors strictly dcache? (Is anything besides just a
barrier needed?)

No flush is needed, the code just loads the function address from memory into CTR, loads R2 and branch to CTR:

	 19c:	e9 21 00 70 	ld      r9,112(r1)
	 1a0:	e9 49 00 00 	ld      r10,0(r9)
	 1a4:	7d 49 03 a6 	mtctr   r10
	 1a8:	e8 49 00 08 	ld      r2,8(r9)
	 1ac:	4e 80 04 21 	bctrl



+	func();
+}
+
  void lkdtm_EXEC_DATA(void)
  {
  	execute_location(data_area, CODE_WRITE);
--
2.31.1







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