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

Kees Cook
2021-10-15T21:35:51Z

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@csgroup.eu>
> ---
> 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");

> + 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?)

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

--
Kees Cook