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

Kees Cook
2021-10-15T21:31:38Z

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@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;
> +}

How about collapsing the "have_function_descriptors()" check into
setup_function_descriptor()?

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;
barrier();
return fdesc;
} else {
return dst;
}
}

> +
> 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

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

--
Kees Cook