Re: [PATCH v1 10/10] lkdtm: Fix execute_[user]_location()

Kees Cook
2021-10-13T07:16:32Z

On Mon, Oct 11, 2021 at 05:25:37PM +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 | 45 +++++++++++++++++++++++++++++++-------
> 1 file changed, 37 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> index da16564e1ecd..1d03cd44c21d 100644
> --- a/drivers/misc/lkdtm/perms.c
> +++ b/drivers/misc/lkdtm/perms.c
> @@ -44,19 +44,42 @@ static noinline void do_overwritten(void)
> return;
> }
>
> +static void *setup_function_descriptor(funct_descr_t *fdesc, void *dst)
> +{
> + int err;
> +
> + if (!__is_defined(HAVE_DEREFERENCE_FUNCTION_DESCRIPTOR))
> + return dst;
> +
> + err = copy_from_kernel_nofault(fdesc, do_nothing, sizeof(*fdesc));
> + if (err < 0)
> + return ERR_PTR(err);
> +
> + fdesc->addr = (unsigned long)dst;
> + barrier();
> +
> + return fdesc;
> +}
> +
> static noinline void execute_location(void *dst, bool write)
> {
> - void (*func)(void) = dst;
> + void (*func)(void);
> + funct_descr_t fdesc;
> + void *do_nothing_text = dereference_symbol_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);
> + func = setup_function_descriptor(&fdesc, dst);
> + if (IS_ERR(func))
> + return;

I think this error case should complain with some details. :) Maybe:

pr_err("FAIL: could not build function descriptor for %px\n", dst);

> +
> + pr_info("attempting bad execution at %px\n", dst);

And then leave this pr_info() as it was, before the
setup_function_descriptor() call.

> func();
> pr_err("FAIL: func returned\n");
> }
> @@ -66,16 +89,22 @@ static void execute_user_location(void *dst)
> int copied;
>
> /* Intentionally crossing kernel/user memory boundary. */
> - void (*func)(void) = dst;
> + void (*func)(void);
> + funct_descr_t fdesc;
> + void *do_nothing_text = dereference_symbol_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);
> + func = setup_function_descriptor(&fdesc, dst);
> + if (IS_ERR(func))
> + return;
> +
> + pr_info("attempting bad execution at %px\n", dst);

Same here.

> func();
> pr_err("FAIL: func returned\n");
> }
> --
> 2.31.1
>

--
Kees Cook