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

Christophe Leroy
2021-10-13T12:00:50Z

Le 13/10/2021 à 09:16, Kees Cook a écrit :
> 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);

Ok, I was going to add that in v2, but after one more thought I realise
that there's no need to use copy_from_kernel_nofault() here, we are
copying a static object into our stack, so a memcpy() will be good enough.

>
>> +
>> + 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
>>
>