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

Christophe Leroy
2021-10-16T06:42:08Z

Le 15/10/2021 à 23:31, Kees Cook a écrit :
> 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;
> }
> }

Ok

>
>> +
>> 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.
>

Ok I move it in a previous patch.