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

[Christophe Leroy <christophe.leroy@csgroup.eu>]

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
>>
>