Re: uprobes: bug in comm/string output?

[Andreas Ziegler <andreas.ziegler@fau.de>]

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Hi,

On 1/17/19 10:47 AM, Masami Hiramatsu wrote:
> On Thu, 17 Jan 2019 09:08:41 +0100
> Andreas Ziegler <andreas.ziegler@fau.de> wrote:
> 
>> On 17.01.19 09:00, Masami Hiramatsu wrote:
>>> On Thu, 17 Jan 2019 15:13:09 +0900
>>> Masami Hiramatsu <mhiramat@kernel.org> wrote:
>>>
>>>> On Wed, 16 Jan 2019 11:16:07 +0100
>>>> Andreas Ziegler <andreas.ziegler@fau.de> wrote:
>>>>
>>>>>
>>>>> I went into this a bit deeper today, and right now it is simply failing
>>>>> to parse the code because there is no FETCH_OP_COMM case in
>>>>> process_fetch_insn() for uprobes so that will return -EILSEQ, leading to
>>>>> a make_data_loc(0, ...) in store_trace_args(). If we just add
>>>>> FETCH_OP_COMM and let val point to current->comm (that's what
>>>>> trace_kprobe.c does), we get an -EFAULT return value from
>>>>> fetch_store_string because strncpy_from_user() checks if the argument is
>>>>> in user space.
>>>>
>>>> Correct. I missed to add OP_COMM support. And uprobe's fetch_store_string
>>>> is only for user space strings.
>>>>
>>>>> So I think we might need a special case for that, something like
>>>>> FETCH_OP_ST_COMM_STRING which is only used for FETCH_OP_COMM and copies
>>>>> current->comm over to the dynamic area. The implementation could be
>>>>> similar to the old fetch_comm_string implementation before your rewrite.
>>>>
>>>> Hmm, instead, I would like to add current->comm checker and only allows
>>>> to copy that. That would be simpler and enough.
>>>>
>>>> Could you test below patch?
>>>>
>>>>
>>>> tracing: uprobes: Re-enable $comm support for uprobe events
>>>>
>>>> From: Masami Hiramatsu <mhiramat@kernel.org>
>>>>
>>>> Since commit 533059281ee5 ("tracing: probeevent: Introduce new
>>>> argument fetching code") dropped the $comm support from uprobe
>>>> events, this re-enable it.
>>
>> this should read 're-enables'.
>>
>>>>
>>>> For $comm support, use strncpy() instead of strncpy_from_user()
>>                               ^
>> we're using strlcpy(), not strncpy().
>>
>>>> to copy current task's comm. Because it is in the kernel space,
>>>> strncpy_from_user() always fails to copy the comm.
>>>> This also use strlen() instead of strlen_user() to measure the
>>                 ^                        ^
>> 'uses', and the function should be 'strnlen_user()'.
>>
>>>> length of the comm.
>>>>
>>>> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
>>>> Reported-by: Andreas Ziegler <andreas.ziegler@fau.de>
>>>> ---
>>>>   kernel/trace/trace_uprobe.c |   13 +++++++++++--
>>>>   1 file changed, 11 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
>>>> index e335576b9411..97d134e83e0f 100644
>>>> --- a/kernel/trace/trace_uprobe.c
>>>> +++ b/kernel/trace/trace_uprobe.c
>>>> @@ -156,7 +156,10 @@ fetch_store_string(unsigned long addr, void *dest, void *base)
>>>>   	if (unlikely(!maxlen))
>>>>   		return -ENOMEM;
>>>>   
>>>> -	ret = strncpy_from_user(dst, src, maxlen);
>>>> +	if (addr == (unsigned long)current->comm)
>>>> +		ret = strlcpy(dst, current->comm, maxlen);
>>>> +	else
>>>> +		ret = strncpy_from_user(dst, src, maxlen);
>>>>   	if (ret >= 0) {
>>>>   		if (ret == maxlen)
>>>>   			dst[ret - 1] = '\0';
>>>> @@ -173,7 +176,10 @@ fetch_store_strlen(unsigned long addr)
>>>>   	int len;
>>>>   	void __user *vaddr = (void __force __user *) addr;
>>>>   
>>>> -	len = strnlen_user(vaddr, MAX_STRING_SIZE);
>>>> +	if (addr == (unsigned long)current->comm)
>>>> +		len = strlen(current->comm);
>>>
>>> To balance with the strnlen_user, we must increse the len in this block.
>>> (strlen doesn't count the final '\0', but strnlen_user counts it)
>>>
>>
>> yes, we need to add a '+ 1' here.
>>
>> With the typos and this one fixed, this is
>>
>> Acked-by: Andreas Ziegler <andreas.ziegler@fau.de>
> 
> Thank you for fixing typo and Ack :)
> 
> Thanks you,
 >
>>
>>> Thank you,
>>>
>>>> +	else
>>>> +		len = strnlen_user(vaddr, MAX_STRING_SIZE);
>>>>   
>>>>   	return (len > MAX_STRING_SIZE) ? 0 : len;
>>>>   }
>>>> @@ -213,6 +219,9 @@ process_fetch_insn(struct fetch_insn *code, struct pt_regs *regs, void *dest,
>>>>   	case FETCH_OP_IMM:
>>>>   		val = code->immediate;
>>>>   		break;
>>>> +	case FETCH_OP_COMM:
>>>> +		val = (unsigned long)current->comm;
>>>> +		break;
>>>>   	case FETCH_OP_FOFFS:
>>>>   		val = translate_user_vaddr(code->immediate);
>>>>   		break;
>>>
>>>
>>
> 
> 
as the original commit breaking $comm support was merged for v4.20 
(which is a stable kernel) and the wrong behaviour with multiple strings 
exists in all longterm/stable releases (tested back to v4.4), do you 
think this should be going into a stable release once it's merged?

I added Greg as he might know the answer to that.

Thanks,

Andreas


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