linux-kernel - Re: uprobes: bug in comm/string output?

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Date:   Thu, 17 Jan 2019 09:08:41 +0100
From:   Andreas Ziegler <andreas.ziegler@....de>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: uprobes: bug in comm/string output?

On 17.01.19 09:00, Masami Hiramatsu wrote:
> On Thu, 17 Jan 2019 15:13:09 +0900
> Masami Hiramatsu <mhiramat@...nel.org> wrote:
> 
>> On Wed, 16 Jan 2019 11:16:07 +0100
>> Andreas Ziegler <andreas.ziegler@....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@...nel.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@...nel.org>
>> Reported-by: Andreas Ziegler <andreas.ziegler@....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@....de>

> 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;
> 
>