Re: uprobes: bug in comm/string output?

[Masami Hiramatsu <mhiramat@kernel.org>]

On Thu, 17 Jan 2019 14:44:41 +0100
Andreas Ziegler <andreas.ziegler@fau.de> wrote:

> 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?

Yes, clearly your patch should go to stable to fix the multiple string
support. Mine is only for v4.20.

Thank you!

> 
> I added Greg as he might know the answer to that.
> 
> Thanks,
> 
> Andreas
> 


-- 
Masami Hiramatsu <mhiramat@kernel.org>