Re: uprobes: bug in comm/string output?

[Masami Hiramatsu]

On Thu, 17 Jan 2019 15:13:09 +0900
Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:

> On Wed, 16 Jan 2019 11:16:07 +0100
> Andreas Ziegler <andreas.ziegler@xxxxxx> 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@xxxxxxxxxx>
> 
> Since commit 533059281ee5 ("tracing: probeevent: Introduce new
> argument fetching code") dropped the $comm support from uprobe
> events, this re-enable it.
> 
> For $comm support, use strncpy() instead of strncpy_from_user()
> 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
> length of the comm.
> 
> Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Reported-by: Andreas Ziegler <andreas.ziegler@xxxxxx>
> ---
>  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)

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;


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>