Re: [PATCH v2 1/3] tracing: uprobes: Re-enable $comm support for uprobe events

[Steven Rostedt]

On Fri, 18 Jan 2019 13:44:25 +0900
Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:

> Since commit 533059281ee5 ("tracing: probeevent: Introduce new
> argument fetching code") dropped the $comm support from uprobe
> events, this re-enables it.
> 
> For $comm support, uses strlcpy() 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 uses strlen() instead of strnlen_user() to measure the
> length of the comm.
> 
> Fixes: 533059281ee5 ("tracing: probeevent: Introduce new argument fetching code")
> Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Reported-by: Andreas Ziegler <andreas.ziegler@xxxxxx>
> Acked-by: Andreas Ziegler <andreas.ziegler@xxxxxx>
> ---
>  kernel/trace/trace_uprobe.c |   15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
> index 3a1d5ab6b4ba..b07e498ccbc6 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);

As user space (although only root) defines the size of the event being
stored, and we could trick addr to be current->comm (although
difficult), we could possibly leak data if maxlen is > TASK_COMM_LEN. I
would feel better if we tested maxlen against TASK_COMM_LEN in this
case.

	if (maxlen > TASK_COMM_LEN)
		maxlen = TASK_COMM_LEN;

Or if we don't think it can happen, add a WARN_ON(maxlen >
TASK_COMM_LEN).

-- Steve


> +	else
> +		ret = strncpy_from_user(dst, src, maxlen);
>  	if (ret >= 0) {
>  		if (ret == maxlen)
>  			dst[ret - 1] = '\0';
> @@ -180,7 +183,12 @@ 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);
> +		if (len)
> +			len++;
> +	} else
> +		len = strnlen_user(vaddr, MAX_STRING_SIZE);
>  
>  	return (len > MAX_STRING_SIZE) ? 0 : len;
>  }
> @@ -220,6 +228,9 @@ 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;