Subject: Re: [PATCH 13/20] maccess: always use strict semantics for probe_kernel_read

From: Masami Hiramatsu <[email-masked]>
Date: Wed, 20 May 2020 20:11:26 +0900

On Tue, 19 May 2020 15:44:42 +0200
Christoph Hellwig <[email-masked]> wrote:

> diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> index 2f6737cc53e6c..82da20e712507 100644
> --- a/kernel/trace/trace_kprobe.c
> +++ b/kernel/trace/trace_kprobe.c
> @@ -1208,7 +1208,13 @@ fetch_store_strlen(unsigned long addr)
>  	u8 c;
>  
>  	do {
> -		ret = probe_kernel_read(&c, (u8 *)addr + len, 1);
> +		if (IS_ENABLED(CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE) &&
> +		    (unsigned long)addr < TASK_SIZE) {
> +			ret = probe_user_read(&c,
> +				(__force u8 __user *)addr + len, 1);
> +		} else {
> +			ret = probe_kernel_read(&c, (u8 *)addr + len, 1);
> +		}
>  		len++;
>  	} while (c && ret == 0 && len < MAX_STRING_SIZE);

To avoid redundant check in the loop, we can use strnlen_user_nofault() out of
the loop. Something like below.

...
	u8 c;

	if (IS_ENABLED(CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE) &&
	    (unsigned long)addr < TASK_SIZE) {
		return strnlen_user_nofault((__force u8 __user *)addr, MAX_STRING_SIZE);

	do {
		ret = probe_kernel_read(&c, (u8 *)addr + len, 1);
		len++;
	} while (c && ret == 0 && len < MAX_STRING_SIZE);
...

This must work because we must not have a string that continues across
kernel　space and user space.

Thank you,

-- 
Masami Hiramatsu <[email-masked]>