Re: [PATCH v2] tracing: Add test for user space strings when filtering on string pointers

[Sven Schnelle]

Hi Steve,

Steven Rostedt <rostedt@xxxxxxxxxxx> writes:

> On Tue, 11 Jan 2022 21:55:53 +0100
> Sven Schnelle <svens@xxxxxxxxxxxxx> wrote:
>
>> > Isn't there also at least one architecture where you can't differentiate
>> > between user and kernel pointers by looking at the address?
>> > (Something like sparc ASI is used for user accesses so both user
>> > and kernel get the full 4G address range. But it isn't sparc (or pdp/11))
>> > ISTR it causing issues with the code for kernel_setsockopt() and
>> > required a separate flag.  
>> 
>> On s390 TASK_SIZE is defined as -PAGE_SIZE, so with the patch above the
>> kernel would always try to fetch it from user space. I think it would be
>> the same for parisc.
>
> As a work around for these cases, would something like this work?

Hmm, i don't see how. On s390, TASK_SIZE is -PAGE_SIZE, which means
0xfffffffffffff000 so i think the if() condition below is always true.

Too bad that the __user attribute is stripped during a normal compile.
But couldn't we add the information whether a pointer belongs to user
or kernel space in the trace event definition? For syscall tracing it's
easy, because pointer types in SYSCALL_DEFINE() and friends are always
userspace pointers?

> -- Steve
>
> diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
> index 91352a64be09..06013822764c 100644
> --- a/kernel/trace/trace_events_filter.c
> +++ b/kernel/trace/trace_events_filter.c
> @@ -676,7 +676,15 @@ static __always_inline char *test_string(char *str)
>  	ubuf = this_cpu_ptr(ustring_per_cpu);
>  	kstr = ubuf->buffer;
>  
> -	if (likely((unsigned long)str >= TASK_SIZE)) {
> +	/*
> +	 * Test the address of ustring_per_cpu against TASK_SIZE, as
> +	 * comparing TASK_SIZE to determine kernel/user space address
> +	 * is not enough on some architectures. If the address is less
> +	 * than TASK_SIZE we know this is the case, in which we should
> +	 * always use the from_kernel variant.
> +	 */
> +	if ((unsigned long)&ustring_per_cpu < (unsigned long)TASK_SIZE ||
> +	    likely((unsigned long)str >= TASK_SIZE)) {
>  		/* For safety, do not trust the string pointer */
>  		if (!strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE))
>  			return NULL;