linux-kernel - RE: [PATCH v2] tracing: Add test for user space strings when filtering on string pointers

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Date:   Mon, 10 Jan 2022 17:11:52 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Steven Rostedt' <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>
CC:     Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pingfan Liu <kernelfans@...il.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Tom Zanussi <zanussi@...nel.org>
Subject: RE: [PATCH v2] tracing: Add test for user space strings when
 filtering on  string pointers

From: Steven Rostedt
> Sent: 10 January 2022 16:56
> 
> From: Steven Rostedt <rostedt@...dmis.org>
> 
> Pingfan reported that the following causes a fault:
> 
>   echo "filename ~ \"cpu\"" > events/syscalls/sys_enter_openat/filter
>   echo 1 > events/syscalls/sys_enter_at/enable
> 
> The reason is that trace event filter treats the user space pointer
> defined by "filename" as a normal pointer to compare against the "cpu"
> string. If the string is not loaded into memory yet, it will trigger a
> fault in kernel space:

If a userspace pointer can end up the kernel structure then presumably
a 'dodgy' user program can supply an arbitrary kernel address instead?
This may give the user the ability to read arbitrary kernel addresses
(including ones that are mapped to PCIe IO addresses).
Doesn't sound good at all.

...
> +	if (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;
> +	} else {
> +		/* user space address? */
> +		ustr = (char __user *)str;
> +		if (!strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE))
> +			return NULL;

Is that check against TASK_SIZE even correct for all architectures?
copy_to/from_user() uses access_ok() - which is architecture dependant.

I think you need to remember where the pointer came from.

	David

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