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

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Date:   Tue, 11 Jan 2022 21:55:53 +0100
From:   Sven Schnelle <svens@...ux.ibm.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     "'Steven Rostedt'" <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        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>, hca@...ux.ibm.com,
        deller@....de
Subject: Re: [PATCH v2] tracing: Add test for user space strings when
 filtering on  string pointers

David Laight <David.Laight@...LAB.COM> writes:

> From: Steven Rostedt
>> Sent: 10 January 2022 17:25
> ...
>> > ...
>> > > +	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.
>> 
>> The problem with access_ok() (which I tried first) is that it can't be used
>> from interrupt context, and this check can happen in interrupt context.
>> Either way, if we pick the wrong one for the arch, the only thing bad that
>> can happen is that it returns "fault" and the filter fails, just like if
>> the pointer was to bad memory.
>
> 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.

> Put that together with something that needs user_access_begin()
> to bracket user accesses and you probably fail big-time.
>
> 	David
>
> -
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