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 21:58:33 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Steven Rostedt' <rostedt@...dmis.org>
CC:     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>
Subject: RE: [PATCH v2] tracing: Add test for user space strings when
 filtering on  string pointers

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.

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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