linux-kernel - Re: [PATCH 2/2] tracing/filters: use strcmp() instead of strncmp()

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Message-ID: <20090530135236.GB5969@nowhere>
Date:	Sat, 30 May 2009 15:52:37 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Tom Zanussi <tzanussi@...il.com>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] tracing/filters: use strcmp() instead of strncmp()

On Sat, May 30, 2009 at 05:06:39PM +0800, Li Zefan wrote:
> Frédéric Weisbecker wrote:
> > 2009/5/29 Li Zefan <lizf@...fujitsu.com>:
> >> Trace filter is not working normally:
> >>
> >>  # echo 'name == et' > tracing/events/irq/irq_handler_entry/filter
> >>  # echo 1 > tracing/events/irq/irq_handler_entry/enable
> >>  # cat trace_pipe
> >>      <idle>-0     [001]  1363.423175: irq_handler_entry: irq=18 handler=eth0
> >>      <idle>-0     [001]  1363.934528: irq_handler_entry: irq=18 handler=eth0
> >>      ...
> >>
> >> It's because we pass to trace_define_field() the information of
> >> __str_loc_##item, but not the actual string, so pred->str_len == field->size
> >> == sizeof(unsigned short), thus it always compare at most 2 bytes when
> >> filtering on __string() field.
> > 
> > 
> > Weird, I was about sure I set the size of each string() to FILTER_MAX_STRING (or
> > something like that).
> > 
> > Anyway this patch looks good but it does more than just fixing the
> > issue, it removes
> > the string len boundary security we had with strncmp() for every
> > string (static and
> > dynamic size).
> > 
> > The potential side effect that comes along this patch would disappear if
> > you just turn strncmp into strcmp only in filter_pred_strloc().
> > 
> > If you do that also for fixed size strings, then it should be done in
> > a second patch,
> > although I guess turning anything here into strcmp is fine because the
> > strings given
> > by the user are always limited in their size. But we never know...
> > 
> 
> I don't think there's any security issue. It's irrelevant how big the user-input
> strings are. The point is those strings are guaranteed to be NULL-terminated.
> Am I missing something?
> 
> And I don't think it's necessary to make 2 patches that each patch converts
> one strncmp to strcmp. But maybe it's better to improve this changelog?


Hmm, you must be right, indeed they seem to be guaranted beeing NULL-terminated
strings.

But at least improve your changelog to explain why your solution is
safe.

Thanks,
Frederic.

 
> > 
> > Thanks,
> > Frederic.
> > 
> > 
> >> Since __string() is dynamic size, we are not able to set field->size to
> >> string length. Thus this patch uses strcmp() instead of strncmp().
> >>
> >> [ Impact: make filter facility working normally for __string() field ]
> >>
> >> Signed-off-by: Li Zefan <lizf@...fujitsu.com>
> >> ---

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