linux-kernel - Re: [PATCH] trace-cmd: prevent print_graph

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Date:	Sun, 13 Jun 2010 17:01:34 -0400
From:	Chase Douglas <chase.douglas@...onical.com>
To:	Valdis.Kletnieks@...edu
Cc:	Steven Rostedt <srostedt@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] trace-cmd: prevent print_graph_duration buffer overflow

On Sun, 2010-06-13 at 16:52 -0400, Valdis.Kletnieks@...edu wrote:
> On Sun, 13 Jun 2010 13:11:48 EDT, Chase Douglas said:
> > Passing n > sizeof(string) to snprintf can cause a glibc buffer overflow
> > condition. We know the exact size of nsecs_str, so use it instead of
> > math that may overflow.
> 
> >  	/* Print nsecs (we don't want to exceed 7 numbers) */
> >  	if ((s->len - len) < 7) {
> > -		snprintf(nsecs_str, 8 - (s->len - len), "%03lu", nsecs_rem);
> > +		snprintf(nsecs_str, sizeof(nsecs_str), "%03lu", nsecs_rem);
> 
> We only get into this code after we've checked that the length is under 7
> characters.  How much overflow can happen as long as the sizeof(nsecs_str) is a
> sane size (like at least 8 chars)?  Probably a better bet would be doing the
> right thing and 'BUILD_BUG_ON(sizeof(nsecs_str) < 8);'?

nsecs_str is a local variable defined just above this block of code as:

	char nsecs_str[5];

I was hitting cases where s->len == 64 and len == 63, leading to the
size argument of snprintf being 7 on a 5 byte string. I didn't delve too
much into the reasoning for the if statement, but I think it's math is
not actually related to the size of nsecs_rem but to some other string
length.

-- Chase

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