`On Tue, 2010-06-15 at 14:49 +0200, Frederic Weisbecker wrote:> On Mon, Jun 14, 2010 at 08:16:03PM -0400, Steven Rostedt wrote:> > On Mon, 2010-06-14 at 17:40 -0400, Valdis.Kletnieks@vt.edu wrote:> > > On Sun, 13 Jun 2010 17:01:34 EDT, Chase Douglas said:> > > > On Sun, 2010-06-13 at 16:52 -0400, Valdis.Kletnieks@vt.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.> > > > > > This is starting to smell like that patch is just papering over a bug...> > > > > > I saw that '8 -' and made the rash assumption that was the size of the array.> > > Is 5 in fact big enough and the  's->len - len' calculation is broken, or> > > should it be bigger?  As you noted, that length calculation is looking a tad> > > sketchy.  (And if we're stuck with '5' because it's a magic number for> > > somebody's formatting purposes, maybe it needs to be a #define?)> > > > > > > Ouch, this is worse than that. this code was cut & pasted almost> > directly from the Linux kernel (kernel/trace/trace_function_graph.c).> > And it looks like any bug here is also a bug there. The difference is> > that if we trigger the bug there we crash the kernel :-p> > > I must be missing the purpose of this patch.> > log10(nsecs_rem) can't exceed 3 characters as it is the rest of> a division per 1000.> > The goal of this:> > 	if (len < 7) {> 		snprintf(nsecs_str, 8 - len, "%03lu", nsecs_rem)> > is to avoid having a duration that exceeds 7 characters, so formatted nsecs> be shrinked on need.> > For example:> > 	75000.567> > would be shrinked to 75000.56, and that's the point.> > if (len < 7) is not a security guard, it is a formatting convenience> to get a fixed column length.> > The security guard is the mathematics that tells us log10(n % 1000) < 4.> In fact nsecs_str could be even of size 4 rather than 5.I agree that there is no *real* security issue here because of thelength of the string that snprintf would generate. However, glibc stillbarfs when you pass in a size parameter larger than the string. Withoutthis patch, trace-cmd is unusable for me; glibc aborts as soon as thecondition is hit. I found this as I was packaging trace-cmd for Ubuntu,so maybe glibc in other distributions is behaving differently?-- Chase`