Messages in this thread | | | Subject | Re: [PATCH] trace-cmd: prevent print_graph_duration buffer overflow | From | Valdis.Kletnieks@vt ... | Date | Mon, 14 Jun 2010 17:40:33 -0400 |
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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?)
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