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    SubjectRe: [PATCH] trace-cmd: prevent print_graph_duration buffer overflow
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    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 the
    length of the string that snprintf would generate. However, glibc still
    barfs when you pass in a size parameter larger than the string. Without
    this patch, trace-cmd is unusable for me; glibc aborts as soon as the
    condition is hit. I found this as I was packaging trace-cmd for Ubuntu,
    so maybe glibc in other distributions is behaving differently?

    -- Chase



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