Re: [Qemu-devel] [PATCH 1/8] qdict: fix unbounded stack for qdict_array_entries

[Kevin Wolf <kwolf@redhat.com>]

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Am 09.03.2016 um 04:04 hat Eric Blake geschrieben:
> On 03/08/2016 07:57 PM, Peter Xu wrote:
> > On Tue, Mar 08, 2016 at 11:19:44AM +0100, Kevin Wolf wrote:
> >> Am 08.03.2016 um 09:22 hat Markus Armbruster geschrieben:
> >>> Same arguments as for PATCH 2, except here an argument on the maximum
> >>> length of subqdict would probably be easier.
> >>
> >> Yes, these are constant string literals in all callers, including the
> >> one non-test case in quorum.
> >>
> >> Let's simply assert a reasonable maximum for subqdict_length. The
> >> minimum we need to allow with the existing callers is 9, and I expect
> >> we'll never get keys longer than 16 characters.
> > 
> > Hi, Kevin, Markus,
> > 
> > The patch should be trying to do as mentioned above. To make it
> > clearer, how about the following one:
> > 
> > diff --git a/qobject/qdict.c b/qobject/qdict.c
> > index 9833bd0..dde99e0 100644
> > --- a/qobject/qdict.c
> > +++ b/qobject/qdict.c
> > @@ -704,17 +704,16 @@ int qdict_array_entries(QDict *src, const char *subqdict)
> >      for (i = 0; i < INT_MAX; i++) {
> >          QObject *subqobj;
> >          int subqdict_entries;
> > -        size_t slen = 32 + subqdict_len;
> > -        char indexstr[slen], prefix[slen];
> > +        char indexstr[128], prefix[128];
> >          size_t snprintf_ret;
> > 
> > -        snprintf_ret = snprintf(indexstr, slen, "%s%u", subqdict, i);
> > -        assert(snprintf_ret < slen);
> > +        snprintf_ret = snprintf(indexstr, ARRAY_SIZE(indexstr), "%s%u", subqdict, i);
> > +        assert(snprintf_ret < ARRAY_SIZE(indexstr));
> 
> sizeof(indexstr) works, and is a bit nicer than ARRAY_SIZE() when
> dealing with char.
> 
> But I'm worried that this can trigger an abort() by someone hammering on
> the command line.  Just because we don't expect any QMP command to
> validate with a key name longer than 128 doesn't mean that we don't have
> to deal with a command line with a garbage key name that long.  What's
> wrong with just using g_strdup_printf() and heap-allocating the result,
> avoiding snprintf() and fixed lengths altogether?

I can only repeat myself, we're not dealing with user data here, but
with constant literal strings. Put an assert(subqdict_len < 32); at the
beginning of the function and be done with it. Any violation of it is
not unexpected user input, but a caller bug.

> Two assertions on the snprintf_ret should make sure we are safe,
> right?

No, asserting after the fact that you haven't just overflown a buffer is
generally not a valid way of error handling (especially if you consider
that compiling with NDEBUG would make the assert disappear).

Of course, the strnlen() would already avoid this, so what the assertion
would really catch is string truncation.

In summary, the behaviour after your patch would still be correct, but
it's pointless, it's less obvious what the reason for the array size is
and it wastes memory on the stack. So I wouldn't do that.

Kevin

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