[Bug 206761] Kernel stack overflow in sysctl handler for kern.binmisc.add

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=206761

--- Comment #14 from CTurt <cturt at hardenedbsd.org> ---
I've taken another look at the code and found another potential bug. I'm not
certain if this is a bug yet, but I'd also like to bring the following code
from `imgact_binmisc_add_entry` to attention:

        /* Make sure we don't have any invalid #'s. */
        p = xbe->xbe_interpreter;
        while (1) {
                p = strchr(p, '#');
                if (!p)
                        break;

                p++;
                switch(*p) {
                case ISM_POUND:
                        /* "##" */
                        p++;
                        break;

                case ISM_OLD_ARGV0:
                        /* "#a" */
                        p++;
                        break;

                case 0:
                default:
                        /* Anything besides the above is invalid. */
                        return (EINVAL);
                }
        }

>From the comment, and usage of a loop, it seems like this code should be
checking that every '#' character in the string follows either another '#' or
an 'a' character, however there is no way that this loop will ever be executed
more than once since all conditions lead to `break` or `return`. In its current
form the code will only validate the first '#' character.

To instead check that _every_ '#' character follows a valid character (and not
just the first '#' character), the `case`s should `continue` the loop as below:

        /* Make sure we don't have any invalid #'s. */
        p = xbe->xbe_interpreter;
        while (1) {
                p = strchr(p, '#');
                if (!p)
                        break;

                p++;
                switch(*p) {
                case ISM_POUND:
                        /* "##" */
                        p++;
                        continue;

                case ISM_OLD_ARGV0:
                        /* "#a" */
                        p++;
                        continue;

                case 0:
                default:
                        /* Anything besides the above is invalid. */
                        return (EINVAL);
                }
        }

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