Commit 244fa852fe27 ("dwm: Fix heap buffer overflow in getatomprop")
introduced a check for dl > 0 before dereferencing the property pointer.
However, I missed that the variable dl is passed to XGetWindowProperty
for both nitems_return and bytes_after_return parameters:

    XGetWindowProperty(..., &dl, &dl, &p)

The final value in dl is bytes_after_return, not nitems_return. For a
successfully read property, bytes_after is typically 0 (indicating all
data was retrieved), so the check `dl > 0` is always false and dwm never
reads any atom properties. So this is safe, but not very helpful :-)

dl is probably just a dummy variable anyway, so fix by using a separate
variable for nitems, and check nitems > 0 as originally intended.

Put the maintainer at the top and bump years (time flies).

When getatomprop() is called, it invokes XGetWindowProperty() to
retrieve an Atom. If the property exists but has zero elements (length
0), Xlib returns Success and sets p to a valid, non-NULL memory address
containing a single null byte.

However, dl (that is, the number of items) is 0. dwm blindly casts p to
Atom* and dereferences it. While Xlib guarantees that p is safe to read
as a string (that is, it is null-terminated), it does _not_ guarantee it
is safe to read as an Atom (an unsigned long).

The Atom type is a typedef for unsigned long. Reading an Atom (which
thus will either likely be 4 or 8 bytes) from a 1-byte allocated buffer
results in a heap buffer overflow. Since property content is user
controlled, this allows any client to trigger an out of bounds read
simply by setting a property with format 32 and length 0.

An example client which reliably crashes dwm under ASAN:

    #include <X11/Xlib.h>
    #include <X11/Xatom.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <unistd.h>

    int main(void) {
        Display *d;
        Window root, w;
        Atom net_wm_state;

        d = XOpenDisplay(NULL);
        if (!d) return 1;

        root = DefaultRootWindow(d);
        w = XCreateSimpleWindow(d, root, 10, 10, 200, 200, 1, 0, 0);
        net_wm_state = XInternAtom(d, "_NET_WM_STATE", False);
        if (net_wm_state == None) return 1;

        XChangeProperty(d, w, net_wm_state, XA_ATOM, 32,
                        PropModeReplace, NULL, 0);
        XMapWindow(d, w);
        XSync(d, False);
        sleep(1);

        XCloseDisplay(d);
        return 0;
    }

In order to avoid this, check that the number of items returned is
greater than zero before dereferencing the pointer.

Because drw_scm_create() allocates it.

Bump the default from 60 to 120.

Original patch by Raymond Cole with some modifications, thanks!

- drw: minor improvement to the nomatches cache
- overhaul utf8decoding and render invalid utf8 sequences as U+FFFD.

Thanks NRK for these improvements!