From: Kees Cook on
The primary exception to Yama's descendant-based PTRACE restrictions
is when an application has a predefined crash handler that is spawned
in parallel with the crashed application (e.g. KDE, Chromium). These
applications want to bypass the common RLIMIT_CORE=0, and gather state
information from the process for upstream problem reporting. When the
main application crashes, it generally has access to the PID of what
will debug it (e.g. when a KDE app crashes, it knows the parent PID of
the debugger that will be spawned).

So, since this programmatic method of PTRACEing is useful, there should be
a way for processes to actively declare who can PTRACE them. This patch
adds a prctl hook for Yama so that processes can exempt themselves from
the PTRACE restrictions in the case of a crash when they know their
debugger's PID.

As a matter of demonstration, here is what the patch to KDE4 would look
like to support Yama, or other PTRACE-restricting LSMs that wanted to grant
a similar exception:

--- kde4libs-4.4.90.orig/kdeui/util/kcrash.cpp 2010-06-28 17:07:28.667869954 -0700
+++ kde4libs-4.4.90/kdeui/util/kcrash.cpp 2010-06-28 17:09:32.089958401 -0700
@@ -41,6 +41,7 @@
#include <sys/wait.h>
#include <sys/un.h>
#include <sys/socket.h>
+#include <sys/prctl.h>
#include <errno.h>

#include <qwindowdefs.h>
@@ -437,6 +438,7 @@
//if the process was started directly, use waitpid(), as it's a child...
while(waitpid(-1, NULL, 0) != pid) {}
} else {
+ prctl(PR_SET_PTRACER, pid, 0, 0, 0);
//...else poll its status using kill()
while(kill(pid, 0) >= 0) {

Kees Cook
Ubuntu Security Team
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