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    Subject[PATCH 0/2] Yama: add PTRACE exception tracking
    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) {
    sleep(1);

    --
    Kees Cook
    Ubuntu Security Team


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