Problem description: On Ubuntu Vivid Linux distribution apport is used for automated sending of client program crash dumps but also of kernel crash dumps. For kernel crashes, upstart or SysV init invokes the program /usr/share/apport/kernel_crashdump at boot to prepare crash dump files for sending. This action is performed with root privileges. As the crash dump directory /var/crash/ is world writable and kernel_crashdump performs file access in unsafe manner, any local user may trigger a denial of service or escalate to root privileges. If symlink and hardlink protection is enabled (which should be the default for any modern system), only denial of service is possible.
Problematic syscall in kernel_crashdump is:
open("/var/crash/linux-image-3.19.0-18-generic.0.crash", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE|O_CLOEXEC, 0666) = 30
open("/var/crash/vmcore.log", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 31
Thus the output file is opened unconditionally and without O_EXCL or O_NOFOLLOW. Also opening of input file does not care about links.
Proof of Concept: