The Linux kernel through 5.0.7, when CONFIG_IA32_AOUT is enabled and ia32_aout is loaded, allows local users to bypass ASLR on setuid a.out programs (if any exist) because install_exec_creds() is called too late in load_aout_binary() in fs/binfmt_aout.c, and thus the ptrace_may_access() check has a race condition when reading /proc/pid/stat. NOTE: the software maintainer disputes that this is a vulnerability because ASLR for a.out format executables has never been supported

Affected: Ubuntu:18.04:LTS: linux-hwe

USN-4006-1 fixed a vulnerability in the Linux kernel for Ubuntu 18.10. This update provides the corresponding updates for the Linux Hardware Enablement (HWE) kernel from Ubuntu 18.10 for Ubuntu 18.04 LTS.

Federico Manuel Bento discovered that the Linux kernel did not properly apply Address Space Layout Randomization (ASLR) in some situations for setuid a.out binaries. A local attacker could use this to improve the chances of exploiting an existing vulnerability in a setuid a.out binary.

As a hardening measure, this update disables a.out support.