CVE-2017-5551

La función simple_set_acl en fs/posix_acl.c en el kernel de Linux en versiones anteriores a 4.9.6 preserva el bit setgid durante una llamada setxattr que implica un sistema de archivos tmpfs, lo que permite a usuarios locales obtener privilegios de grupo aprovechando la existencia de un programa setgid con restricciones sobre los permisos de ejecución. NOTA: esta vulnerabilidad existe debido a una solución incompleta para CVE-2016-7097.

A vulnerability was found in the Linux kernel in 'tmpfs' file system. When file permissions are modified via 'chmod' and the user is not in the owning group or capable of CAP_FSETID, the setgid bit is cleared in inode_change_ok(). Setting a POSIX ACL via 'setxattr' sets the file permissions as well as the new ACL, but doesn't clear the setgid bit in a similar way; this allows to bypass the check in 'chmod'.

USN-3358-1 fixed vulnerabilities in the Linux kernel for Ubuntu 17.04. This update provides the corresponding updates for the Linux Hardware Enablement kernel from Ubuntu 17.04 for Ubuntu 16.04 LTS. Please note that this update changes the Linux HWE kernel to the 4.10 based kernel from Ubuntu 17.04, superseding the 4.8 based HWE kernel from Ubuntu 16.10. Ben Harris discovered that the Linux kernel would strip extended privilege attributes of files when performing a failed unprivileged system call. A local attacker could use this to cause a denial of service. Various other issues were also addressed.