From: "Serge E. Hallyn" An unprivileged process must be able to kill a setuid root program started by the same user. This is legacy behavior needed for instance for xinit to kill X when the window manager exits. When an unprivileged user runs a setuid root program in !SECURE_NOROOT mode, fP, fI, and fE are set full on, so pP' and pE' are full on. Then cap_task_kill() prevents the user from signaling the setuid root task. This is a change in behavior compared to when !CONFIG_SECURITY_FILE_CAPABILITIES. This patch introduces a special check into cap_task_kill() just to check whether a non-root user is signaling a setuid root program started by the same user. If so, then signal is allowed. Signed-off-by: Serge E. Hallyn Cc: Andrew Morgan Cc: Stephen Smalley Cc: Chris Wright Cc: James Morris Cc: Casey Schaufler Signed-off-by: Andrew Morton --- security/commoncap.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff -puN security/commoncap.c~file-capabilities-dont-prevent-signaling-setuid-root security/commoncap.c --- a/security/commoncap.c~file-capabilities-dont-prevent-signaling-setuid-root +++ a/security/commoncap.c @@ -526,6 +526,15 @@ int cap_task_kill(struct task_struct *p, if (info != SEND_SIG_NOINFO && (is_si_special(info) || SI_FROMKERNEL(info))) return 0; + /* + * Running a setuid root program raises your capabilities. + * Killing your own setuid root processes was previously + * allowed. + * We must preserve legacy signal behavior in this case. + */ + if (p->euid == 0 && p->uid == current->uid) + return 0; + /* sigcont is permitted within same session */ if (sig == SIGCONT && (task_session_nr(current) == task_session_nr(p))) return 0; _